xref: /llvm-project/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp (revision 05da2fe52162c80dfa18aedf70cf73cb11201811)
1 ///===- SimpleLoopUnswitch.cpp - Hoist loop-invariant control flow ---------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 
9 #include "llvm/Transforms/Scalar/SimpleLoopUnswitch.h"
10 #include "llvm/ADT/DenseMap.h"
11 #include "llvm/ADT/STLExtras.h"
12 #include "llvm/ADT/Sequence.h"
13 #include "llvm/ADT/SetVector.h"
14 #include "llvm/ADT/SmallPtrSet.h"
15 #include "llvm/ADT/SmallVector.h"
16 #include "llvm/ADT/Statistic.h"
17 #include "llvm/ADT/Twine.h"
18 #include "llvm/Analysis/AssumptionCache.h"
19 #include "llvm/Analysis/CFG.h"
20 #include "llvm/Analysis/CodeMetrics.h"
21 #include "llvm/Analysis/GuardUtils.h"
22 #include "llvm/Analysis/InstructionSimplify.h"
23 #include "llvm/Analysis/LoopAnalysisManager.h"
24 #include "llvm/Analysis/LoopInfo.h"
25 #include "llvm/Analysis/LoopIterator.h"
26 #include "llvm/Analysis/LoopPass.h"
27 #include "llvm/Analysis/MemorySSA.h"
28 #include "llvm/Analysis/MemorySSAUpdater.h"
29 #include "llvm/Analysis/Utils/Local.h"
30 #include "llvm/IR/BasicBlock.h"
31 #include "llvm/IR/Constant.h"
32 #include "llvm/IR/Constants.h"
33 #include "llvm/IR/Dominators.h"
34 #include "llvm/IR/Function.h"
35 #include "llvm/IR/InstrTypes.h"
36 #include "llvm/IR/Instruction.h"
37 #include "llvm/IR/Instructions.h"
38 #include "llvm/IR/IntrinsicInst.h"
39 #include "llvm/IR/Use.h"
40 #include "llvm/IR/Value.h"
41 #include "llvm/InitializePasses.h"
42 #include "llvm/Pass.h"
43 #include "llvm/Support/Casting.h"
44 #include "llvm/Support/Debug.h"
45 #include "llvm/Support/ErrorHandling.h"
46 #include "llvm/Support/GenericDomTree.h"
47 #include "llvm/Support/raw_ostream.h"
48 #include "llvm/Transforms/Scalar/SimpleLoopUnswitch.h"
49 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
50 #include "llvm/Transforms/Utils/Cloning.h"
51 #include "llvm/Transforms/Utils/LoopUtils.h"
52 #include "llvm/Transforms/Utils/ValueMapper.h"
53 #include <algorithm>
54 #include <cassert>
55 #include <iterator>
56 #include <numeric>
57 #include <utility>
58 
59 #define DEBUG_TYPE "simple-loop-unswitch"
60 
61 using namespace llvm;
62 
63 STATISTIC(NumBranches, "Number of branches unswitched");
64 STATISTIC(NumSwitches, "Number of switches unswitched");
65 STATISTIC(NumGuards, "Number of guards turned into branches for unswitching");
66 STATISTIC(NumTrivial, "Number of unswitches that are trivial");
67 STATISTIC(
68     NumCostMultiplierSkipped,
69     "Number of unswitch candidates that had their cost multiplier skipped");
70 
71 static cl::opt<bool> EnableNonTrivialUnswitch(
72     "enable-nontrivial-unswitch", cl::init(false), cl::Hidden,
73     cl::desc("Forcibly enables non-trivial loop unswitching rather than "
74              "following the configuration passed into the pass."));
75 
76 static cl::opt<int>
77     UnswitchThreshold("unswitch-threshold", cl::init(50), cl::Hidden,
78                       cl::desc("The cost threshold for unswitching a loop."));
79 
80 static cl::opt<bool> EnableUnswitchCostMultiplier(
81     "enable-unswitch-cost-multiplier", cl::init(true), cl::Hidden,
82     cl::desc("Enable unswitch cost multiplier that prohibits exponential "
83              "explosion in nontrivial unswitch."));
84 static cl::opt<int> UnswitchSiblingsToplevelDiv(
85     "unswitch-siblings-toplevel-div", cl::init(2), cl::Hidden,
86     cl::desc("Toplevel siblings divisor for cost multiplier."));
87 static cl::opt<int> UnswitchNumInitialUnscaledCandidates(
88     "unswitch-num-initial-unscaled-candidates", cl::init(8), cl::Hidden,
89     cl::desc("Number of unswitch candidates that are ignored when calculating "
90              "cost multiplier."));
91 static cl::opt<bool> UnswitchGuards(
92     "simple-loop-unswitch-guards", cl::init(true), cl::Hidden,
93     cl::desc("If enabled, simple loop unswitching will also consider "
94              "llvm.experimental.guard intrinsics as unswitch candidates."));
95 
96 /// Collect all of the loop invariant input values transitively used by the
97 /// homogeneous instruction graph from a given root.
98 ///
99 /// This essentially walks from a root recursively through loop variant operands
100 /// which have the exact same opcode and finds all inputs which are loop
101 /// invariant. For some operations these can be re-associated and unswitched out
102 /// of the loop entirely.
103 static TinyPtrVector<Value *>
104 collectHomogenousInstGraphLoopInvariants(Loop &L, Instruction &Root,
105                                          LoopInfo &LI) {
106   assert(!L.isLoopInvariant(&Root) &&
107          "Only need to walk the graph if root itself is not invariant.");
108   TinyPtrVector<Value *> Invariants;
109 
110   // Build a worklist and recurse through operators collecting invariants.
111   SmallVector<Instruction *, 4> Worklist;
112   SmallPtrSet<Instruction *, 8> Visited;
113   Worklist.push_back(&Root);
114   Visited.insert(&Root);
115   do {
116     Instruction &I = *Worklist.pop_back_val();
117     for (Value *OpV : I.operand_values()) {
118       // Skip constants as unswitching isn't interesting for them.
119       if (isa<Constant>(OpV))
120         continue;
121 
122       // Add it to our result if loop invariant.
123       if (L.isLoopInvariant(OpV)) {
124         Invariants.push_back(OpV);
125         continue;
126       }
127 
128       // If not an instruction with the same opcode, nothing we can do.
129       Instruction *OpI = dyn_cast<Instruction>(OpV);
130       if (!OpI || OpI->getOpcode() != Root.getOpcode())
131         continue;
132 
133       // Visit this operand.
134       if (Visited.insert(OpI).second)
135         Worklist.push_back(OpI);
136     }
137   } while (!Worklist.empty());
138 
139   return Invariants;
140 }
141 
142 static void replaceLoopInvariantUses(Loop &L, Value *Invariant,
143                                      Constant &Replacement) {
144   assert(!isa<Constant>(Invariant) && "Why are we unswitching on a constant?");
145 
146   // Replace uses of LIC in the loop with the given constant.
147   for (auto UI = Invariant->use_begin(), UE = Invariant->use_end(); UI != UE;) {
148     // Grab the use and walk past it so we can clobber it in the use list.
149     Use *U = &*UI++;
150     Instruction *UserI = dyn_cast<Instruction>(U->getUser());
151 
152     // Replace this use within the loop body.
153     if (UserI && L.contains(UserI))
154       U->set(&Replacement);
155   }
156 }
157 
158 /// Check that all the LCSSA PHI nodes in the loop exit block have trivial
159 /// incoming values along this edge.
160 static bool areLoopExitPHIsLoopInvariant(Loop &L, BasicBlock &ExitingBB,
161                                          BasicBlock &ExitBB) {
162   for (Instruction &I : ExitBB) {
163     auto *PN = dyn_cast<PHINode>(&I);
164     if (!PN)
165       // No more PHIs to check.
166       return true;
167 
168     // If the incoming value for this edge isn't loop invariant the unswitch
169     // won't be trivial.
170     if (!L.isLoopInvariant(PN->getIncomingValueForBlock(&ExitingBB)))
171       return false;
172   }
173   llvm_unreachable("Basic blocks should never be empty!");
174 }
175 
176 /// Insert code to test a set of loop invariant values, and conditionally branch
177 /// on them.
178 static void buildPartialUnswitchConditionalBranch(BasicBlock &BB,
179                                                   ArrayRef<Value *> Invariants,
180                                                   bool Direction,
181                                                   BasicBlock &UnswitchedSucc,
182                                                   BasicBlock &NormalSucc) {
183   IRBuilder<> IRB(&BB);
184 
185   Value *Cond = Direction ? IRB.CreateOr(Invariants) :
186     IRB.CreateAnd(Invariants);
187   IRB.CreateCondBr(Cond, Direction ? &UnswitchedSucc : &NormalSucc,
188                    Direction ? &NormalSucc : &UnswitchedSucc);
189 }
190 
191 /// Rewrite the PHI nodes in an unswitched loop exit basic block.
192 ///
193 /// Requires that the loop exit and unswitched basic block are the same, and
194 /// that the exiting block was a unique predecessor of that block. Rewrites the
195 /// PHI nodes in that block such that what were LCSSA PHI nodes become trivial
196 /// PHI nodes from the old preheader that now contains the unswitched
197 /// terminator.
198 static void rewritePHINodesForUnswitchedExitBlock(BasicBlock &UnswitchedBB,
199                                                   BasicBlock &OldExitingBB,
200                                                   BasicBlock &OldPH) {
201   for (PHINode &PN : UnswitchedBB.phis()) {
202     // When the loop exit is directly unswitched we just need to update the
203     // incoming basic block. We loop to handle weird cases with repeated
204     // incoming blocks, but expect to typically only have one operand here.
205     for (auto i : seq<int>(0, PN.getNumOperands())) {
206       assert(PN.getIncomingBlock(i) == &OldExitingBB &&
207              "Found incoming block different from unique predecessor!");
208       PN.setIncomingBlock(i, &OldPH);
209     }
210   }
211 }
212 
213 /// Rewrite the PHI nodes in the loop exit basic block and the split off
214 /// unswitched block.
215 ///
216 /// Because the exit block remains an exit from the loop, this rewrites the
217 /// LCSSA PHI nodes in it to remove the unswitched edge and introduces PHI
218 /// nodes into the unswitched basic block to select between the value in the
219 /// old preheader and the loop exit.
220 static void rewritePHINodesForExitAndUnswitchedBlocks(BasicBlock &ExitBB,
221                                                       BasicBlock &UnswitchedBB,
222                                                       BasicBlock &OldExitingBB,
223                                                       BasicBlock &OldPH,
224                                                       bool FullUnswitch) {
225   assert(&ExitBB != &UnswitchedBB &&
226          "Must have different loop exit and unswitched blocks!");
227   Instruction *InsertPt = &*UnswitchedBB.begin();
228   for (PHINode &PN : ExitBB.phis()) {
229     auto *NewPN = PHINode::Create(PN.getType(), /*NumReservedValues*/ 2,
230                                   PN.getName() + ".split", InsertPt);
231 
232     // Walk backwards over the old PHI node's inputs to minimize the cost of
233     // removing each one. We have to do this weird loop manually so that we
234     // create the same number of new incoming edges in the new PHI as we expect
235     // each case-based edge to be included in the unswitched switch in some
236     // cases.
237     // FIXME: This is really, really gross. It would be much cleaner if LLVM
238     // allowed us to create a single entry for a predecessor block without
239     // having separate entries for each "edge" even though these edges are
240     // required to produce identical results.
241     for (int i = PN.getNumIncomingValues() - 1; i >= 0; --i) {
242       if (PN.getIncomingBlock(i) != &OldExitingBB)
243         continue;
244 
245       Value *Incoming = PN.getIncomingValue(i);
246       if (FullUnswitch)
247         // No more edge from the old exiting block to the exit block.
248         PN.removeIncomingValue(i);
249 
250       NewPN->addIncoming(Incoming, &OldPH);
251     }
252 
253     // Now replace the old PHI with the new one and wire the old one in as an
254     // input to the new one.
255     PN.replaceAllUsesWith(NewPN);
256     NewPN->addIncoming(&PN, &ExitBB);
257   }
258 }
259 
260 /// Hoist the current loop up to the innermost loop containing a remaining exit.
261 ///
262 /// Because we've removed an exit from the loop, we may have changed the set of
263 /// loops reachable and need to move the current loop up the loop nest or even
264 /// to an entirely separate nest.
265 static void hoistLoopToNewParent(Loop &L, BasicBlock &Preheader,
266                                  DominatorTree &DT, LoopInfo &LI,
267                                  MemorySSAUpdater *MSSAU) {
268   // If the loop is already at the top level, we can't hoist it anywhere.
269   Loop *OldParentL = L.getParentLoop();
270   if (!OldParentL)
271     return;
272 
273   SmallVector<BasicBlock *, 4> Exits;
274   L.getExitBlocks(Exits);
275   Loop *NewParentL = nullptr;
276   for (auto *ExitBB : Exits)
277     if (Loop *ExitL = LI.getLoopFor(ExitBB))
278       if (!NewParentL || NewParentL->contains(ExitL))
279         NewParentL = ExitL;
280 
281   if (NewParentL == OldParentL)
282     return;
283 
284   // The new parent loop (if different) should always contain the old one.
285   if (NewParentL)
286     assert(NewParentL->contains(OldParentL) &&
287            "Can only hoist this loop up the nest!");
288 
289   // The preheader will need to move with the body of this loop. However,
290   // because it isn't in this loop we also need to update the primary loop map.
291   assert(OldParentL == LI.getLoopFor(&Preheader) &&
292          "Parent loop of this loop should contain this loop's preheader!");
293   LI.changeLoopFor(&Preheader, NewParentL);
294 
295   // Remove this loop from its old parent.
296   OldParentL->removeChildLoop(&L);
297 
298   // Add the loop either to the new parent or as a top-level loop.
299   if (NewParentL)
300     NewParentL->addChildLoop(&L);
301   else
302     LI.addTopLevelLoop(&L);
303 
304   // Remove this loops blocks from the old parent and every other loop up the
305   // nest until reaching the new parent. Also update all of these
306   // no-longer-containing loops to reflect the nesting change.
307   for (Loop *OldContainingL = OldParentL; OldContainingL != NewParentL;
308        OldContainingL = OldContainingL->getParentLoop()) {
309     llvm::erase_if(OldContainingL->getBlocksVector(),
310                    [&](const BasicBlock *BB) {
311                      return BB == &Preheader || L.contains(BB);
312                    });
313 
314     OldContainingL->getBlocksSet().erase(&Preheader);
315     for (BasicBlock *BB : L.blocks())
316       OldContainingL->getBlocksSet().erase(BB);
317 
318     // Because we just hoisted a loop out of this one, we have essentially
319     // created new exit paths from it. That means we need to form LCSSA PHI
320     // nodes for values used in the no-longer-nested loop.
321     formLCSSA(*OldContainingL, DT, &LI, nullptr);
322 
323     // We shouldn't need to form dedicated exits because the exit introduced
324     // here is the (just split by unswitching) preheader. However, after trivial
325     // unswitching it is possible to get new non-dedicated exits out of parent
326     // loop so let's conservatively form dedicated exit blocks and figure out
327     // if we can optimize later.
328     formDedicatedExitBlocks(OldContainingL, &DT, &LI, MSSAU,
329                             /*PreserveLCSSA*/ true);
330   }
331 }
332 
333 /// Unswitch a trivial branch if the condition is loop invariant.
334 ///
335 /// This routine should only be called when loop code leading to the branch has
336 /// been validated as trivial (no side effects). This routine checks if the
337 /// condition is invariant and one of the successors is a loop exit. This
338 /// allows us to unswitch without duplicating the loop, making it trivial.
339 ///
340 /// If this routine fails to unswitch the branch it returns false.
341 ///
342 /// If the branch can be unswitched, this routine splits the preheader and
343 /// hoists the branch above that split. Preserves loop simplified form
344 /// (splitting the exit block as necessary). It simplifies the branch within
345 /// the loop to an unconditional branch but doesn't remove it entirely. Further
346 /// cleanup can be done with some simplify-cfg like pass.
347 ///
348 /// If `SE` is not null, it will be updated based on the potential loop SCEVs
349 /// invalidated by this.
350 static bool unswitchTrivialBranch(Loop &L, BranchInst &BI, DominatorTree &DT,
351                                   LoopInfo &LI, ScalarEvolution *SE,
352                                   MemorySSAUpdater *MSSAU) {
353   assert(BI.isConditional() && "Can only unswitch a conditional branch!");
354   LLVM_DEBUG(dbgs() << "  Trying to unswitch branch: " << BI << "\n");
355 
356   // The loop invariant values that we want to unswitch.
357   TinyPtrVector<Value *> Invariants;
358 
359   // When true, we're fully unswitching the branch rather than just unswitching
360   // some input conditions to the branch.
361   bool FullUnswitch = false;
362 
363   if (L.isLoopInvariant(BI.getCondition())) {
364     Invariants.push_back(BI.getCondition());
365     FullUnswitch = true;
366   } else {
367     if (auto *CondInst = dyn_cast<Instruction>(BI.getCondition()))
368       Invariants = collectHomogenousInstGraphLoopInvariants(L, *CondInst, LI);
369     if (Invariants.empty())
370       // Couldn't find invariant inputs!
371       return false;
372   }
373 
374   // Check that one of the branch's successors exits, and which one.
375   bool ExitDirection = true;
376   int LoopExitSuccIdx = 0;
377   auto *LoopExitBB = BI.getSuccessor(0);
378   if (L.contains(LoopExitBB)) {
379     ExitDirection = false;
380     LoopExitSuccIdx = 1;
381     LoopExitBB = BI.getSuccessor(1);
382     if (L.contains(LoopExitBB))
383       return false;
384   }
385   auto *ContinueBB = BI.getSuccessor(1 - LoopExitSuccIdx);
386   auto *ParentBB = BI.getParent();
387   if (!areLoopExitPHIsLoopInvariant(L, *ParentBB, *LoopExitBB))
388     return false;
389 
390   // When unswitching only part of the branch's condition, we need the exit
391   // block to be reached directly from the partially unswitched input. This can
392   // be done when the exit block is along the true edge and the branch condition
393   // is a graph of `or` operations, or the exit block is along the false edge
394   // and the condition is a graph of `and` operations.
395   if (!FullUnswitch) {
396     if (ExitDirection) {
397       if (cast<Instruction>(BI.getCondition())->getOpcode() != Instruction::Or)
398         return false;
399     } else {
400       if (cast<Instruction>(BI.getCondition())->getOpcode() != Instruction::And)
401         return false;
402     }
403   }
404 
405   LLVM_DEBUG({
406     dbgs() << "    unswitching trivial invariant conditions for: " << BI
407            << "\n";
408     for (Value *Invariant : Invariants) {
409       dbgs() << "      " << *Invariant << " == true";
410       if (Invariant != Invariants.back())
411         dbgs() << " ||";
412       dbgs() << "\n";
413     }
414   });
415 
416   // If we have scalar evolutions, we need to invalidate them including this
417   // loop and the loop containing the exit block.
418   if (SE) {
419     if (Loop *ExitL = LI.getLoopFor(LoopExitBB))
420       SE->forgetLoop(ExitL);
421     else
422       // Forget the entire nest as this exits the entire nest.
423       SE->forgetTopmostLoop(&L);
424   }
425 
426   if (MSSAU && VerifyMemorySSA)
427     MSSAU->getMemorySSA()->verifyMemorySSA();
428 
429   // Split the preheader, so that we know that there is a safe place to insert
430   // the conditional branch. We will change the preheader to have a conditional
431   // branch on LoopCond.
432   BasicBlock *OldPH = L.getLoopPreheader();
433   BasicBlock *NewPH = SplitEdge(OldPH, L.getHeader(), &DT, &LI, MSSAU);
434 
435   // Now that we have a place to insert the conditional branch, create a place
436   // to branch to: this is the exit block out of the loop that we are
437   // unswitching. We need to split this if there are other loop predecessors.
438   // Because the loop is in simplified form, *any* other predecessor is enough.
439   BasicBlock *UnswitchedBB;
440   if (FullUnswitch && LoopExitBB->getUniquePredecessor()) {
441     assert(LoopExitBB->getUniquePredecessor() == BI.getParent() &&
442            "A branch's parent isn't a predecessor!");
443     UnswitchedBB = LoopExitBB;
444   } else {
445     UnswitchedBB =
446         SplitBlock(LoopExitBB, &LoopExitBB->front(), &DT, &LI, MSSAU);
447   }
448 
449   if (MSSAU && VerifyMemorySSA)
450     MSSAU->getMemorySSA()->verifyMemorySSA();
451 
452   // Actually move the invariant uses into the unswitched position. If possible,
453   // we do this by moving the instructions, but when doing partial unswitching
454   // we do it by building a new merge of the values in the unswitched position.
455   OldPH->getTerminator()->eraseFromParent();
456   if (FullUnswitch) {
457     // If fully unswitching, we can use the existing branch instruction.
458     // Splice it into the old PH to gate reaching the new preheader and re-point
459     // its successors.
460     OldPH->getInstList().splice(OldPH->end(), BI.getParent()->getInstList(),
461                                 BI);
462     if (MSSAU) {
463       // Temporarily clone the terminator, to make MSSA update cheaper by
464       // separating "insert edge" updates from "remove edge" ones.
465       ParentBB->getInstList().push_back(BI.clone());
466     } else {
467       // Create a new unconditional branch that will continue the loop as a new
468       // terminator.
469       BranchInst::Create(ContinueBB, ParentBB);
470     }
471     BI.setSuccessor(LoopExitSuccIdx, UnswitchedBB);
472     BI.setSuccessor(1 - LoopExitSuccIdx, NewPH);
473   } else {
474     // Only unswitching a subset of inputs to the condition, so we will need to
475     // build a new branch that merges the invariant inputs.
476     if (ExitDirection)
477       assert(cast<Instruction>(BI.getCondition())->getOpcode() ==
478                  Instruction::Or &&
479              "Must have an `or` of `i1`s for the condition!");
480     else
481       assert(cast<Instruction>(BI.getCondition())->getOpcode() ==
482                  Instruction::And &&
483              "Must have an `and` of `i1`s for the condition!");
484     buildPartialUnswitchConditionalBranch(*OldPH, Invariants, ExitDirection,
485                                           *UnswitchedBB, *NewPH);
486   }
487 
488   // Update the dominator tree with the added edge.
489   DT.insertEdge(OldPH, UnswitchedBB);
490 
491   // After the dominator tree was updated with the added edge, update MemorySSA
492   // if available.
493   if (MSSAU) {
494     SmallVector<CFGUpdate, 1> Updates;
495     Updates.push_back({cfg::UpdateKind::Insert, OldPH, UnswitchedBB});
496     MSSAU->applyInsertUpdates(Updates, DT);
497   }
498 
499   // Finish updating dominator tree and memory ssa for full unswitch.
500   if (FullUnswitch) {
501     if (MSSAU) {
502       // Remove the cloned branch instruction.
503       ParentBB->getTerminator()->eraseFromParent();
504       // Create unconditional branch now.
505       BranchInst::Create(ContinueBB, ParentBB);
506       MSSAU->removeEdge(ParentBB, LoopExitBB);
507     }
508     DT.deleteEdge(ParentBB, LoopExitBB);
509   }
510 
511   if (MSSAU && VerifyMemorySSA)
512     MSSAU->getMemorySSA()->verifyMemorySSA();
513 
514   // Rewrite the relevant PHI nodes.
515   if (UnswitchedBB == LoopExitBB)
516     rewritePHINodesForUnswitchedExitBlock(*UnswitchedBB, *ParentBB, *OldPH);
517   else
518     rewritePHINodesForExitAndUnswitchedBlocks(*LoopExitBB, *UnswitchedBB,
519                                               *ParentBB, *OldPH, FullUnswitch);
520 
521   // The constant we can replace all of our invariants with inside the loop
522   // body. If any of the invariants have a value other than this the loop won't
523   // be entered.
524   ConstantInt *Replacement = ExitDirection
525                                  ? ConstantInt::getFalse(BI.getContext())
526                                  : ConstantInt::getTrue(BI.getContext());
527 
528   // Since this is an i1 condition we can also trivially replace uses of it
529   // within the loop with a constant.
530   for (Value *Invariant : Invariants)
531     replaceLoopInvariantUses(L, Invariant, *Replacement);
532 
533   // If this was full unswitching, we may have changed the nesting relationship
534   // for this loop so hoist it to its correct parent if needed.
535   if (FullUnswitch)
536     hoistLoopToNewParent(L, *NewPH, DT, LI, MSSAU);
537 
538   if (MSSAU && VerifyMemorySSA)
539     MSSAU->getMemorySSA()->verifyMemorySSA();
540 
541   LLVM_DEBUG(dbgs() << "    done: unswitching trivial branch...\n");
542   ++NumTrivial;
543   ++NumBranches;
544   return true;
545 }
546 
547 /// Unswitch a trivial switch if the condition is loop invariant.
548 ///
549 /// This routine should only be called when loop code leading to the switch has
550 /// been validated as trivial (no side effects). This routine checks if the
551 /// condition is invariant and that at least one of the successors is a loop
552 /// exit. This allows us to unswitch without duplicating the loop, making it
553 /// trivial.
554 ///
555 /// If this routine fails to unswitch the switch it returns false.
556 ///
557 /// If the switch can be unswitched, this routine splits the preheader and
558 /// copies the switch above that split. If the default case is one of the
559 /// exiting cases, it copies the non-exiting cases and points them at the new
560 /// preheader. If the default case is not exiting, it copies the exiting cases
561 /// and points the default at the preheader. It preserves loop simplified form
562 /// (splitting the exit blocks as necessary). It simplifies the switch within
563 /// the loop by removing now-dead cases. If the default case is one of those
564 /// unswitched, it replaces its destination with a new basic block containing
565 /// only unreachable. Such basic blocks, while technically loop exits, are not
566 /// considered for unswitching so this is a stable transform and the same
567 /// switch will not be revisited. If after unswitching there is only a single
568 /// in-loop successor, the switch is further simplified to an unconditional
569 /// branch. Still more cleanup can be done with some simplify-cfg like pass.
570 ///
571 /// If `SE` is not null, it will be updated based on the potential loop SCEVs
572 /// invalidated by this.
573 static bool unswitchTrivialSwitch(Loop &L, SwitchInst &SI, DominatorTree &DT,
574                                   LoopInfo &LI, ScalarEvolution *SE,
575                                   MemorySSAUpdater *MSSAU) {
576   LLVM_DEBUG(dbgs() << "  Trying to unswitch switch: " << SI << "\n");
577   Value *LoopCond = SI.getCondition();
578 
579   // If this isn't switching on an invariant condition, we can't unswitch it.
580   if (!L.isLoopInvariant(LoopCond))
581     return false;
582 
583   auto *ParentBB = SI.getParent();
584 
585   SmallVector<int, 4> ExitCaseIndices;
586   for (auto Case : SI.cases()) {
587     auto *SuccBB = Case.getCaseSuccessor();
588     if (!L.contains(SuccBB) &&
589         areLoopExitPHIsLoopInvariant(L, *ParentBB, *SuccBB))
590       ExitCaseIndices.push_back(Case.getCaseIndex());
591   }
592   BasicBlock *DefaultExitBB = nullptr;
593   SwitchInstProfUpdateWrapper::CaseWeightOpt DefaultCaseWeight =
594       SwitchInstProfUpdateWrapper::getSuccessorWeight(SI, 0);
595   if (!L.contains(SI.getDefaultDest()) &&
596       areLoopExitPHIsLoopInvariant(L, *ParentBB, *SI.getDefaultDest()) &&
597       !isa<UnreachableInst>(SI.getDefaultDest()->getTerminator())) {
598     DefaultExitBB = SI.getDefaultDest();
599   } else if (ExitCaseIndices.empty())
600     return false;
601 
602   LLVM_DEBUG(dbgs() << "    unswitching trivial switch...\n");
603 
604   if (MSSAU && VerifyMemorySSA)
605     MSSAU->getMemorySSA()->verifyMemorySSA();
606 
607   // We may need to invalidate SCEVs for the outermost loop reached by any of
608   // the exits.
609   Loop *OuterL = &L;
610 
611   if (DefaultExitBB) {
612     // Clear out the default destination temporarily to allow accurate
613     // predecessor lists to be examined below.
614     SI.setDefaultDest(nullptr);
615     // Check the loop containing this exit.
616     Loop *ExitL = LI.getLoopFor(DefaultExitBB);
617     if (!ExitL || ExitL->contains(OuterL))
618       OuterL = ExitL;
619   }
620 
621   // Store the exit cases into a separate data structure and remove them from
622   // the switch.
623   SmallVector<std::tuple<ConstantInt *, BasicBlock *,
624                          SwitchInstProfUpdateWrapper::CaseWeightOpt>,
625               4> ExitCases;
626   ExitCases.reserve(ExitCaseIndices.size());
627   SwitchInstProfUpdateWrapper SIW(SI);
628   // We walk the case indices backwards so that we remove the last case first
629   // and don't disrupt the earlier indices.
630   for (unsigned Index : reverse(ExitCaseIndices)) {
631     auto CaseI = SI.case_begin() + Index;
632     // Compute the outer loop from this exit.
633     Loop *ExitL = LI.getLoopFor(CaseI->getCaseSuccessor());
634     if (!ExitL || ExitL->contains(OuterL))
635       OuterL = ExitL;
636     // Save the value of this case.
637     auto W = SIW.getSuccessorWeight(CaseI->getSuccessorIndex());
638     ExitCases.emplace_back(CaseI->getCaseValue(), CaseI->getCaseSuccessor(), W);
639     // Delete the unswitched cases.
640     SIW.removeCase(CaseI);
641   }
642 
643   if (SE) {
644     if (OuterL)
645       SE->forgetLoop(OuterL);
646     else
647       SE->forgetTopmostLoop(&L);
648   }
649 
650   // Check if after this all of the remaining cases point at the same
651   // successor.
652   BasicBlock *CommonSuccBB = nullptr;
653   if (SI.getNumCases() > 0 &&
654       std::all_of(std::next(SI.case_begin()), SI.case_end(),
655                   [&SI](const SwitchInst::CaseHandle &Case) {
656                     return Case.getCaseSuccessor() ==
657                            SI.case_begin()->getCaseSuccessor();
658                   }))
659     CommonSuccBB = SI.case_begin()->getCaseSuccessor();
660   if (!DefaultExitBB) {
661     // If we're not unswitching the default, we need it to match any cases to
662     // have a common successor or if we have no cases it is the common
663     // successor.
664     if (SI.getNumCases() == 0)
665       CommonSuccBB = SI.getDefaultDest();
666     else if (SI.getDefaultDest() != CommonSuccBB)
667       CommonSuccBB = nullptr;
668   }
669 
670   // Split the preheader, so that we know that there is a safe place to insert
671   // the switch.
672   BasicBlock *OldPH = L.getLoopPreheader();
673   BasicBlock *NewPH = SplitEdge(OldPH, L.getHeader(), &DT, &LI, MSSAU);
674   OldPH->getTerminator()->eraseFromParent();
675 
676   // Now add the unswitched switch.
677   auto *NewSI = SwitchInst::Create(LoopCond, NewPH, ExitCases.size(), OldPH);
678   SwitchInstProfUpdateWrapper NewSIW(*NewSI);
679 
680   // Rewrite the IR for the unswitched basic blocks. This requires two steps.
681   // First, we split any exit blocks with remaining in-loop predecessors. Then
682   // we update the PHIs in one of two ways depending on if there was a split.
683   // We walk in reverse so that we split in the same order as the cases
684   // appeared. This is purely for convenience of reading the resulting IR, but
685   // it doesn't cost anything really.
686   SmallPtrSet<BasicBlock *, 2> UnswitchedExitBBs;
687   SmallDenseMap<BasicBlock *, BasicBlock *, 2> SplitExitBBMap;
688   // Handle the default exit if necessary.
689   // FIXME: It'd be great if we could merge this with the loop below but LLVM's
690   // ranges aren't quite powerful enough yet.
691   if (DefaultExitBB) {
692     if (pred_empty(DefaultExitBB)) {
693       UnswitchedExitBBs.insert(DefaultExitBB);
694       rewritePHINodesForUnswitchedExitBlock(*DefaultExitBB, *ParentBB, *OldPH);
695     } else {
696       auto *SplitBB =
697           SplitBlock(DefaultExitBB, &DefaultExitBB->front(), &DT, &LI, MSSAU);
698       rewritePHINodesForExitAndUnswitchedBlocks(*DefaultExitBB, *SplitBB,
699                                                 *ParentBB, *OldPH,
700                                                 /*FullUnswitch*/ true);
701       DefaultExitBB = SplitExitBBMap[DefaultExitBB] = SplitBB;
702     }
703   }
704   // Note that we must use a reference in the for loop so that we update the
705   // container.
706   for (auto &ExitCase : reverse(ExitCases)) {
707     // Grab a reference to the exit block in the pair so that we can update it.
708     BasicBlock *ExitBB = std::get<1>(ExitCase);
709 
710     // If this case is the last edge into the exit block, we can simply reuse it
711     // as it will no longer be a loop exit. No mapping necessary.
712     if (pred_empty(ExitBB)) {
713       // Only rewrite once.
714       if (UnswitchedExitBBs.insert(ExitBB).second)
715         rewritePHINodesForUnswitchedExitBlock(*ExitBB, *ParentBB, *OldPH);
716       continue;
717     }
718 
719     // Otherwise we need to split the exit block so that we retain an exit
720     // block from the loop and a target for the unswitched condition.
721     BasicBlock *&SplitExitBB = SplitExitBBMap[ExitBB];
722     if (!SplitExitBB) {
723       // If this is the first time we see this, do the split and remember it.
724       SplitExitBB = SplitBlock(ExitBB, &ExitBB->front(), &DT, &LI, MSSAU);
725       rewritePHINodesForExitAndUnswitchedBlocks(*ExitBB, *SplitExitBB,
726                                                 *ParentBB, *OldPH,
727                                                 /*FullUnswitch*/ true);
728     }
729     // Update the case pair to point to the split block.
730     std::get<1>(ExitCase) = SplitExitBB;
731   }
732 
733   // Now add the unswitched cases. We do this in reverse order as we built them
734   // in reverse order.
735   for (auto &ExitCase : reverse(ExitCases)) {
736     ConstantInt *CaseVal = std::get<0>(ExitCase);
737     BasicBlock *UnswitchedBB = std::get<1>(ExitCase);
738 
739     NewSIW.addCase(CaseVal, UnswitchedBB, std::get<2>(ExitCase));
740   }
741 
742   // If the default was unswitched, re-point it and add explicit cases for
743   // entering the loop.
744   if (DefaultExitBB) {
745     NewSIW->setDefaultDest(DefaultExitBB);
746     NewSIW.setSuccessorWeight(0, DefaultCaseWeight);
747 
748     // We removed all the exit cases, so we just copy the cases to the
749     // unswitched switch.
750     for (const auto &Case : SI.cases())
751       NewSIW.addCase(Case.getCaseValue(), NewPH,
752                      SIW.getSuccessorWeight(Case.getSuccessorIndex()));
753   } else if (DefaultCaseWeight) {
754     // We have to set branch weight of the default case.
755     uint64_t SW = *DefaultCaseWeight;
756     for (const auto &Case : SI.cases()) {
757       auto W = SIW.getSuccessorWeight(Case.getSuccessorIndex());
758       assert(W &&
759              "case weight must be defined as default case weight is defined");
760       SW += *W;
761     }
762     NewSIW.setSuccessorWeight(0, SW);
763   }
764 
765   // If we ended up with a common successor for every path through the switch
766   // after unswitching, rewrite it to an unconditional branch to make it easy
767   // to recognize. Otherwise we potentially have to recognize the default case
768   // pointing at unreachable and other complexity.
769   if (CommonSuccBB) {
770     BasicBlock *BB = SI.getParent();
771     // We may have had multiple edges to this common successor block, so remove
772     // them as predecessors. We skip the first one, either the default or the
773     // actual first case.
774     bool SkippedFirst = DefaultExitBB == nullptr;
775     for (auto Case : SI.cases()) {
776       assert(Case.getCaseSuccessor() == CommonSuccBB &&
777              "Non-common successor!");
778       (void)Case;
779       if (!SkippedFirst) {
780         SkippedFirst = true;
781         continue;
782       }
783       CommonSuccBB->removePredecessor(BB,
784                                       /*KeepOneInputPHIs*/ true);
785     }
786     // Now nuke the switch and replace it with a direct branch.
787     SIW.eraseFromParent();
788     BranchInst::Create(CommonSuccBB, BB);
789   } else if (DefaultExitBB) {
790     assert(SI.getNumCases() > 0 &&
791            "If we had no cases we'd have a common successor!");
792     // Move the last case to the default successor. This is valid as if the
793     // default got unswitched it cannot be reached. This has the advantage of
794     // being simple and keeping the number of edges from this switch to
795     // successors the same, and avoiding any PHI update complexity.
796     auto LastCaseI = std::prev(SI.case_end());
797 
798     SI.setDefaultDest(LastCaseI->getCaseSuccessor());
799     SIW.setSuccessorWeight(
800         0, SIW.getSuccessorWeight(LastCaseI->getSuccessorIndex()));
801     SIW.removeCase(LastCaseI);
802   }
803 
804   // Walk the unswitched exit blocks and the unswitched split blocks and update
805   // the dominator tree based on the CFG edits. While we are walking unordered
806   // containers here, the API for applyUpdates takes an unordered list of
807   // updates and requires them to not contain duplicates.
808   SmallVector<DominatorTree::UpdateType, 4> DTUpdates;
809   for (auto *UnswitchedExitBB : UnswitchedExitBBs) {
810     DTUpdates.push_back({DT.Delete, ParentBB, UnswitchedExitBB});
811     DTUpdates.push_back({DT.Insert, OldPH, UnswitchedExitBB});
812   }
813   for (auto SplitUnswitchedPair : SplitExitBBMap) {
814     DTUpdates.push_back({DT.Delete, ParentBB, SplitUnswitchedPair.first});
815     DTUpdates.push_back({DT.Insert, OldPH, SplitUnswitchedPair.second});
816   }
817   DT.applyUpdates(DTUpdates);
818 
819   if (MSSAU) {
820     MSSAU->applyUpdates(DTUpdates, DT);
821     if (VerifyMemorySSA)
822       MSSAU->getMemorySSA()->verifyMemorySSA();
823   }
824 
825   assert(DT.verify(DominatorTree::VerificationLevel::Fast));
826 
827   // We may have changed the nesting relationship for this loop so hoist it to
828   // its correct parent if needed.
829   hoistLoopToNewParent(L, *NewPH, DT, LI, MSSAU);
830 
831   if (MSSAU && VerifyMemorySSA)
832     MSSAU->getMemorySSA()->verifyMemorySSA();
833 
834   ++NumTrivial;
835   ++NumSwitches;
836   LLVM_DEBUG(dbgs() << "    done: unswitching trivial switch...\n");
837   return true;
838 }
839 
840 /// This routine scans the loop to find a branch or switch which occurs before
841 /// any side effects occur. These can potentially be unswitched without
842 /// duplicating the loop. If a branch or switch is successfully unswitched the
843 /// scanning continues to see if subsequent branches or switches have become
844 /// trivial. Once all trivial candidates have been unswitched, this routine
845 /// returns.
846 ///
847 /// The return value indicates whether anything was unswitched (and therefore
848 /// changed).
849 ///
850 /// If `SE` is not null, it will be updated based on the potential loop SCEVs
851 /// invalidated by this.
852 static bool unswitchAllTrivialConditions(Loop &L, DominatorTree &DT,
853                                          LoopInfo &LI, ScalarEvolution *SE,
854                                          MemorySSAUpdater *MSSAU) {
855   bool Changed = false;
856 
857   // If loop header has only one reachable successor we should keep looking for
858   // trivial condition candidates in the successor as well. An alternative is
859   // to constant fold conditions and merge successors into loop header (then we
860   // only need to check header's terminator). The reason for not doing this in
861   // LoopUnswitch pass is that it could potentially break LoopPassManager's
862   // invariants. Folding dead branches could either eliminate the current loop
863   // or make other loops unreachable. LCSSA form might also not be preserved
864   // after deleting branches. The following code keeps traversing loop header's
865   // successors until it finds the trivial condition candidate (condition that
866   // is not a constant). Since unswitching generates branches with constant
867   // conditions, this scenario could be very common in practice.
868   BasicBlock *CurrentBB = L.getHeader();
869   SmallPtrSet<BasicBlock *, 8> Visited;
870   Visited.insert(CurrentBB);
871   do {
872     // Check if there are any side-effecting instructions (e.g. stores, calls,
873     // volatile loads) in the part of the loop that the code *would* execute
874     // without unswitching.
875     if (MSSAU) // Possible early exit with MSSA
876       if (auto *Defs = MSSAU->getMemorySSA()->getBlockDefs(CurrentBB))
877         if (!isa<MemoryPhi>(*Defs->begin()) || (++Defs->begin() != Defs->end()))
878           return Changed;
879     if (llvm::any_of(*CurrentBB,
880                      [](Instruction &I) { return I.mayHaveSideEffects(); }))
881       return Changed;
882 
883     Instruction *CurrentTerm = CurrentBB->getTerminator();
884 
885     if (auto *SI = dyn_cast<SwitchInst>(CurrentTerm)) {
886       // Don't bother trying to unswitch past a switch with a constant
887       // condition. This should be removed prior to running this pass by
888       // simplify-cfg.
889       if (isa<Constant>(SI->getCondition()))
890         return Changed;
891 
892       if (!unswitchTrivialSwitch(L, *SI, DT, LI, SE, MSSAU))
893         // Couldn't unswitch this one so we're done.
894         return Changed;
895 
896       // Mark that we managed to unswitch something.
897       Changed = true;
898 
899       // If unswitching turned the terminator into an unconditional branch then
900       // we can continue. The unswitching logic specifically works to fold any
901       // cases it can into an unconditional branch to make it easier to
902       // recognize here.
903       auto *BI = dyn_cast<BranchInst>(CurrentBB->getTerminator());
904       if (!BI || BI->isConditional())
905         return Changed;
906 
907       CurrentBB = BI->getSuccessor(0);
908       continue;
909     }
910 
911     auto *BI = dyn_cast<BranchInst>(CurrentTerm);
912     if (!BI)
913       // We do not understand other terminator instructions.
914       return Changed;
915 
916     // Don't bother trying to unswitch past an unconditional branch or a branch
917     // with a constant value. These should be removed by simplify-cfg prior to
918     // running this pass.
919     if (!BI->isConditional() || isa<Constant>(BI->getCondition()))
920       return Changed;
921 
922     // Found a trivial condition candidate: non-foldable conditional branch. If
923     // we fail to unswitch this, we can't do anything else that is trivial.
924     if (!unswitchTrivialBranch(L, *BI, DT, LI, SE, MSSAU))
925       return Changed;
926 
927     // Mark that we managed to unswitch something.
928     Changed = true;
929 
930     // If we only unswitched some of the conditions feeding the branch, we won't
931     // have collapsed it to a single successor.
932     BI = cast<BranchInst>(CurrentBB->getTerminator());
933     if (BI->isConditional())
934       return Changed;
935 
936     // Follow the newly unconditional branch into its successor.
937     CurrentBB = BI->getSuccessor(0);
938 
939     // When continuing, if we exit the loop or reach a previous visited block,
940     // then we can not reach any trivial condition candidates (unfoldable
941     // branch instructions or switch instructions) and no unswitch can happen.
942   } while (L.contains(CurrentBB) && Visited.insert(CurrentBB).second);
943 
944   return Changed;
945 }
946 
947 /// Build the cloned blocks for an unswitched copy of the given loop.
948 ///
949 /// The cloned blocks are inserted before the loop preheader (`LoopPH`) and
950 /// after the split block (`SplitBB`) that will be used to select between the
951 /// cloned and original loop.
952 ///
953 /// This routine handles cloning all of the necessary loop blocks and exit
954 /// blocks including rewriting their instructions and the relevant PHI nodes.
955 /// Any loop blocks or exit blocks which are dominated by a different successor
956 /// than the one for this clone of the loop blocks can be trivially skipped. We
957 /// use the `DominatingSucc` map to determine whether a block satisfies that
958 /// property with a simple map lookup.
959 ///
960 /// It also correctly creates the unconditional branch in the cloned
961 /// unswitched parent block to only point at the unswitched successor.
962 ///
963 /// This does not handle most of the necessary updates to `LoopInfo`. Only exit
964 /// block splitting is correctly reflected in `LoopInfo`, essentially all of
965 /// the cloned blocks (and their loops) are left without full `LoopInfo`
966 /// updates. This also doesn't fully update `DominatorTree`. It adds the cloned
967 /// blocks to them but doesn't create the cloned `DominatorTree` structure and
968 /// instead the caller must recompute an accurate DT. It *does* correctly
969 /// update the `AssumptionCache` provided in `AC`.
970 static BasicBlock *buildClonedLoopBlocks(
971     Loop &L, BasicBlock *LoopPH, BasicBlock *SplitBB,
972     ArrayRef<BasicBlock *> ExitBlocks, BasicBlock *ParentBB,
973     BasicBlock *UnswitchedSuccBB, BasicBlock *ContinueSuccBB,
974     const SmallDenseMap<BasicBlock *, BasicBlock *, 16> &DominatingSucc,
975     ValueToValueMapTy &VMap,
976     SmallVectorImpl<DominatorTree::UpdateType> &DTUpdates, AssumptionCache &AC,
977     DominatorTree &DT, LoopInfo &LI, MemorySSAUpdater *MSSAU) {
978   SmallVector<BasicBlock *, 4> NewBlocks;
979   NewBlocks.reserve(L.getNumBlocks() + ExitBlocks.size());
980 
981   // We will need to clone a bunch of blocks, wrap up the clone operation in
982   // a helper.
983   auto CloneBlock = [&](BasicBlock *OldBB) {
984     // Clone the basic block and insert it before the new preheader.
985     BasicBlock *NewBB = CloneBasicBlock(OldBB, VMap, ".us", OldBB->getParent());
986     NewBB->moveBefore(LoopPH);
987 
988     // Record this block and the mapping.
989     NewBlocks.push_back(NewBB);
990     VMap[OldBB] = NewBB;
991 
992     return NewBB;
993   };
994 
995   // We skip cloning blocks when they have a dominating succ that is not the
996   // succ we are cloning for.
997   auto SkipBlock = [&](BasicBlock *BB) {
998     auto It = DominatingSucc.find(BB);
999     return It != DominatingSucc.end() && It->second != UnswitchedSuccBB;
1000   };
1001 
1002   // First, clone the preheader.
1003   auto *ClonedPH = CloneBlock(LoopPH);
1004 
1005   // Then clone all the loop blocks, skipping the ones that aren't necessary.
1006   for (auto *LoopBB : L.blocks())
1007     if (!SkipBlock(LoopBB))
1008       CloneBlock(LoopBB);
1009 
1010   // Split all the loop exit edges so that when we clone the exit blocks, if
1011   // any of the exit blocks are *also* a preheader for some other loop, we
1012   // don't create multiple predecessors entering the loop header.
1013   for (auto *ExitBB : ExitBlocks) {
1014     if (SkipBlock(ExitBB))
1015       continue;
1016 
1017     // When we are going to clone an exit, we don't need to clone all the
1018     // instructions in the exit block and we want to ensure we have an easy
1019     // place to merge the CFG, so split the exit first. This is always safe to
1020     // do because there cannot be any non-loop predecessors of a loop exit in
1021     // loop simplified form.
1022     auto *MergeBB = SplitBlock(ExitBB, &ExitBB->front(), &DT, &LI, MSSAU);
1023 
1024     // Rearrange the names to make it easier to write test cases by having the
1025     // exit block carry the suffix rather than the merge block carrying the
1026     // suffix.
1027     MergeBB->takeName(ExitBB);
1028     ExitBB->setName(Twine(MergeBB->getName()) + ".split");
1029 
1030     // Now clone the original exit block.
1031     auto *ClonedExitBB = CloneBlock(ExitBB);
1032     assert(ClonedExitBB->getTerminator()->getNumSuccessors() == 1 &&
1033            "Exit block should have been split to have one successor!");
1034     assert(ClonedExitBB->getTerminator()->getSuccessor(0) == MergeBB &&
1035            "Cloned exit block has the wrong successor!");
1036 
1037     // Remap any cloned instructions and create a merge phi node for them.
1038     for (auto ZippedInsts : llvm::zip_first(
1039              llvm::make_range(ExitBB->begin(), std::prev(ExitBB->end())),
1040              llvm::make_range(ClonedExitBB->begin(),
1041                               std::prev(ClonedExitBB->end())))) {
1042       Instruction &I = std::get<0>(ZippedInsts);
1043       Instruction &ClonedI = std::get<1>(ZippedInsts);
1044 
1045       // The only instructions in the exit block should be PHI nodes and
1046       // potentially a landing pad.
1047       assert(
1048           (isa<PHINode>(I) || isa<LandingPadInst>(I) || isa<CatchPadInst>(I)) &&
1049           "Bad instruction in exit block!");
1050       // We should have a value map between the instruction and its clone.
1051       assert(VMap.lookup(&I) == &ClonedI && "Mismatch in the value map!");
1052 
1053       auto *MergePN =
1054           PHINode::Create(I.getType(), /*NumReservedValues*/ 2, ".us-phi",
1055                           &*MergeBB->getFirstInsertionPt());
1056       I.replaceAllUsesWith(MergePN);
1057       MergePN->addIncoming(&I, ExitBB);
1058       MergePN->addIncoming(&ClonedI, ClonedExitBB);
1059     }
1060   }
1061 
1062   // Rewrite the instructions in the cloned blocks to refer to the instructions
1063   // in the cloned blocks. We have to do this as a second pass so that we have
1064   // everything available. Also, we have inserted new instructions which may
1065   // include assume intrinsics, so we update the assumption cache while
1066   // processing this.
1067   for (auto *ClonedBB : NewBlocks)
1068     for (Instruction &I : *ClonedBB) {
1069       RemapInstruction(&I, VMap,
1070                        RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
1071       if (auto *II = dyn_cast<IntrinsicInst>(&I))
1072         if (II->getIntrinsicID() == Intrinsic::assume)
1073           AC.registerAssumption(II);
1074     }
1075 
1076   // Update any PHI nodes in the cloned successors of the skipped blocks to not
1077   // have spurious incoming values.
1078   for (auto *LoopBB : L.blocks())
1079     if (SkipBlock(LoopBB))
1080       for (auto *SuccBB : successors(LoopBB))
1081         if (auto *ClonedSuccBB = cast_or_null<BasicBlock>(VMap.lookup(SuccBB)))
1082           for (PHINode &PN : ClonedSuccBB->phis())
1083             PN.removeIncomingValue(LoopBB, /*DeletePHIIfEmpty*/ false);
1084 
1085   // Remove the cloned parent as a predecessor of any successor we ended up
1086   // cloning other than the unswitched one.
1087   auto *ClonedParentBB = cast<BasicBlock>(VMap.lookup(ParentBB));
1088   for (auto *SuccBB : successors(ParentBB)) {
1089     if (SuccBB == UnswitchedSuccBB)
1090       continue;
1091 
1092     auto *ClonedSuccBB = cast_or_null<BasicBlock>(VMap.lookup(SuccBB));
1093     if (!ClonedSuccBB)
1094       continue;
1095 
1096     ClonedSuccBB->removePredecessor(ClonedParentBB,
1097                                     /*KeepOneInputPHIs*/ true);
1098   }
1099 
1100   // Replace the cloned branch with an unconditional branch to the cloned
1101   // unswitched successor.
1102   auto *ClonedSuccBB = cast<BasicBlock>(VMap.lookup(UnswitchedSuccBB));
1103   ClonedParentBB->getTerminator()->eraseFromParent();
1104   BranchInst::Create(ClonedSuccBB, ClonedParentBB);
1105 
1106   // If there are duplicate entries in the PHI nodes because of multiple edges
1107   // to the unswitched successor, we need to nuke all but one as we replaced it
1108   // with a direct branch.
1109   for (PHINode &PN : ClonedSuccBB->phis()) {
1110     bool Found = false;
1111     // Loop over the incoming operands backwards so we can easily delete as we
1112     // go without invalidating the index.
1113     for (int i = PN.getNumOperands() - 1; i >= 0; --i) {
1114       if (PN.getIncomingBlock(i) != ClonedParentBB)
1115         continue;
1116       if (!Found) {
1117         Found = true;
1118         continue;
1119       }
1120       PN.removeIncomingValue(i, /*DeletePHIIfEmpty*/ false);
1121     }
1122   }
1123 
1124   // Record the domtree updates for the new blocks.
1125   SmallPtrSet<BasicBlock *, 4> SuccSet;
1126   for (auto *ClonedBB : NewBlocks) {
1127     for (auto *SuccBB : successors(ClonedBB))
1128       if (SuccSet.insert(SuccBB).second)
1129         DTUpdates.push_back({DominatorTree::Insert, ClonedBB, SuccBB});
1130     SuccSet.clear();
1131   }
1132 
1133   return ClonedPH;
1134 }
1135 
1136 /// Recursively clone the specified loop and all of its children.
1137 ///
1138 /// The target parent loop for the clone should be provided, or can be null if
1139 /// the clone is a top-level loop. While cloning, all the blocks are mapped
1140 /// with the provided value map. The entire original loop must be present in
1141 /// the value map. The cloned loop is returned.
1142 static Loop *cloneLoopNest(Loop &OrigRootL, Loop *RootParentL,
1143                            const ValueToValueMapTy &VMap, LoopInfo &LI) {
1144   auto AddClonedBlocksToLoop = [&](Loop &OrigL, Loop &ClonedL) {
1145     assert(ClonedL.getBlocks().empty() && "Must start with an empty loop!");
1146     ClonedL.reserveBlocks(OrigL.getNumBlocks());
1147     for (auto *BB : OrigL.blocks()) {
1148       auto *ClonedBB = cast<BasicBlock>(VMap.lookup(BB));
1149       ClonedL.addBlockEntry(ClonedBB);
1150       if (LI.getLoopFor(BB) == &OrigL)
1151         LI.changeLoopFor(ClonedBB, &ClonedL);
1152     }
1153   };
1154 
1155   // We specially handle the first loop because it may get cloned into
1156   // a different parent and because we most commonly are cloning leaf loops.
1157   Loop *ClonedRootL = LI.AllocateLoop();
1158   if (RootParentL)
1159     RootParentL->addChildLoop(ClonedRootL);
1160   else
1161     LI.addTopLevelLoop(ClonedRootL);
1162   AddClonedBlocksToLoop(OrigRootL, *ClonedRootL);
1163 
1164   if (OrigRootL.empty())
1165     return ClonedRootL;
1166 
1167   // If we have a nest, we can quickly clone the entire loop nest using an
1168   // iterative approach because it is a tree. We keep the cloned parent in the
1169   // data structure to avoid repeatedly querying through a map to find it.
1170   SmallVector<std::pair<Loop *, Loop *>, 16> LoopsToClone;
1171   // Build up the loops to clone in reverse order as we'll clone them from the
1172   // back.
1173   for (Loop *ChildL : llvm::reverse(OrigRootL))
1174     LoopsToClone.push_back({ClonedRootL, ChildL});
1175   do {
1176     Loop *ClonedParentL, *L;
1177     std::tie(ClonedParentL, L) = LoopsToClone.pop_back_val();
1178     Loop *ClonedL = LI.AllocateLoop();
1179     ClonedParentL->addChildLoop(ClonedL);
1180     AddClonedBlocksToLoop(*L, *ClonedL);
1181     for (Loop *ChildL : llvm::reverse(*L))
1182       LoopsToClone.push_back({ClonedL, ChildL});
1183   } while (!LoopsToClone.empty());
1184 
1185   return ClonedRootL;
1186 }
1187 
1188 /// Build the cloned loops of an original loop from unswitching.
1189 ///
1190 /// Because unswitching simplifies the CFG of the loop, this isn't a trivial
1191 /// operation. We need to re-verify that there even is a loop (as the backedge
1192 /// may not have been cloned), and even if there are remaining backedges the
1193 /// backedge set may be different. However, we know that each child loop is
1194 /// undisturbed, we only need to find where to place each child loop within
1195 /// either any parent loop or within a cloned version of the original loop.
1196 ///
1197 /// Because child loops may end up cloned outside of any cloned version of the
1198 /// original loop, multiple cloned sibling loops may be created. All of them
1199 /// are returned so that the newly introduced loop nest roots can be
1200 /// identified.
1201 static void buildClonedLoops(Loop &OrigL, ArrayRef<BasicBlock *> ExitBlocks,
1202                              const ValueToValueMapTy &VMap, LoopInfo &LI,
1203                              SmallVectorImpl<Loop *> &NonChildClonedLoops) {
1204   Loop *ClonedL = nullptr;
1205 
1206   auto *OrigPH = OrigL.getLoopPreheader();
1207   auto *OrigHeader = OrigL.getHeader();
1208 
1209   auto *ClonedPH = cast<BasicBlock>(VMap.lookup(OrigPH));
1210   auto *ClonedHeader = cast<BasicBlock>(VMap.lookup(OrigHeader));
1211 
1212   // We need to know the loops of the cloned exit blocks to even compute the
1213   // accurate parent loop. If we only clone exits to some parent of the
1214   // original parent, we want to clone into that outer loop. We also keep track
1215   // of the loops that our cloned exit blocks participate in.
1216   Loop *ParentL = nullptr;
1217   SmallVector<BasicBlock *, 4> ClonedExitsInLoops;
1218   SmallDenseMap<BasicBlock *, Loop *, 16> ExitLoopMap;
1219   ClonedExitsInLoops.reserve(ExitBlocks.size());
1220   for (auto *ExitBB : ExitBlocks)
1221     if (auto *ClonedExitBB = cast_or_null<BasicBlock>(VMap.lookup(ExitBB)))
1222       if (Loop *ExitL = LI.getLoopFor(ExitBB)) {
1223         ExitLoopMap[ClonedExitBB] = ExitL;
1224         ClonedExitsInLoops.push_back(ClonedExitBB);
1225         if (!ParentL || (ParentL != ExitL && ParentL->contains(ExitL)))
1226           ParentL = ExitL;
1227       }
1228   assert((!ParentL || ParentL == OrigL.getParentLoop() ||
1229           ParentL->contains(OrigL.getParentLoop())) &&
1230          "The computed parent loop should always contain (or be) the parent of "
1231          "the original loop.");
1232 
1233   // We build the set of blocks dominated by the cloned header from the set of
1234   // cloned blocks out of the original loop. While not all of these will
1235   // necessarily be in the cloned loop, it is enough to establish that they
1236   // aren't in unreachable cycles, etc.
1237   SmallSetVector<BasicBlock *, 16> ClonedLoopBlocks;
1238   for (auto *BB : OrigL.blocks())
1239     if (auto *ClonedBB = cast_or_null<BasicBlock>(VMap.lookup(BB)))
1240       ClonedLoopBlocks.insert(ClonedBB);
1241 
1242   // Rebuild the set of blocks that will end up in the cloned loop. We may have
1243   // skipped cloning some region of this loop which can in turn skip some of
1244   // the backedges so we have to rebuild the blocks in the loop based on the
1245   // backedges that remain after cloning.
1246   SmallVector<BasicBlock *, 16> Worklist;
1247   SmallPtrSet<BasicBlock *, 16> BlocksInClonedLoop;
1248   for (auto *Pred : predecessors(ClonedHeader)) {
1249     // The only possible non-loop header predecessor is the preheader because
1250     // we know we cloned the loop in simplified form.
1251     if (Pred == ClonedPH)
1252       continue;
1253 
1254     // Because the loop was in simplified form, the only non-loop predecessor
1255     // should be the preheader.
1256     assert(ClonedLoopBlocks.count(Pred) && "Found a predecessor of the loop "
1257                                            "header other than the preheader "
1258                                            "that is not part of the loop!");
1259 
1260     // Insert this block into the loop set and on the first visit (and if it
1261     // isn't the header we're currently walking) put it into the worklist to
1262     // recurse through.
1263     if (BlocksInClonedLoop.insert(Pred).second && Pred != ClonedHeader)
1264       Worklist.push_back(Pred);
1265   }
1266 
1267   // If we had any backedges then there *is* a cloned loop. Put the header into
1268   // the loop set and then walk the worklist backwards to find all the blocks
1269   // that remain within the loop after cloning.
1270   if (!BlocksInClonedLoop.empty()) {
1271     BlocksInClonedLoop.insert(ClonedHeader);
1272 
1273     while (!Worklist.empty()) {
1274       BasicBlock *BB = Worklist.pop_back_val();
1275       assert(BlocksInClonedLoop.count(BB) &&
1276              "Didn't put block into the loop set!");
1277 
1278       // Insert any predecessors that are in the possible set into the cloned
1279       // set, and if the insert is successful, add them to the worklist. Note
1280       // that we filter on the blocks that are definitely reachable via the
1281       // backedge to the loop header so we may prune out dead code within the
1282       // cloned loop.
1283       for (auto *Pred : predecessors(BB))
1284         if (ClonedLoopBlocks.count(Pred) &&
1285             BlocksInClonedLoop.insert(Pred).second)
1286           Worklist.push_back(Pred);
1287     }
1288 
1289     ClonedL = LI.AllocateLoop();
1290     if (ParentL) {
1291       ParentL->addBasicBlockToLoop(ClonedPH, LI);
1292       ParentL->addChildLoop(ClonedL);
1293     } else {
1294       LI.addTopLevelLoop(ClonedL);
1295     }
1296     NonChildClonedLoops.push_back(ClonedL);
1297 
1298     ClonedL->reserveBlocks(BlocksInClonedLoop.size());
1299     // We don't want to just add the cloned loop blocks based on how we
1300     // discovered them. The original order of blocks was carefully built in
1301     // a way that doesn't rely on predecessor ordering. Rather than re-invent
1302     // that logic, we just re-walk the original blocks (and those of the child
1303     // loops) and filter them as we add them into the cloned loop.
1304     for (auto *BB : OrigL.blocks()) {
1305       auto *ClonedBB = cast_or_null<BasicBlock>(VMap.lookup(BB));
1306       if (!ClonedBB || !BlocksInClonedLoop.count(ClonedBB))
1307         continue;
1308 
1309       // Directly add the blocks that are only in this loop.
1310       if (LI.getLoopFor(BB) == &OrigL) {
1311         ClonedL->addBasicBlockToLoop(ClonedBB, LI);
1312         continue;
1313       }
1314 
1315       // We want to manually add it to this loop and parents.
1316       // Registering it with LoopInfo will happen when we clone the top
1317       // loop for this block.
1318       for (Loop *PL = ClonedL; PL; PL = PL->getParentLoop())
1319         PL->addBlockEntry(ClonedBB);
1320     }
1321 
1322     // Now add each child loop whose header remains within the cloned loop. All
1323     // of the blocks within the loop must satisfy the same constraints as the
1324     // header so once we pass the header checks we can just clone the entire
1325     // child loop nest.
1326     for (Loop *ChildL : OrigL) {
1327       auto *ClonedChildHeader =
1328           cast_or_null<BasicBlock>(VMap.lookup(ChildL->getHeader()));
1329       if (!ClonedChildHeader || !BlocksInClonedLoop.count(ClonedChildHeader))
1330         continue;
1331 
1332 #ifndef NDEBUG
1333       // We should never have a cloned child loop header but fail to have
1334       // all of the blocks for that child loop.
1335       for (auto *ChildLoopBB : ChildL->blocks())
1336         assert(BlocksInClonedLoop.count(
1337                    cast<BasicBlock>(VMap.lookup(ChildLoopBB))) &&
1338                "Child cloned loop has a header within the cloned outer "
1339                "loop but not all of its blocks!");
1340 #endif
1341 
1342       cloneLoopNest(*ChildL, ClonedL, VMap, LI);
1343     }
1344   }
1345 
1346   // Now that we've handled all the components of the original loop that were
1347   // cloned into a new loop, we still need to handle anything from the original
1348   // loop that wasn't in a cloned loop.
1349 
1350   // Figure out what blocks are left to place within any loop nest containing
1351   // the unswitched loop. If we never formed a loop, the cloned PH is one of
1352   // them.
1353   SmallPtrSet<BasicBlock *, 16> UnloopedBlockSet;
1354   if (BlocksInClonedLoop.empty())
1355     UnloopedBlockSet.insert(ClonedPH);
1356   for (auto *ClonedBB : ClonedLoopBlocks)
1357     if (!BlocksInClonedLoop.count(ClonedBB))
1358       UnloopedBlockSet.insert(ClonedBB);
1359 
1360   // Copy the cloned exits and sort them in ascending loop depth, we'll work
1361   // backwards across these to process them inside out. The order shouldn't
1362   // matter as we're just trying to build up the map from inside-out; we use
1363   // the map in a more stably ordered way below.
1364   auto OrderedClonedExitsInLoops = ClonedExitsInLoops;
1365   llvm::sort(OrderedClonedExitsInLoops, [&](BasicBlock *LHS, BasicBlock *RHS) {
1366     return ExitLoopMap.lookup(LHS)->getLoopDepth() <
1367            ExitLoopMap.lookup(RHS)->getLoopDepth();
1368   });
1369 
1370   // Populate the existing ExitLoopMap with everything reachable from each
1371   // exit, starting from the inner most exit.
1372   while (!UnloopedBlockSet.empty() && !OrderedClonedExitsInLoops.empty()) {
1373     assert(Worklist.empty() && "Didn't clear worklist!");
1374 
1375     BasicBlock *ExitBB = OrderedClonedExitsInLoops.pop_back_val();
1376     Loop *ExitL = ExitLoopMap.lookup(ExitBB);
1377 
1378     // Walk the CFG back until we hit the cloned PH adding everything reachable
1379     // and in the unlooped set to this exit block's loop.
1380     Worklist.push_back(ExitBB);
1381     do {
1382       BasicBlock *BB = Worklist.pop_back_val();
1383       // We can stop recursing at the cloned preheader (if we get there).
1384       if (BB == ClonedPH)
1385         continue;
1386 
1387       for (BasicBlock *PredBB : predecessors(BB)) {
1388         // If this pred has already been moved to our set or is part of some
1389         // (inner) loop, no update needed.
1390         if (!UnloopedBlockSet.erase(PredBB)) {
1391           assert(
1392               (BlocksInClonedLoop.count(PredBB) || ExitLoopMap.count(PredBB)) &&
1393               "Predecessor not mapped to a loop!");
1394           continue;
1395         }
1396 
1397         // We just insert into the loop set here. We'll add these blocks to the
1398         // exit loop after we build up the set in an order that doesn't rely on
1399         // predecessor order (which in turn relies on use list order).
1400         bool Inserted = ExitLoopMap.insert({PredBB, ExitL}).second;
1401         (void)Inserted;
1402         assert(Inserted && "Should only visit an unlooped block once!");
1403 
1404         // And recurse through to its predecessors.
1405         Worklist.push_back(PredBB);
1406       }
1407     } while (!Worklist.empty());
1408   }
1409 
1410   // Now that the ExitLoopMap gives as  mapping for all the non-looping cloned
1411   // blocks to their outer loops, walk the cloned blocks and the cloned exits
1412   // in their original order adding them to the correct loop.
1413 
1414   // We need a stable insertion order. We use the order of the original loop
1415   // order and map into the correct parent loop.
1416   for (auto *BB : llvm::concat<BasicBlock *const>(
1417            makeArrayRef(ClonedPH), ClonedLoopBlocks, ClonedExitsInLoops))
1418     if (Loop *OuterL = ExitLoopMap.lookup(BB))
1419       OuterL->addBasicBlockToLoop(BB, LI);
1420 
1421 #ifndef NDEBUG
1422   for (auto &BBAndL : ExitLoopMap) {
1423     auto *BB = BBAndL.first;
1424     auto *OuterL = BBAndL.second;
1425     assert(LI.getLoopFor(BB) == OuterL &&
1426            "Failed to put all blocks into outer loops!");
1427   }
1428 #endif
1429 
1430   // Now that all the blocks are placed into the correct containing loop in the
1431   // absence of child loops, find all the potentially cloned child loops and
1432   // clone them into whatever outer loop we placed their header into.
1433   for (Loop *ChildL : OrigL) {
1434     auto *ClonedChildHeader =
1435         cast_or_null<BasicBlock>(VMap.lookup(ChildL->getHeader()));
1436     if (!ClonedChildHeader || BlocksInClonedLoop.count(ClonedChildHeader))
1437       continue;
1438 
1439 #ifndef NDEBUG
1440     for (auto *ChildLoopBB : ChildL->blocks())
1441       assert(VMap.count(ChildLoopBB) &&
1442              "Cloned a child loop header but not all of that loops blocks!");
1443 #endif
1444 
1445     NonChildClonedLoops.push_back(cloneLoopNest(
1446         *ChildL, ExitLoopMap.lookup(ClonedChildHeader), VMap, LI));
1447   }
1448 }
1449 
1450 static void
1451 deleteDeadClonedBlocks(Loop &L, ArrayRef<BasicBlock *> ExitBlocks,
1452                        ArrayRef<std::unique_ptr<ValueToValueMapTy>> VMaps,
1453                        DominatorTree &DT, MemorySSAUpdater *MSSAU) {
1454   // Find all the dead clones, and remove them from their successors.
1455   SmallVector<BasicBlock *, 16> DeadBlocks;
1456   for (BasicBlock *BB : llvm::concat<BasicBlock *const>(L.blocks(), ExitBlocks))
1457     for (auto &VMap : VMaps)
1458       if (BasicBlock *ClonedBB = cast_or_null<BasicBlock>(VMap->lookup(BB)))
1459         if (!DT.isReachableFromEntry(ClonedBB)) {
1460           for (BasicBlock *SuccBB : successors(ClonedBB))
1461             SuccBB->removePredecessor(ClonedBB);
1462           DeadBlocks.push_back(ClonedBB);
1463         }
1464 
1465   // Remove all MemorySSA in the dead blocks
1466   if (MSSAU) {
1467     SmallSetVector<BasicBlock *, 8> DeadBlockSet(DeadBlocks.begin(),
1468                                                  DeadBlocks.end());
1469     MSSAU->removeBlocks(DeadBlockSet);
1470   }
1471 
1472   // Drop any remaining references to break cycles.
1473   for (BasicBlock *BB : DeadBlocks)
1474     BB->dropAllReferences();
1475   // Erase them from the IR.
1476   for (BasicBlock *BB : DeadBlocks)
1477     BB->eraseFromParent();
1478 }
1479 
1480 static void deleteDeadBlocksFromLoop(Loop &L,
1481                                      SmallVectorImpl<BasicBlock *> &ExitBlocks,
1482                                      DominatorTree &DT, LoopInfo &LI,
1483                                      MemorySSAUpdater *MSSAU) {
1484   // Find all the dead blocks tied to this loop, and remove them from their
1485   // successors.
1486   SmallSetVector<BasicBlock *, 8> DeadBlockSet;
1487 
1488   // Start with loop/exit blocks and get a transitive closure of reachable dead
1489   // blocks.
1490   SmallVector<BasicBlock *, 16> DeathCandidates(ExitBlocks.begin(),
1491                                                 ExitBlocks.end());
1492   DeathCandidates.append(L.blocks().begin(), L.blocks().end());
1493   while (!DeathCandidates.empty()) {
1494     auto *BB = DeathCandidates.pop_back_val();
1495     if (!DeadBlockSet.count(BB) && !DT.isReachableFromEntry(BB)) {
1496       for (BasicBlock *SuccBB : successors(BB)) {
1497         SuccBB->removePredecessor(BB);
1498         DeathCandidates.push_back(SuccBB);
1499       }
1500       DeadBlockSet.insert(BB);
1501     }
1502   }
1503 
1504   // Remove all MemorySSA in the dead blocks
1505   if (MSSAU)
1506     MSSAU->removeBlocks(DeadBlockSet);
1507 
1508   // Filter out the dead blocks from the exit blocks list so that it can be
1509   // used in the caller.
1510   llvm::erase_if(ExitBlocks,
1511                  [&](BasicBlock *BB) { return DeadBlockSet.count(BB); });
1512 
1513   // Walk from this loop up through its parents removing all of the dead blocks.
1514   for (Loop *ParentL = &L; ParentL; ParentL = ParentL->getParentLoop()) {
1515     for (auto *BB : DeadBlockSet)
1516       ParentL->getBlocksSet().erase(BB);
1517     llvm::erase_if(ParentL->getBlocksVector(),
1518                    [&](BasicBlock *BB) { return DeadBlockSet.count(BB); });
1519   }
1520 
1521   // Now delete the dead child loops. This raw delete will clear them
1522   // recursively.
1523   llvm::erase_if(L.getSubLoopsVector(), [&](Loop *ChildL) {
1524     if (!DeadBlockSet.count(ChildL->getHeader()))
1525       return false;
1526 
1527     assert(llvm::all_of(ChildL->blocks(),
1528                         [&](BasicBlock *ChildBB) {
1529                           return DeadBlockSet.count(ChildBB);
1530                         }) &&
1531            "If the child loop header is dead all blocks in the child loop must "
1532            "be dead as well!");
1533     LI.destroy(ChildL);
1534     return true;
1535   });
1536 
1537   // Remove the loop mappings for the dead blocks and drop all the references
1538   // from these blocks to others to handle cyclic references as we start
1539   // deleting the blocks themselves.
1540   for (auto *BB : DeadBlockSet) {
1541     // Check that the dominator tree has already been updated.
1542     assert(!DT.getNode(BB) && "Should already have cleared domtree!");
1543     LI.changeLoopFor(BB, nullptr);
1544     BB->dropAllReferences();
1545   }
1546 
1547   // Actually delete the blocks now that they've been fully unhooked from the
1548   // IR.
1549   for (auto *BB : DeadBlockSet)
1550     BB->eraseFromParent();
1551 }
1552 
1553 /// Recompute the set of blocks in a loop after unswitching.
1554 ///
1555 /// This walks from the original headers predecessors to rebuild the loop. We
1556 /// take advantage of the fact that new blocks can't have been added, and so we
1557 /// filter by the original loop's blocks. This also handles potentially
1558 /// unreachable code that we don't want to explore but might be found examining
1559 /// the predecessors of the header.
1560 ///
1561 /// If the original loop is no longer a loop, this will return an empty set. If
1562 /// it remains a loop, all the blocks within it will be added to the set
1563 /// (including those blocks in inner loops).
1564 static SmallPtrSet<const BasicBlock *, 16> recomputeLoopBlockSet(Loop &L,
1565                                                                  LoopInfo &LI) {
1566   SmallPtrSet<const BasicBlock *, 16> LoopBlockSet;
1567 
1568   auto *PH = L.getLoopPreheader();
1569   auto *Header = L.getHeader();
1570 
1571   // A worklist to use while walking backwards from the header.
1572   SmallVector<BasicBlock *, 16> Worklist;
1573 
1574   // First walk the predecessors of the header to find the backedges. This will
1575   // form the basis of our walk.
1576   for (auto *Pred : predecessors(Header)) {
1577     // Skip the preheader.
1578     if (Pred == PH)
1579       continue;
1580 
1581     // Because the loop was in simplified form, the only non-loop predecessor
1582     // is the preheader.
1583     assert(L.contains(Pred) && "Found a predecessor of the loop header other "
1584                                "than the preheader that is not part of the "
1585                                "loop!");
1586 
1587     // Insert this block into the loop set and on the first visit and, if it
1588     // isn't the header we're currently walking, put it into the worklist to
1589     // recurse through.
1590     if (LoopBlockSet.insert(Pred).second && Pred != Header)
1591       Worklist.push_back(Pred);
1592   }
1593 
1594   // If no backedges were found, we're done.
1595   if (LoopBlockSet.empty())
1596     return LoopBlockSet;
1597 
1598   // We found backedges, recurse through them to identify the loop blocks.
1599   while (!Worklist.empty()) {
1600     BasicBlock *BB = Worklist.pop_back_val();
1601     assert(LoopBlockSet.count(BB) && "Didn't put block into the loop set!");
1602 
1603     // No need to walk past the header.
1604     if (BB == Header)
1605       continue;
1606 
1607     // Because we know the inner loop structure remains valid we can use the
1608     // loop structure to jump immediately across the entire nested loop.
1609     // Further, because it is in loop simplified form, we can directly jump
1610     // to its preheader afterward.
1611     if (Loop *InnerL = LI.getLoopFor(BB))
1612       if (InnerL != &L) {
1613         assert(L.contains(InnerL) &&
1614                "Should not reach a loop *outside* this loop!");
1615         // The preheader is the only possible predecessor of the loop so
1616         // insert it into the set and check whether it was already handled.
1617         auto *InnerPH = InnerL->getLoopPreheader();
1618         assert(L.contains(InnerPH) && "Cannot contain an inner loop block "
1619                                       "but not contain the inner loop "
1620                                       "preheader!");
1621         if (!LoopBlockSet.insert(InnerPH).second)
1622           // The only way to reach the preheader is through the loop body
1623           // itself so if it has been visited the loop is already handled.
1624           continue;
1625 
1626         // Insert all of the blocks (other than those already present) into
1627         // the loop set. We expect at least the block that led us to find the
1628         // inner loop to be in the block set, but we may also have other loop
1629         // blocks if they were already enqueued as predecessors of some other
1630         // outer loop block.
1631         for (auto *InnerBB : InnerL->blocks()) {
1632           if (InnerBB == BB) {
1633             assert(LoopBlockSet.count(InnerBB) &&
1634                    "Block should already be in the set!");
1635             continue;
1636           }
1637 
1638           LoopBlockSet.insert(InnerBB);
1639         }
1640 
1641         // Add the preheader to the worklist so we will continue past the
1642         // loop body.
1643         Worklist.push_back(InnerPH);
1644         continue;
1645       }
1646 
1647     // Insert any predecessors that were in the original loop into the new
1648     // set, and if the insert is successful, add them to the worklist.
1649     for (auto *Pred : predecessors(BB))
1650       if (L.contains(Pred) && LoopBlockSet.insert(Pred).second)
1651         Worklist.push_back(Pred);
1652   }
1653 
1654   assert(LoopBlockSet.count(Header) && "Cannot fail to add the header!");
1655 
1656   // We've found all the blocks participating in the loop, return our completed
1657   // set.
1658   return LoopBlockSet;
1659 }
1660 
1661 /// Rebuild a loop after unswitching removes some subset of blocks and edges.
1662 ///
1663 /// The removal may have removed some child loops entirely but cannot have
1664 /// disturbed any remaining child loops. However, they may need to be hoisted
1665 /// to the parent loop (or to be top-level loops). The original loop may be
1666 /// completely removed.
1667 ///
1668 /// The sibling loops resulting from this update are returned. If the original
1669 /// loop remains a valid loop, it will be the first entry in this list with all
1670 /// of the newly sibling loops following it.
1671 ///
1672 /// Returns true if the loop remains a loop after unswitching, and false if it
1673 /// is no longer a loop after unswitching (and should not continue to be
1674 /// referenced).
1675 static bool rebuildLoopAfterUnswitch(Loop &L, ArrayRef<BasicBlock *> ExitBlocks,
1676                                      LoopInfo &LI,
1677                                      SmallVectorImpl<Loop *> &HoistedLoops) {
1678   auto *PH = L.getLoopPreheader();
1679 
1680   // Compute the actual parent loop from the exit blocks. Because we may have
1681   // pruned some exits the loop may be different from the original parent.
1682   Loop *ParentL = nullptr;
1683   SmallVector<Loop *, 4> ExitLoops;
1684   SmallVector<BasicBlock *, 4> ExitsInLoops;
1685   ExitsInLoops.reserve(ExitBlocks.size());
1686   for (auto *ExitBB : ExitBlocks)
1687     if (Loop *ExitL = LI.getLoopFor(ExitBB)) {
1688       ExitLoops.push_back(ExitL);
1689       ExitsInLoops.push_back(ExitBB);
1690       if (!ParentL || (ParentL != ExitL && ParentL->contains(ExitL)))
1691         ParentL = ExitL;
1692     }
1693 
1694   // Recompute the blocks participating in this loop. This may be empty if it
1695   // is no longer a loop.
1696   auto LoopBlockSet = recomputeLoopBlockSet(L, LI);
1697 
1698   // If we still have a loop, we need to re-set the loop's parent as the exit
1699   // block set changing may have moved it within the loop nest. Note that this
1700   // can only happen when this loop has a parent as it can only hoist the loop
1701   // *up* the nest.
1702   if (!LoopBlockSet.empty() && L.getParentLoop() != ParentL) {
1703     // Remove this loop's (original) blocks from all of the intervening loops.
1704     for (Loop *IL = L.getParentLoop(); IL != ParentL;
1705          IL = IL->getParentLoop()) {
1706       IL->getBlocksSet().erase(PH);
1707       for (auto *BB : L.blocks())
1708         IL->getBlocksSet().erase(BB);
1709       llvm::erase_if(IL->getBlocksVector(), [&](BasicBlock *BB) {
1710         return BB == PH || L.contains(BB);
1711       });
1712     }
1713 
1714     LI.changeLoopFor(PH, ParentL);
1715     L.getParentLoop()->removeChildLoop(&L);
1716     if (ParentL)
1717       ParentL->addChildLoop(&L);
1718     else
1719       LI.addTopLevelLoop(&L);
1720   }
1721 
1722   // Now we update all the blocks which are no longer within the loop.
1723   auto &Blocks = L.getBlocksVector();
1724   auto BlocksSplitI =
1725       LoopBlockSet.empty()
1726           ? Blocks.begin()
1727           : std::stable_partition(
1728                 Blocks.begin(), Blocks.end(),
1729                 [&](BasicBlock *BB) { return LoopBlockSet.count(BB); });
1730 
1731   // Before we erase the list of unlooped blocks, build a set of them.
1732   SmallPtrSet<BasicBlock *, 16> UnloopedBlocks(BlocksSplitI, Blocks.end());
1733   if (LoopBlockSet.empty())
1734     UnloopedBlocks.insert(PH);
1735 
1736   // Now erase these blocks from the loop.
1737   for (auto *BB : make_range(BlocksSplitI, Blocks.end()))
1738     L.getBlocksSet().erase(BB);
1739   Blocks.erase(BlocksSplitI, Blocks.end());
1740 
1741   // Sort the exits in ascending loop depth, we'll work backwards across these
1742   // to process them inside out.
1743   llvm::stable_sort(ExitsInLoops, [&](BasicBlock *LHS, BasicBlock *RHS) {
1744     return LI.getLoopDepth(LHS) < LI.getLoopDepth(RHS);
1745   });
1746 
1747   // We'll build up a set for each exit loop.
1748   SmallPtrSet<BasicBlock *, 16> NewExitLoopBlocks;
1749   Loop *PrevExitL = L.getParentLoop(); // The deepest possible exit loop.
1750 
1751   auto RemoveUnloopedBlocksFromLoop =
1752       [](Loop &L, SmallPtrSetImpl<BasicBlock *> &UnloopedBlocks) {
1753         for (auto *BB : UnloopedBlocks)
1754           L.getBlocksSet().erase(BB);
1755         llvm::erase_if(L.getBlocksVector(), [&](BasicBlock *BB) {
1756           return UnloopedBlocks.count(BB);
1757         });
1758       };
1759 
1760   SmallVector<BasicBlock *, 16> Worklist;
1761   while (!UnloopedBlocks.empty() && !ExitsInLoops.empty()) {
1762     assert(Worklist.empty() && "Didn't clear worklist!");
1763     assert(NewExitLoopBlocks.empty() && "Didn't clear loop set!");
1764 
1765     // Grab the next exit block, in decreasing loop depth order.
1766     BasicBlock *ExitBB = ExitsInLoops.pop_back_val();
1767     Loop &ExitL = *LI.getLoopFor(ExitBB);
1768     assert(ExitL.contains(&L) && "Exit loop must contain the inner loop!");
1769 
1770     // Erase all of the unlooped blocks from the loops between the previous
1771     // exit loop and this exit loop. This works because the ExitInLoops list is
1772     // sorted in increasing order of loop depth and thus we visit loops in
1773     // decreasing order of loop depth.
1774     for (; PrevExitL != &ExitL; PrevExitL = PrevExitL->getParentLoop())
1775       RemoveUnloopedBlocksFromLoop(*PrevExitL, UnloopedBlocks);
1776 
1777     // Walk the CFG back until we hit the cloned PH adding everything reachable
1778     // and in the unlooped set to this exit block's loop.
1779     Worklist.push_back(ExitBB);
1780     do {
1781       BasicBlock *BB = Worklist.pop_back_val();
1782       // We can stop recursing at the cloned preheader (if we get there).
1783       if (BB == PH)
1784         continue;
1785 
1786       for (BasicBlock *PredBB : predecessors(BB)) {
1787         // If this pred has already been moved to our set or is part of some
1788         // (inner) loop, no update needed.
1789         if (!UnloopedBlocks.erase(PredBB)) {
1790           assert((NewExitLoopBlocks.count(PredBB) ||
1791                   ExitL.contains(LI.getLoopFor(PredBB))) &&
1792                  "Predecessor not in a nested loop (or already visited)!");
1793           continue;
1794         }
1795 
1796         // We just insert into the loop set here. We'll add these blocks to the
1797         // exit loop after we build up the set in a deterministic order rather
1798         // than the predecessor-influenced visit order.
1799         bool Inserted = NewExitLoopBlocks.insert(PredBB).second;
1800         (void)Inserted;
1801         assert(Inserted && "Should only visit an unlooped block once!");
1802 
1803         // And recurse through to its predecessors.
1804         Worklist.push_back(PredBB);
1805       }
1806     } while (!Worklist.empty());
1807 
1808     // If blocks in this exit loop were directly part of the original loop (as
1809     // opposed to a child loop) update the map to point to this exit loop. This
1810     // just updates a map and so the fact that the order is unstable is fine.
1811     for (auto *BB : NewExitLoopBlocks)
1812       if (Loop *BBL = LI.getLoopFor(BB))
1813         if (BBL == &L || !L.contains(BBL))
1814           LI.changeLoopFor(BB, &ExitL);
1815 
1816     // We will remove the remaining unlooped blocks from this loop in the next
1817     // iteration or below.
1818     NewExitLoopBlocks.clear();
1819   }
1820 
1821   // Any remaining unlooped blocks are no longer part of any loop unless they
1822   // are part of some child loop.
1823   for (; PrevExitL; PrevExitL = PrevExitL->getParentLoop())
1824     RemoveUnloopedBlocksFromLoop(*PrevExitL, UnloopedBlocks);
1825   for (auto *BB : UnloopedBlocks)
1826     if (Loop *BBL = LI.getLoopFor(BB))
1827       if (BBL == &L || !L.contains(BBL))
1828         LI.changeLoopFor(BB, nullptr);
1829 
1830   // Sink all the child loops whose headers are no longer in the loop set to
1831   // the parent (or to be top level loops). We reach into the loop and directly
1832   // update its subloop vector to make this batch update efficient.
1833   auto &SubLoops = L.getSubLoopsVector();
1834   auto SubLoopsSplitI =
1835       LoopBlockSet.empty()
1836           ? SubLoops.begin()
1837           : std::stable_partition(
1838                 SubLoops.begin(), SubLoops.end(), [&](Loop *SubL) {
1839                   return LoopBlockSet.count(SubL->getHeader());
1840                 });
1841   for (auto *HoistedL : make_range(SubLoopsSplitI, SubLoops.end())) {
1842     HoistedLoops.push_back(HoistedL);
1843     HoistedL->setParentLoop(nullptr);
1844 
1845     // To compute the new parent of this hoisted loop we look at where we
1846     // placed the preheader above. We can't lookup the header itself because we
1847     // retained the mapping from the header to the hoisted loop. But the
1848     // preheader and header should have the exact same new parent computed
1849     // based on the set of exit blocks from the original loop as the preheader
1850     // is a predecessor of the header and so reached in the reverse walk. And
1851     // because the loops were all in simplified form the preheader of the
1852     // hoisted loop can't be part of some *other* loop.
1853     if (auto *NewParentL = LI.getLoopFor(HoistedL->getLoopPreheader()))
1854       NewParentL->addChildLoop(HoistedL);
1855     else
1856       LI.addTopLevelLoop(HoistedL);
1857   }
1858   SubLoops.erase(SubLoopsSplitI, SubLoops.end());
1859 
1860   // Actually delete the loop if nothing remained within it.
1861   if (Blocks.empty()) {
1862     assert(SubLoops.empty() &&
1863            "Failed to remove all subloops from the original loop!");
1864     if (Loop *ParentL = L.getParentLoop())
1865       ParentL->removeChildLoop(llvm::find(*ParentL, &L));
1866     else
1867       LI.removeLoop(llvm::find(LI, &L));
1868     LI.destroy(&L);
1869     return false;
1870   }
1871 
1872   return true;
1873 }
1874 
1875 /// Helper to visit a dominator subtree, invoking a callable on each node.
1876 ///
1877 /// Returning false at any point will stop walking past that node of the tree.
1878 template <typename CallableT>
1879 void visitDomSubTree(DominatorTree &DT, BasicBlock *BB, CallableT Callable) {
1880   SmallVector<DomTreeNode *, 4> DomWorklist;
1881   DomWorklist.push_back(DT[BB]);
1882 #ifndef NDEBUG
1883   SmallPtrSet<DomTreeNode *, 4> Visited;
1884   Visited.insert(DT[BB]);
1885 #endif
1886   do {
1887     DomTreeNode *N = DomWorklist.pop_back_val();
1888 
1889     // Visit this node.
1890     if (!Callable(N->getBlock()))
1891       continue;
1892 
1893     // Accumulate the child nodes.
1894     for (DomTreeNode *ChildN : *N) {
1895       assert(Visited.insert(ChildN).second &&
1896              "Cannot visit a node twice when walking a tree!");
1897       DomWorklist.push_back(ChildN);
1898     }
1899   } while (!DomWorklist.empty());
1900 }
1901 
1902 static void unswitchNontrivialInvariants(
1903     Loop &L, Instruction &TI, ArrayRef<Value *> Invariants,
1904     SmallVectorImpl<BasicBlock *> &ExitBlocks, DominatorTree &DT, LoopInfo &LI,
1905     AssumptionCache &AC, function_ref<void(bool, ArrayRef<Loop *>)> UnswitchCB,
1906     ScalarEvolution *SE, MemorySSAUpdater *MSSAU) {
1907   auto *ParentBB = TI.getParent();
1908   BranchInst *BI = dyn_cast<BranchInst>(&TI);
1909   SwitchInst *SI = BI ? nullptr : cast<SwitchInst>(&TI);
1910 
1911   // We can only unswitch switches, conditional branches with an invariant
1912   // condition, or combining invariant conditions with an instruction.
1913   assert((SI || (BI && BI->isConditional())) &&
1914          "Can only unswitch switches and conditional branch!");
1915   bool FullUnswitch = SI || BI->getCondition() == Invariants[0];
1916   if (FullUnswitch)
1917     assert(Invariants.size() == 1 &&
1918            "Cannot have other invariants with full unswitching!");
1919   else
1920     assert(isa<Instruction>(BI->getCondition()) &&
1921            "Partial unswitching requires an instruction as the condition!");
1922 
1923   if (MSSAU && VerifyMemorySSA)
1924     MSSAU->getMemorySSA()->verifyMemorySSA();
1925 
1926   // Constant and BBs tracking the cloned and continuing successor. When we are
1927   // unswitching the entire condition, this can just be trivially chosen to
1928   // unswitch towards `true`. However, when we are unswitching a set of
1929   // invariants combined with `and` or `or`, the combining operation determines
1930   // the best direction to unswitch: we want to unswitch the direction that will
1931   // collapse the branch.
1932   bool Direction = true;
1933   int ClonedSucc = 0;
1934   if (!FullUnswitch) {
1935     if (cast<Instruction>(BI->getCondition())->getOpcode() != Instruction::Or) {
1936       assert(cast<Instruction>(BI->getCondition())->getOpcode() ==
1937                  Instruction::And &&
1938              "Only `or` and `and` instructions can combine invariants being "
1939              "unswitched.");
1940       Direction = false;
1941       ClonedSucc = 1;
1942     }
1943   }
1944 
1945   BasicBlock *RetainedSuccBB =
1946       BI ? BI->getSuccessor(1 - ClonedSucc) : SI->getDefaultDest();
1947   SmallSetVector<BasicBlock *, 4> UnswitchedSuccBBs;
1948   if (BI)
1949     UnswitchedSuccBBs.insert(BI->getSuccessor(ClonedSucc));
1950   else
1951     for (auto Case : SI->cases())
1952       if (Case.getCaseSuccessor() != RetainedSuccBB)
1953         UnswitchedSuccBBs.insert(Case.getCaseSuccessor());
1954 
1955   assert(!UnswitchedSuccBBs.count(RetainedSuccBB) &&
1956          "Should not unswitch the same successor we are retaining!");
1957 
1958   // The branch should be in this exact loop. Any inner loop's invariant branch
1959   // should be handled by unswitching that inner loop. The caller of this
1960   // routine should filter out any candidates that remain (but were skipped for
1961   // whatever reason).
1962   assert(LI.getLoopFor(ParentBB) == &L && "Branch in an inner loop!");
1963 
1964   // Compute the parent loop now before we start hacking on things.
1965   Loop *ParentL = L.getParentLoop();
1966   // Get blocks in RPO order for MSSA update, before changing the CFG.
1967   LoopBlocksRPO LBRPO(&L);
1968   if (MSSAU)
1969     LBRPO.perform(&LI);
1970 
1971   // Compute the outer-most loop containing one of our exit blocks. This is the
1972   // furthest up our loopnest which can be mutated, which we will use below to
1973   // update things.
1974   Loop *OuterExitL = &L;
1975   for (auto *ExitBB : ExitBlocks) {
1976     Loop *NewOuterExitL = LI.getLoopFor(ExitBB);
1977     if (!NewOuterExitL) {
1978       // We exited the entire nest with this block, so we're done.
1979       OuterExitL = nullptr;
1980       break;
1981     }
1982     if (NewOuterExitL != OuterExitL && NewOuterExitL->contains(OuterExitL))
1983       OuterExitL = NewOuterExitL;
1984   }
1985 
1986   // At this point, we're definitely going to unswitch something so invalidate
1987   // any cached information in ScalarEvolution for the outer most loop
1988   // containing an exit block and all nested loops.
1989   if (SE) {
1990     if (OuterExitL)
1991       SE->forgetLoop(OuterExitL);
1992     else
1993       SE->forgetTopmostLoop(&L);
1994   }
1995 
1996   // If the edge from this terminator to a successor dominates that successor,
1997   // store a map from each block in its dominator subtree to it. This lets us
1998   // tell when cloning for a particular successor if a block is dominated by
1999   // some *other* successor with a single data structure. We use this to
2000   // significantly reduce cloning.
2001   SmallDenseMap<BasicBlock *, BasicBlock *, 16> DominatingSucc;
2002   for (auto *SuccBB : llvm::concat<BasicBlock *const>(
2003            makeArrayRef(RetainedSuccBB), UnswitchedSuccBBs))
2004     if (SuccBB->getUniquePredecessor() ||
2005         llvm::all_of(predecessors(SuccBB), [&](BasicBlock *PredBB) {
2006           return PredBB == ParentBB || DT.dominates(SuccBB, PredBB);
2007         }))
2008       visitDomSubTree(DT, SuccBB, [&](BasicBlock *BB) {
2009         DominatingSucc[BB] = SuccBB;
2010         return true;
2011       });
2012 
2013   // Split the preheader, so that we know that there is a safe place to insert
2014   // the conditional branch. We will change the preheader to have a conditional
2015   // branch on LoopCond. The original preheader will become the split point
2016   // between the unswitched versions, and we will have a new preheader for the
2017   // original loop.
2018   BasicBlock *SplitBB = L.getLoopPreheader();
2019   BasicBlock *LoopPH = SplitEdge(SplitBB, L.getHeader(), &DT, &LI, MSSAU);
2020 
2021   // Keep track of the dominator tree updates needed.
2022   SmallVector<DominatorTree::UpdateType, 4> DTUpdates;
2023 
2024   // Clone the loop for each unswitched successor.
2025   SmallVector<std::unique_ptr<ValueToValueMapTy>, 4> VMaps;
2026   VMaps.reserve(UnswitchedSuccBBs.size());
2027   SmallDenseMap<BasicBlock *, BasicBlock *, 4> ClonedPHs;
2028   for (auto *SuccBB : UnswitchedSuccBBs) {
2029     VMaps.emplace_back(new ValueToValueMapTy());
2030     ClonedPHs[SuccBB] = buildClonedLoopBlocks(
2031         L, LoopPH, SplitBB, ExitBlocks, ParentBB, SuccBB, RetainedSuccBB,
2032         DominatingSucc, *VMaps.back(), DTUpdates, AC, DT, LI, MSSAU);
2033   }
2034 
2035   // The stitching of the branched code back together depends on whether we're
2036   // doing full unswitching or not with the exception that we always want to
2037   // nuke the initial terminator placed in the split block.
2038   SplitBB->getTerminator()->eraseFromParent();
2039   if (FullUnswitch) {
2040     // Splice the terminator from the original loop and rewrite its
2041     // successors.
2042     SplitBB->getInstList().splice(SplitBB->end(), ParentBB->getInstList(), TI);
2043 
2044     // Keep a clone of the terminator for MSSA updates.
2045     Instruction *NewTI = TI.clone();
2046     ParentBB->getInstList().push_back(NewTI);
2047 
2048     // First wire up the moved terminator to the preheaders.
2049     if (BI) {
2050       BasicBlock *ClonedPH = ClonedPHs.begin()->second;
2051       BI->setSuccessor(ClonedSucc, ClonedPH);
2052       BI->setSuccessor(1 - ClonedSucc, LoopPH);
2053       DTUpdates.push_back({DominatorTree::Insert, SplitBB, ClonedPH});
2054     } else {
2055       assert(SI && "Must either be a branch or switch!");
2056 
2057       // Walk the cases and directly update their successors.
2058       assert(SI->getDefaultDest() == RetainedSuccBB &&
2059              "Not retaining default successor!");
2060       SI->setDefaultDest(LoopPH);
2061       for (auto &Case : SI->cases())
2062         if (Case.getCaseSuccessor() == RetainedSuccBB)
2063           Case.setSuccessor(LoopPH);
2064         else
2065           Case.setSuccessor(ClonedPHs.find(Case.getCaseSuccessor())->second);
2066 
2067       // We need to use the set to populate domtree updates as even when there
2068       // are multiple cases pointing at the same successor we only want to
2069       // remove and insert one edge in the domtree.
2070       for (BasicBlock *SuccBB : UnswitchedSuccBBs)
2071         DTUpdates.push_back(
2072             {DominatorTree::Insert, SplitBB, ClonedPHs.find(SuccBB)->second});
2073     }
2074 
2075     if (MSSAU) {
2076       DT.applyUpdates(DTUpdates);
2077       DTUpdates.clear();
2078 
2079       // Remove all but one edge to the retained block and all unswitched
2080       // blocks. This is to avoid having duplicate entries in the cloned Phis,
2081       // when we know we only keep a single edge for each case.
2082       MSSAU->removeDuplicatePhiEdgesBetween(ParentBB, RetainedSuccBB);
2083       for (BasicBlock *SuccBB : UnswitchedSuccBBs)
2084         MSSAU->removeDuplicatePhiEdgesBetween(ParentBB, SuccBB);
2085 
2086       for (auto &VMap : VMaps)
2087         MSSAU->updateForClonedLoop(LBRPO, ExitBlocks, *VMap,
2088                                    /*IgnoreIncomingWithNoClones=*/true);
2089       MSSAU->updateExitBlocksForClonedLoop(ExitBlocks, VMaps, DT);
2090 
2091       // Remove all edges to unswitched blocks.
2092       for (BasicBlock *SuccBB : UnswitchedSuccBBs)
2093         MSSAU->removeEdge(ParentBB, SuccBB);
2094     }
2095 
2096     // Now unhook the successor relationship as we'll be replacing
2097     // the terminator with a direct branch. This is much simpler for branches
2098     // than switches so we handle those first.
2099     if (BI) {
2100       // Remove the parent as a predecessor of the unswitched successor.
2101       assert(UnswitchedSuccBBs.size() == 1 &&
2102              "Only one possible unswitched block for a branch!");
2103       BasicBlock *UnswitchedSuccBB = *UnswitchedSuccBBs.begin();
2104       UnswitchedSuccBB->removePredecessor(ParentBB,
2105                                           /*KeepOneInputPHIs*/ true);
2106       DTUpdates.push_back({DominatorTree::Delete, ParentBB, UnswitchedSuccBB});
2107     } else {
2108       // Note that we actually want to remove the parent block as a predecessor
2109       // of *every* case successor. The case successor is either unswitched,
2110       // completely eliminating an edge from the parent to that successor, or it
2111       // is a duplicate edge to the retained successor as the retained successor
2112       // is always the default successor and as we'll replace this with a direct
2113       // branch we no longer need the duplicate entries in the PHI nodes.
2114       SwitchInst *NewSI = cast<SwitchInst>(NewTI);
2115       assert(NewSI->getDefaultDest() == RetainedSuccBB &&
2116              "Not retaining default successor!");
2117       for (auto &Case : NewSI->cases())
2118         Case.getCaseSuccessor()->removePredecessor(
2119             ParentBB,
2120             /*KeepOneInputPHIs*/ true);
2121 
2122       // We need to use the set to populate domtree updates as even when there
2123       // are multiple cases pointing at the same successor we only want to
2124       // remove and insert one edge in the domtree.
2125       for (BasicBlock *SuccBB : UnswitchedSuccBBs)
2126         DTUpdates.push_back({DominatorTree::Delete, ParentBB, SuccBB});
2127     }
2128 
2129     // After MSSAU update, remove the cloned terminator instruction NewTI.
2130     ParentBB->getTerminator()->eraseFromParent();
2131 
2132     // Create a new unconditional branch to the continuing block (as opposed to
2133     // the one cloned).
2134     BranchInst::Create(RetainedSuccBB, ParentBB);
2135   } else {
2136     assert(BI && "Only branches have partial unswitching.");
2137     assert(UnswitchedSuccBBs.size() == 1 &&
2138            "Only one possible unswitched block for a branch!");
2139     BasicBlock *ClonedPH = ClonedPHs.begin()->second;
2140     // When doing a partial unswitch, we have to do a bit more work to build up
2141     // the branch in the split block.
2142     buildPartialUnswitchConditionalBranch(*SplitBB, Invariants, Direction,
2143                                           *ClonedPH, *LoopPH);
2144     DTUpdates.push_back({DominatorTree::Insert, SplitBB, ClonedPH});
2145 
2146     if (MSSAU) {
2147       DT.applyUpdates(DTUpdates);
2148       DTUpdates.clear();
2149 
2150       // Perform MSSA cloning updates.
2151       for (auto &VMap : VMaps)
2152         MSSAU->updateForClonedLoop(LBRPO, ExitBlocks, *VMap,
2153                                    /*IgnoreIncomingWithNoClones=*/true);
2154       MSSAU->updateExitBlocksForClonedLoop(ExitBlocks, VMaps, DT);
2155     }
2156   }
2157 
2158   // Apply the updates accumulated above to get an up-to-date dominator tree.
2159   DT.applyUpdates(DTUpdates);
2160 
2161   // Now that we have an accurate dominator tree, first delete the dead cloned
2162   // blocks so that we can accurately build any cloned loops. It is important to
2163   // not delete the blocks from the original loop yet because we still want to
2164   // reference the original loop to understand the cloned loop's structure.
2165   deleteDeadClonedBlocks(L, ExitBlocks, VMaps, DT, MSSAU);
2166 
2167   // Build the cloned loop structure itself. This may be substantially
2168   // different from the original structure due to the simplified CFG. This also
2169   // handles inserting all the cloned blocks into the correct loops.
2170   SmallVector<Loop *, 4> NonChildClonedLoops;
2171   for (std::unique_ptr<ValueToValueMapTy> &VMap : VMaps)
2172     buildClonedLoops(L, ExitBlocks, *VMap, LI, NonChildClonedLoops);
2173 
2174   // Now that our cloned loops have been built, we can update the original loop.
2175   // First we delete the dead blocks from it and then we rebuild the loop
2176   // structure taking these deletions into account.
2177   deleteDeadBlocksFromLoop(L, ExitBlocks, DT, LI, MSSAU);
2178 
2179   if (MSSAU && VerifyMemorySSA)
2180     MSSAU->getMemorySSA()->verifyMemorySSA();
2181 
2182   SmallVector<Loop *, 4> HoistedLoops;
2183   bool IsStillLoop = rebuildLoopAfterUnswitch(L, ExitBlocks, LI, HoistedLoops);
2184 
2185   if (MSSAU && VerifyMemorySSA)
2186     MSSAU->getMemorySSA()->verifyMemorySSA();
2187 
2188   // This transformation has a high risk of corrupting the dominator tree, and
2189   // the below steps to rebuild loop structures will result in hard to debug
2190   // errors in that case so verify that the dominator tree is sane first.
2191   // FIXME: Remove this when the bugs stop showing up and rely on existing
2192   // verification steps.
2193   assert(DT.verify(DominatorTree::VerificationLevel::Fast));
2194 
2195   if (BI) {
2196     // If we unswitched a branch which collapses the condition to a known
2197     // constant we want to replace all the uses of the invariants within both
2198     // the original and cloned blocks. We do this here so that we can use the
2199     // now updated dominator tree to identify which side the users are on.
2200     assert(UnswitchedSuccBBs.size() == 1 &&
2201            "Only one possible unswitched block for a branch!");
2202     BasicBlock *ClonedPH = ClonedPHs.begin()->second;
2203 
2204     // When considering multiple partially-unswitched invariants
2205     // we cant just go replace them with constants in both branches.
2206     //
2207     // For 'AND' we infer that true branch ("continue") means true
2208     // for each invariant operand.
2209     // For 'OR' we can infer that false branch ("continue") means false
2210     // for each invariant operand.
2211     // So it happens that for multiple-partial case we dont replace
2212     // in the unswitched branch.
2213     bool ReplaceUnswitched = FullUnswitch || (Invariants.size() == 1);
2214 
2215     ConstantInt *UnswitchedReplacement =
2216         Direction ? ConstantInt::getTrue(BI->getContext())
2217                   : ConstantInt::getFalse(BI->getContext());
2218     ConstantInt *ContinueReplacement =
2219         Direction ? ConstantInt::getFalse(BI->getContext())
2220                   : ConstantInt::getTrue(BI->getContext());
2221     for (Value *Invariant : Invariants)
2222       for (auto UI = Invariant->use_begin(), UE = Invariant->use_end();
2223            UI != UE;) {
2224         // Grab the use and walk past it so we can clobber it in the use list.
2225         Use *U = &*UI++;
2226         Instruction *UserI = dyn_cast<Instruction>(U->getUser());
2227         if (!UserI)
2228           continue;
2229 
2230         // Replace it with the 'continue' side if in the main loop body, and the
2231         // unswitched if in the cloned blocks.
2232         if (DT.dominates(LoopPH, UserI->getParent()))
2233           U->set(ContinueReplacement);
2234         else if (ReplaceUnswitched &&
2235                  DT.dominates(ClonedPH, UserI->getParent()))
2236           U->set(UnswitchedReplacement);
2237       }
2238   }
2239 
2240   // We can change which blocks are exit blocks of all the cloned sibling
2241   // loops, the current loop, and any parent loops which shared exit blocks
2242   // with the current loop. As a consequence, we need to re-form LCSSA for
2243   // them. But we shouldn't need to re-form LCSSA for any child loops.
2244   // FIXME: This could be made more efficient by tracking which exit blocks are
2245   // new, and focusing on them, but that isn't likely to be necessary.
2246   //
2247   // In order to reasonably rebuild LCSSA we need to walk inside-out across the
2248   // loop nest and update every loop that could have had its exits changed. We
2249   // also need to cover any intervening loops. We add all of these loops to
2250   // a list and sort them by loop depth to achieve this without updating
2251   // unnecessary loops.
2252   auto UpdateLoop = [&](Loop &UpdateL) {
2253 #ifndef NDEBUG
2254     UpdateL.verifyLoop();
2255     for (Loop *ChildL : UpdateL) {
2256       ChildL->verifyLoop();
2257       assert(ChildL->isRecursivelyLCSSAForm(DT, LI) &&
2258              "Perturbed a child loop's LCSSA form!");
2259     }
2260 #endif
2261     // First build LCSSA for this loop so that we can preserve it when
2262     // forming dedicated exits. We don't want to perturb some other loop's
2263     // LCSSA while doing that CFG edit.
2264     formLCSSA(UpdateL, DT, &LI, nullptr);
2265 
2266     // For loops reached by this loop's original exit blocks we may
2267     // introduced new, non-dedicated exits. At least try to re-form dedicated
2268     // exits for these loops. This may fail if they couldn't have dedicated
2269     // exits to start with.
2270     formDedicatedExitBlocks(&UpdateL, &DT, &LI, MSSAU, /*PreserveLCSSA*/ true);
2271   };
2272 
2273   // For non-child cloned loops and hoisted loops, we just need to update LCSSA
2274   // and we can do it in any order as they don't nest relative to each other.
2275   //
2276   // Also check if any of the loops we have updated have become top-level loops
2277   // as that will necessitate widening the outer loop scope.
2278   for (Loop *UpdatedL :
2279        llvm::concat<Loop *>(NonChildClonedLoops, HoistedLoops)) {
2280     UpdateLoop(*UpdatedL);
2281     if (!UpdatedL->getParentLoop())
2282       OuterExitL = nullptr;
2283   }
2284   if (IsStillLoop) {
2285     UpdateLoop(L);
2286     if (!L.getParentLoop())
2287       OuterExitL = nullptr;
2288   }
2289 
2290   // If the original loop had exit blocks, walk up through the outer most loop
2291   // of those exit blocks to update LCSSA and form updated dedicated exits.
2292   if (OuterExitL != &L)
2293     for (Loop *OuterL = ParentL; OuterL != OuterExitL;
2294          OuterL = OuterL->getParentLoop())
2295       UpdateLoop(*OuterL);
2296 
2297 #ifndef NDEBUG
2298   // Verify the entire loop structure to catch any incorrect updates before we
2299   // progress in the pass pipeline.
2300   LI.verify(DT);
2301 #endif
2302 
2303   // Now that we've unswitched something, make callbacks to report the changes.
2304   // For that we need to merge together the updated loops and the cloned loops
2305   // and check whether the original loop survived.
2306   SmallVector<Loop *, 4> SibLoops;
2307   for (Loop *UpdatedL : llvm::concat<Loop *>(NonChildClonedLoops, HoistedLoops))
2308     if (UpdatedL->getParentLoop() == ParentL)
2309       SibLoops.push_back(UpdatedL);
2310   UnswitchCB(IsStillLoop, SibLoops);
2311 
2312   if (MSSAU && VerifyMemorySSA)
2313     MSSAU->getMemorySSA()->verifyMemorySSA();
2314 
2315   if (BI)
2316     ++NumBranches;
2317   else
2318     ++NumSwitches;
2319 }
2320 
2321 /// Recursively compute the cost of a dominator subtree based on the per-block
2322 /// cost map provided.
2323 ///
2324 /// The recursive computation is memozied into the provided DT-indexed cost map
2325 /// to allow querying it for most nodes in the domtree without it becoming
2326 /// quadratic.
2327 static int
2328 computeDomSubtreeCost(DomTreeNode &N,
2329                       const SmallDenseMap<BasicBlock *, int, 4> &BBCostMap,
2330                       SmallDenseMap<DomTreeNode *, int, 4> &DTCostMap) {
2331   // Don't accumulate cost (or recurse through) blocks not in our block cost
2332   // map and thus not part of the duplication cost being considered.
2333   auto BBCostIt = BBCostMap.find(N.getBlock());
2334   if (BBCostIt == BBCostMap.end())
2335     return 0;
2336 
2337   // Lookup this node to see if we already computed its cost.
2338   auto DTCostIt = DTCostMap.find(&N);
2339   if (DTCostIt != DTCostMap.end())
2340     return DTCostIt->second;
2341 
2342   // If not, we have to compute it. We can't use insert above and update
2343   // because computing the cost may insert more things into the map.
2344   int Cost = std::accumulate(
2345       N.begin(), N.end(), BBCostIt->second, [&](int Sum, DomTreeNode *ChildN) {
2346         return Sum + computeDomSubtreeCost(*ChildN, BBCostMap, DTCostMap);
2347       });
2348   bool Inserted = DTCostMap.insert({&N, Cost}).second;
2349   (void)Inserted;
2350   assert(Inserted && "Should not insert a node while visiting children!");
2351   return Cost;
2352 }
2353 
2354 /// Turns a llvm.experimental.guard intrinsic into implicit control flow branch,
2355 /// making the following replacement:
2356 ///
2357 ///   --code before guard--
2358 ///   call void (i1, ...) @llvm.experimental.guard(i1 %cond) [ "deopt"() ]
2359 ///   --code after guard--
2360 ///
2361 /// into
2362 ///
2363 ///   --code before guard--
2364 ///   br i1 %cond, label %guarded, label %deopt
2365 ///
2366 /// guarded:
2367 ///   --code after guard--
2368 ///
2369 /// deopt:
2370 ///   call void (i1, ...) @llvm.experimental.guard(i1 false) [ "deopt"() ]
2371 ///   unreachable
2372 ///
2373 /// It also makes all relevant DT and LI updates, so that all structures are in
2374 /// valid state after this transform.
2375 static BranchInst *
2376 turnGuardIntoBranch(IntrinsicInst *GI, Loop &L,
2377                     SmallVectorImpl<BasicBlock *> &ExitBlocks,
2378                     DominatorTree &DT, LoopInfo &LI, MemorySSAUpdater *MSSAU) {
2379   SmallVector<DominatorTree::UpdateType, 4> DTUpdates;
2380   LLVM_DEBUG(dbgs() << "Turning " << *GI << " into a branch.\n");
2381   BasicBlock *CheckBB = GI->getParent();
2382 
2383   if (MSSAU && VerifyMemorySSA)
2384      MSSAU->getMemorySSA()->verifyMemorySSA();
2385 
2386   // Remove all CheckBB's successors from DomTree. A block can be seen among
2387   // successors more than once, but for DomTree it should be added only once.
2388   SmallPtrSet<BasicBlock *, 4> Successors;
2389   for (auto *Succ : successors(CheckBB))
2390     if (Successors.insert(Succ).second)
2391       DTUpdates.push_back({DominatorTree::Delete, CheckBB, Succ});
2392 
2393   Instruction *DeoptBlockTerm =
2394       SplitBlockAndInsertIfThen(GI->getArgOperand(0), GI, true);
2395   BranchInst *CheckBI = cast<BranchInst>(CheckBB->getTerminator());
2396   // SplitBlockAndInsertIfThen inserts control flow that branches to
2397   // DeoptBlockTerm if the condition is true.  We want the opposite.
2398   CheckBI->swapSuccessors();
2399 
2400   BasicBlock *GuardedBlock = CheckBI->getSuccessor(0);
2401   GuardedBlock->setName("guarded");
2402   CheckBI->getSuccessor(1)->setName("deopt");
2403   BasicBlock *DeoptBlock = CheckBI->getSuccessor(1);
2404 
2405   // We now have a new exit block.
2406   ExitBlocks.push_back(CheckBI->getSuccessor(1));
2407 
2408   if (MSSAU)
2409     MSSAU->moveAllAfterSpliceBlocks(CheckBB, GuardedBlock, GI);
2410 
2411   GI->moveBefore(DeoptBlockTerm);
2412   GI->setArgOperand(0, ConstantInt::getFalse(GI->getContext()));
2413 
2414   // Add new successors of CheckBB into DomTree.
2415   for (auto *Succ : successors(CheckBB))
2416     DTUpdates.push_back({DominatorTree::Insert, CheckBB, Succ});
2417 
2418   // Now the blocks that used to be CheckBB's successors are GuardedBlock's
2419   // successors.
2420   for (auto *Succ : Successors)
2421     DTUpdates.push_back({DominatorTree::Insert, GuardedBlock, Succ});
2422 
2423   // Make proper changes to DT.
2424   DT.applyUpdates(DTUpdates);
2425   // Inform LI of a new loop block.
2426   L.addBasicBlockToLoop(GuardedBlock, LI);
2427 
2428   if (MSSAU) {
2429     MemoryDef *MD = cast<MemoryDef>(MSSAU->getMemorySSA()->getMemoryAccess(GI));
2430     MSSAU->moveToPlace(MD, DeoptBlock, MemorySSA::End);
2431     if (VerifyMemorySSA)
2432       MSSAU->getMemorySSA()->verifyMemorySSA();
2433   }
2434 
2435   ++NumGuards;
2436   return CheckBI;
2437 }
2438 
2439 /// Cost multiplier is a way to limit potentially exponential behavior
2440 /// of loop-unswitch. Cost is multipied in proportion of 2^number of unswitch
2441 /// candidates available. Also accounting for the number of "sibling" loops with
2442 /// the idea to account for previous unswitches that already happened on this
2443 /// cluster of loops. There was an attempt to keep this formula simple,
2444 /// just enough to limit the worst case behavior. Even if it is not that simple
2445 /// now it is still not an attempt to provide a detailed heuristic size
2446 /// prediction.
2447 ///
2448 /// TODO: Make a proper accounting of "explosion" effect for all kinds of
2449 /// unswitch candidates, making adequate predictions instead of wild guesses.
2450 /// That requires knowing not just the number of "remaining" candidates but
2451 /// also costs of unswitching for each of these candidates.
2452 static int calculateUnswitchCostMultiplier(
2453     Instruction &TI, Loop &L, LoopInfo &LI, DominatorTree &DT,
2454     ArrayRef<std::pair<Instruction *, TinyPtrVector<Value *>>>
2455         UnswitchCandidates) {
2456 
2457   // Guards and other exiting conditions do not contribute to exponential
2458   // explosion as soon as they dominate the latch (otherwise there might be
2459   // another path to the latch remaining that does not allow to eliminate the
2460   // loop copy on unswitch).
2461   BasicBlock *Latch = L.getLoopLatch();
2462   BasicBlock *CondBlock = TI.getParent();
2463   if (DT.dominates(CondBlock, Latch) &&
2464       (isGuard(&TI) ||
2465        llvm::count_if(successors(&TI), [&L](BasicBlock *SuccBB) {
2466          return L.contains(SuccBB);
2467        }) <= 1)) {
2468     NumCostMultiplierSkipped++;
2469     return 1;
2470   }
2471 
2472   auto *ParentL = L.getParentLoop();
2473   int SiblingsCount = (ParentL ? ParentL->getSubLoopsVector().size()
2474                                : std::distance(LI.begin(), LI.end()));
2475   // Count amount of clones that all the candidates might cause during
2476   // unswitching. Branch/guard counts as 1, switch counts as log2 of its cases.
2477   int UnswitchedClones = 0;
2478   for (auto Candidate : UnswitchCandidates) {
2479     Instruction *CI = Candidate.first;
2480     BasicBlock *CondBlock = CI->getParent();
2481     bool SkipExitingSuccessors = DT.dominates(CondBlock, Latch);
2482     if (isGuard(CI)) {
2483       if (!SkipExitingSuccessors)
2484         UnswitchedClones++;
2485       continue;
2486     }
2487     int NonExitingSuccessors = llvm::count_if(
2488         successors(CondBlock), [SkipExitingSuccessors, &L](BasicBlock *SuccBB) {
2489           return !SkipExitingSuccessors || L.contains(SuccBB);
2490         });
2491     UnswitchedClones += Log2_32(NonExitingSuccessors);
2492   }
2493 
2494   // Ignore up to the "unscaled candidates" number of unswitch candidates
2495   // when calculating the power-of-two scaling of the cost. The main idea
2496   // with this control is to allow a small number of unswitches to happen
2497   // and rely more on siblings multiplier (see below) when the number
2498   // of candidates is small.
2499   unsigned ClonesPower =
2500       std::max(UnswitchedClones - (int)UnswitchNumInitialUnscaledCandidates, 0);
2501 
2502   // Allowing top-level loops to spread a bit more than nested ones.
2503   int SiblingsMultiplier =
2504       std::max((ParentL ? SiblingsCount
2505                         : SiblingsCount / (int)UnswitchSiblingsToplevelDiv),
2506                1);
2507   // Compute the cost multiplier in a way that won't overflow by saturating
2508   // at an upper bound.
2509   int CostMultiplier;
2510   if (ClonesPower > Log2_32(UnswitchThreshold) ||
2511       SiblingsMultiplier > UnswitchThreshold)
2512     CostMultiplier = UnswitchThreshold;
2513   else
2514     CostMultiplier = std::min(SiblingsMultiplier * (1 << ClonesPower),
2515                               (int)UnswitchThreshold);
2516 
2517   LLVM_DEBUG(dbgs() << "  Computed multiplier  " << CostMultiplier
2518                     << " (siblings " << SiblingsMultiplier << " * clones "
2519                     << (1 << ClonesPower) << ")"
2520                     << " for unswitch candidate: " << TI << "\n");
2521   return CostMultiplier;
2522 }
2523 
2524 static bool
2525 unswitchBestCondition(Loop &L, DominatorTree &DT, LoopInfo &LI,
2526                       AssumptionCache &AC, TargetTransformInfo &TTI,
2527                       function_ref<void(bool, ArrayRef<Loop *>)> UnswitchCB,
2528                       ScalarEvolution *SE, MemorySSAUpdater *MSSAU) {
2529   // Collect all invariant conditions within this loop (as opposed to an inner
2530   // loop which would be handled when visiting that inner loop).
2531   SmallVector<std::pair<Instruction *, TinyPtrVector<Value *>>, 4>
2532       UnswitchCandidates;
2533 
2534   // Whether or not we should also collect guards in the loop.
2535   bool CollectGuards = false;
2536   if (UnswitchGuards) {
2537     auto *GuardDecl = L.getHeader()->getParent()->getParent()->getFunction(
2538         Intrinsic::getName(Intrinsic::experimental_guard));
2539     if (GuardDecl && !GuardDecl->use_empty())
2540       CollectGuards = true;
2541   }
2542 
2543   for (auto *BB : L.blocks()) {
2544     if (LI.getLoopFor(BB) != &L)
2545       continue;
2546 
2547     if (CollectGuards)
2548       for (auto &I : *BB)
2549         if (isGuard(&I)) {
2550           auto *Cond = cast<IntrinsicInst>(&I)->getArgOperand(0);
2551           // TODO: Support AND, OR conditions and partial unswitching.
2552           if (!isa<Constant>(Cond) && L.isLoopInvariant(Cond))
2553             UnswitchCandidates.push_back({&I, {Cond}});
2554         }
2555 
2556     if (auto *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
2557       // We can only consider fully loop-invariant switch conditions as we need
2558       // to completely eliminate the switch after unswitching.
2559       if (!isa<Constant>(SI->getCondition()) &&
2560           L.isLoopInvariant(SI->getCondition()) && !BB->getUniqueSuccessor())
2561         UnswitchCandidates.push_back({SI, {SI->getCondition()}});
2562       continue;
2563     }
2564 
2565     auto *BI = dyn_cast<BranchInst>(BB->getTerminator());
2566     if (!BI || !BI->isConditional() || isa<Constant>(BI->getCondition()) ||
2567         BI->getSuccessor(0) == BI->getSuccessor(1))
2568       continue;
2569 
2570     if (L.isLoopInvariant(BI->getCondition())) {
2571       UnswitchCandidates.push_back({BI, {BI->getCondition()}});
2572       continue;
2573     }
2574 
2575     Instruction &CondI = *cast<Instruction>(BI->getCondition());
2576     if (CondI.getOpcode() != Instruction::And &&
2577       CondI.getOpcode() != Instruction::Or)
2578       continue;
2579 
2580     TinyPtrVector<Value *> Invariants =
2581         collectHomogenousInstGraphLoopInvariants(L, CondI, LI);
2582     if (Invariants.empty())
2583       continue;
2584 
2585     UnswitchCandidates.push_back({BI, std::move(Invariants)});
2586   }
2587 
2588   // If we didn't find any candidates, we're done.
2589   if (UnswitchCandidates.empty())
2590     return false;
2591 
2592   // Check if there are irreducible CFG cycles in this loop. If so, we cannot
2593   // easily unswitch non-trivial edges out of the loop. Doing so might turn the
2594   // irreducible control flow into reducible control flow and introduce new
2595   // loops "out of thin air". If we ever discover important use cases for doing
2596   // this, we can add support to loop unswitch, but it is a lot of complexity
2597   // for what seems little or no real world benefit.
2598   LoopBlocksRPO RPOT(&L);
2599   RPOT.perform(&LI);
2600   if (containsIrreducibleCFG<const BasicBlock *>(RPOT, LI))
2601     return false;
2602 
2603   SmallVector<BasicBlock *, 4> ExitBlocks;
2604   L.getUniqueExitBlocks(ExitBlocks);
2605 
2606   // We cannot unswitch if exit blocks contain a cleanuppad instruction as we
2607   // don't know how to split those exit blocks.
2608   // FIXME: We should teach SplitBlock to handle this and remove this
2609   // restriction.
2610   for (auto *ExitBB : ExitBlocks)
2611     if (isa<CleanupPadInst>(ExitBB->getFirstNonPHI())) {
2612       dbgs() << "Cannot unswitch because of cleanuppad in exit block\n";
2613       return false;
2614     }
2615 
2616   LLVM_DEBUG(
2617       dbgs() << "Considering " << UnswitchCandidates.size()
2618              << " non-trivial loop invariant conditions for unswitching.\n");
2619 
2620   // Given that unswitching these terminators will require duplicating parts of
2621   // the loop, so we need to be able to model that cost. Compute the ephemeral
2622   // values and set up a data structure to hold per-BB costs. We cache each
2623   // block's cost so that we don't recompute this when considering different
2624   // subsets of the loop for duplication during unswitching.
2625   SmallPtrSet<const Value *, 4> EphValues;
2626   CodeMetrics::collectEphemeralValues(&L, &AC, EphValues);
2627   SmallDenseMap<BasicBlock *, int, 4> BBCostMap;
2628 
2629   // Compute the cost of each block, as well as the total loop cost. Also, bail
2630   // out if we see instructions which are incompatible with loop unswitching
2631   // (convergent, noduplicate, or cross-basic-block tokens).
2632   // FIXME: We might be able to safely handle some of these in non-duplicated
2633   // regions.
2634   int LoopCost = 0;
2635   for (auto *BB : L.blocks()) {
2636     int Cost = 0;
2637     for (auto &I : *BB) {
2638       if (EphValues.count(&I))
2639         continue;
2640 
2641       if (I.getType()->isTokenTy() && I.isUsedOutsideOfBlock(BB))
2642         return false;
2643       if (auto CS = CallSite(&I))
2644         if (CS.isConvergent() || CS.cannotDuplicate())
2645           return false;
2646 
2647       Cost += TTI.getUserCost(&I);
2648     }
2649     assert(Cost >= 0 && "Must not have negative costs!");
2650     LoopCost += Cost;
2651     assert(LoopCost >= 0 && "Must not have negative loop costs!");
2652     BBCostMap[BB] = Cost;
2653   }
2654   LLVM_DEBUG(dbgs() << "  Total loop cost: " << LoopCost << "\n");
2655 
2656   // Now we find the best candidate by searching for the one with the following
2657   // properties in order:
2658   //
2659   // 1) An unswitching cost below the threshold
2660   // 2) The smallest number of duplicated unswitch candidates (to avoid
2661   //    creating redundant subsequent unswitching)
2662   // 3) The smallest cost after unswitching.
2663   //
2664   // We prioritize reducing fanout of unswitch candidates provided the cost
2665   // remains below the threshold because this has a multiplicative effect.
2666   //
2667   // This requires memoizing each dominator subtree to avoid redundant work.
2668   //
2669   // FIXME: Need to actually do the number of candidates part above.
2670   SmallDenseMap<DomTreeNode *, int, 4> DTCostMap;
2671   // Given a terminator which might be unswitched, computes the non-duplicated
2672   // cost for that terminator.
2673   auto ComputeUnswitchedCost = [&](Instruction &TI, bool FullUnswitch) {
2674     BasicBlock &BB = *TI.getParent();
2675     SmallPtrSet<BasicBlock *, 4> Visited;
2676 
2677     int Cost = LoopCost;
2678     for (BasicBlock *SuccBB : successors(&BB)) {
2679       // Don't count successors more than once.
2680       if (!Visited.insert(SuccBB).second)
2681         continue;
2682 
2683       // If this is a partial unswitch candidate, then it must be a conditional
2684       // branch with a condition of either `or` or `and`. In that case, one of
2685       // the successors is necessarily duplicated, so don't even try to remove
2686       // its cost.
2687       if (!FullUnswitch) {
2688         auto &BI = cast<BranchInst>(TI);
2689         if (cast<Instruction>(BI.getCondition())->getOpcode() ==
2690             Instruction::And) {
2691           if (SuccBB == BI.getSuccessor(1))
2692             continue;
2693         } else {
2694           assert(cast<Instruction>(BI.getCondition())->getOpcode() ==
2695                      Instruction::Or &&
2696                  "Only `and` and `or` conditions can result in a partial "
2697                  "unswitch!");
2698           if (SuccBB == BI.getSuccessor(0))
2699             continue;
2700         }
2701       }
2702 
2703       // This successor's domtree will not need to be duplicated after
2704       // unswitching if the edge to the successor dominates it (and thus the
2705       // entire tree). This essentially means there is no other path into this
2706       // subtree and so it will end up live in only one clone of the loop.
2707       if (SuccBB->getUniquePredecessor() ||
2708           llvm::all_of(predecessors(SuccBB), [&](BasicBlock *PredBB) {
2709             return PredBB == &BB || DT.dominates(SuccBB, PredBB);
2710           })) {
2711         Cost -= computeDomSubtreeCost(*DT[SuccBB], BBCostMap, DTCostMap);
2712         assert(Cost >= 0 &&
2713                "Non-duplicated cost should never exceed total loop cost!");
2714       }
2715     }
2716 
2717     // Now scale the cost by the number of unique successors minus one. We
2718     // subtract one because there is already at least one copy of the entire
2719     // loop. This is computing the new cost of unswitching a condition.
2720     // Note that guards always have 2 unique successors that are implicit and
2721     // will be materialized if we decide to unswitch it.
2722     int SuccessorsCount = isGuard(&TI) ? 2 : Visited.size();
2723     assert(SuccessorsCount > 1 &&
2724            "Cannot unswitch a condition without multiple distinct successors!");
2725     return Cost * (SuccessorsCount - 1);
2726   };
2727   Instruction *BestUnswitchTI = nullptr;
2728   int BestUnswitchCost = 0;
2729   ArrayRef<Value *> BestUnswitchInvariants;
2730   for (auto &TerminatorAndInvariants : UnswitchCandidates) {
2731     Instruction &TI = *TerminatorAndInvariants.first;
2732     ArrayRef<Value *> Invariants = TerminatorAndInvariants.second;
2733     BranchInst *BI = dyn_cast<BranchInst>(&TI);
2734     int CandidateCost = ComputeUnswitchedCost(
2735         TI, /*FullUnswitch*/ !BI || (Invariants.size() == 1 &&
2736                                      Invariants[0] == BI->getCondition()));
2737     // Calculate cost multiplier which is a tool to limit potentially
2738     // exponential behavior of loop-unswitch.
2739     if (EnableUnswitchCostMultiplier) {
2740       int CostMultiplier =
2741           calculateUnswitchCostMultiplier(TI, L, LI, DT, UnswitchCandidates);
2742       assert(
2743           (CostMultiplier > 0 && CostMultiplier <= UnswitchThreshold) &&
2744           "cost multiplier needs to be in the range of 1..UnswitchThreshold");
2745       CandidateCost *= CostMultiplier;
2746       LLVM_DEBUG(dbgs() << "  Computed cost of " << CandidateCost
2747                         << " (multiplier: " << CostMultiplier << ")"
2748                         << " for unswitch candidate: " << TI << "\n");
2749     } else {
2750       LLVM_DEBUG(dbgs() << "  Computed cost of " << CandidateCost
2751                         << " for unswitch candidate: " << TI << "\n");
2752     }
2753 
2754     if (!BestUnswitchTI || CandidateCost < BestUnswitchCost) {
2755       BestUnswitchTI = &TI;
2756       BestUnswitchCost = CandidateCost;
2757       BestUnswitchInvariants = Invariants;
2758     }
2759   }
2760   assert(BestUnswitchTI && "Failed to find loop unswitch candidate");
2761 
2762   if (BestUnswitchCost >= UnswitchThreshold) {
2763     LLVM_DEBUG(dbgs() << "Cannot unswitch, lowest cost found: "
2764                       << BestUnswitchCost << "\n");
2765     return false;
2766   }
2767 
2768   // If the best candidate is a guard, turn it into a branch.
2769   if (isGuard(BestUnswitchTI))
2770     BestUnswitchTI = turnGuardIntoBranch(cast<IntrinsicInst>(BestUnswitchTI), L,
2771                                          ExitBlocks, DT, LI, MSSAU);
2772 
2773   LLVM_DEBUG(dbgs() << "  Unswitching non-trivial (cost = "
2774                     << BestUnswitchCost << ") terminator: " << *BestUnswitchTI
2775                     << "\n");
2776   unswitchNontrivialInvariants(L, *BestUnswitchTI, BestUnswitchInvariants,
2777                                ExitBlocks, DT, LI, AC, UnswitchCB, SE, MSSAU);
2778   return true;
2779 }
2780 
2781 /// Unswitch control flow predicated on loop invariant conditions.
2782 ///
2783 /// This first hoists all branches or switches which are trivial (IE, do not
2784 /// require duplicating any part of the loop) out of the loop body. It then
2785 /// looks at other loop invariant control flows and tries to unswitch those as
2786 /// well by cloning the loop if the result is small enough.
2787 ///
2788 /// The `DT`, `LI`, `AC`, `TTI` parameters are required analyses that are also
2789 /// updated based on the unswitch.
2790 /// The `MSSA` analysis is also updated if valid (i.e. its use is enabled).
2791 ///
2792 /// If either `NonTrivial` is true or the flag `EnableNonTrivialUnswitch` is
2793 /// true, we will attempt to do non-trivial unswitching as well as trivial
2794 /// unswitching.
2795 ///
2796 /// The `UnswitchCB` callback provided will be run after unswitching is
2797 /// complete, with the first parameter set to `true` if the provided loop
2798 /// remains a loop, and a list of new sibling loops created.
2799 ///
2800 /// If `SE` is non-null, we will update that analysis based on the unswitching
2801 /// done.
2802 static bool unswitchLoop(Loop &L, DominatorTree &DT, LoopInfo &LI,
2803                          AssumptionCache &AC, TargetTransformInfo &TTI,
2804                          bool NonTrivial,
2805                          function_ref<void(bool, ArrayRef<Loop *>)> UnswitchCB,
2806                          ScalarEvolution *SE, MemorySSAUpdater *MSSAU) {
2807   assert(L.isRecursivelyLCSSAForm(DT, LI) &&
2808          "Loops must be in LCSSA form before unswitching.");
2809   bool Changed = false;
2810 
2811   // Must be in loop simplified form: we need a preheader and dedicated exits.
2812   if (!L.isLoopSimplifyForm())
2813     return false;
2814 
2815   // Try trivial unswitch first before loop over other basic blocks in the loop.
2816   if (unswitchAllTrivialConditions(L, DT, LI, SE, MSSAU)) {
2817     // If we unswitched successfully we will want to clean up the loop before
2818     // processing it further so just mark it as unswitched and return.
2819     UnswitchCB(/*CurrentLoopValid*/ true, {});
2820     return true;
2821   }
2822 
2823   // If we're not doing non-trivial unswitching, we're done. We both accept
2824   // a parameter but also check a local flag that can be used for testing
2825   // a debugging.
2826   if (!NonTrivial && !EnableNonTrivialUnswitch)
2827     return false;
2828 
2829   // For non-trivial unswitching, because it often creates new loops, we rely on
2830   // the pass manager to iterate on the loops rather than trying to immediately
2831   // reach a fixed point. There is no substantial advantage to iterating
2832   // internally, and if any of the new loops are simplified enough to contain
2833   // trivial unswitching we want to prefer those.
2834 
2835   // Try to unswitch the best invariant condition. We prefer this full unswitch to
2836   // a partial unswitch when possible below the threshold.
2837   if (unswitchBestCondition(L, DT, LI, AC, TTI, UnswitchCB, SE, MSSAU))
2838     return true;
2839 
2840   // No other opportunities to unswitch.
2841   return Changed;
2842 }
2843 
2844 PreservedAnalyses SimpleLoopUnswitchPass::run(Loop &L, LoopAnalysisManager &AM,
2845                                               LoopStandardAnalysisResults &AR,
2846                                               LPMUpdater &U) {
2847   Function &F = *L.getHeader()->getParent();
2848   (void)F;
2849 
2850   LLVM_DEBUG(dbgs() << "Unswitching loop in " << F.getName() << ": " << L
2851                     << "\n");
2852 
2853   // Save the current loop name in a variable so that we can report it even
2854   // after it has been deleted.
2855   std::string LoopName = L.getName();
2856 
2857   auto UnswitchCB = [&L, &U, &LoopName](bool CurrentLoopValid,
2858                                         ArrayRef<Loop *> NewLoops) {
2859     // If we did a non-trivial unswitch, we have added new (cloned) loops.
2860     if (!NewLoops.empty())
2861       U.addSiblingLoops(NewLoops);
2862 
2863     // If the current loop remains valid, we should revisit it to catch any
2864     // other unswitch opportunities. Otherwise, we need to mark it as deleted.
2865     if (CurrentLoopValid)
2866       U.revisitCurrentLoop();
2867     else
2868       U.markLoopAsDeleted(L, LoopName);
2869   };
2870 
2871   Optional<MemorySSAUpdater> MSSAU;
2872   if (AR.MSSA) {
2873     MSSAU = MemorySSAUpdater(AR.MSSA);
2874     if (VerifyMemorySSA)
2875       AR.MSSA->verifyMemorySSA();
2876   }
2877   if (!unswitchLoop(L, AR.DT, AR.LI, AR.AC, AR.TTI, NonTrivial, UnswitchCB,
2878                     &AR.SE, MSSAU.hasValue() ? MSSAU.getPointer() : nullptr))
2879     return PreservedAnalyses::all();
2880 
2881   if (AR.MSSA && VerifyMemorySSA)
2882     AR.MSSA->verifyMemorySSA();
2883 
2884   // Historically this pass has had issues with the dominator tree so verify it
2885   // in asserts builds.
2886   assert(AR.DT.verify(DominatorTree::VerificationLevel::Fast));
2887 
2888   auto PA = getLoopPassPreservedAnalyses();
2889   if (AR.MSSA)
2890     PA.preserve<MemorySSAAnalysis>();
2891   return PA;
2892 }
2893 
2894 namespace {
2895 
2896 class SimpleLoopUnswitchLegacyPass : public LoopPass {
2897   bool NonTrivial;
2898 
2899 public:
2900   static char ID; // Pass ID, replacement for typeid
2901 
2902   explicit SimpleLoopUnswitchLegacyPass(bool NonTrivial = false)
2903       : LoopPass(ID), NonTrivial(NonTrivial) {
2904     initializeSimpleLoopUnswitchLegacyPassPass(
2905         *PassRegistry::getPassRegistry());
2906   }
2907 
2908   bool runOnLoop(Loop *L, LPPassManager &LPM) override;
2909 
2910   void getAnalysisUsage(AnalysisUsage &AU) const override {
2911     AU.addRequired<AssumptionCacheTracker>();
2912     AU.addRequired<TargetTransformInfoWrapperPass>();
2913     if (EnableMSSALoopDependency) {
2914       AU.addRequired<MemorySSAWrapperPass>();
2915       AU.addPreserved<MemorySSAWrapperPass>();
2916     }
2917     getLoopAnalysisUsage(AU);
2918   }
2919 };
2920 
2921 } // end anonymous namespace
2922 
2923 bool SimpleLoopUnswitchLegacyPass::runOnLoop(Loop *L, LPPassManager &LPM) {
2924   if (skipLoop(L))
2925     return false;
2926 
2927   Function &F = *L->getHeader()->getParent();
2928 
2929   LLVM_DEBUG(dbgs() << "Unswitching loop in " << F.getName() << ": " << *L
2930                     << "\n");
2931 
2932   auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
2933   auto &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
2934   auto &AC = getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
2935   auto &TTI = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
2936   MemorySSA *MSSA = nullptr;
2937   Optional<MemorySSAUpdater> MSSAU;
2938   if (EnableMSSALoopDependency) {
2939     MSSA = &getAnalysis<MemorySSAWrapperPass>().getMSSA();
2940     MSSAU = MemorySSAUpdater(MSSA);
2941   }
2942 
2943   auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>();
2944   auto *SE = SEWP ? &SEWP->getSE() : nullptr;
2945 
2946   auto UnswitchCB = [&L, &LPM](bool CurrentLoopValid,
2947                                ArrayRef<Loop *> NewLoops) {
2948     // If we did a non-trivial unswitch, we have added new (cloned) loops.
2949     for (auto *NewL : NewLoops)
2950       LPM.addLoop(*NewL);
2951 
2952     // If the current loop remains valid, re-add it to the queue. This is
2953     // a little wasteful as we'll finish processing the current loop as well,
2954     // but it is the best we can do in the old PM.
2955     if (CurrentLoopValid)
2956       LPM.addLoop(*L);
2957     else
2958       LPM.markLoopAsDeleted(*L);
2959   };
2960 
2961   if (MSSA && VerifyMemorySSA)
2962     MSSA->verifyMemorySSA();
2963 
2964   bool Changed = unswitchLoop(*L, DT, LI, AC, TTI, NonTrivial, UnswitchCB, SE,
2965                               MSSAU.hasValue() ? MSSAU.getPointer() : nullptr);
2966 
2967   if (MSSA && VerifyMemorySSA)
2968     MSSA->verifyMemorySSA();
2969 
2970   // If anything was unswitched, also clear any cached information about this
2971   // loop.
2972   LPM.deleteSimpleAnalysisLoop(L);
2973 
2974   // Historically this pass has had issues with the dominator tree so verify it
2975   // in asserts builds.
2976   assert(DT.verify(DominatorTree::VerificationLevel::Fast));
2977 
2978   return Changed;
2979 }
2980 
2981 char SimpleLoopUnswitchLegacyPass::ID = 0;
2982 INITIALIZE_PASS_BEGIN(SimpleLoopUnswitchLegacyPass, "simple-loop-unswitch",
2983                       "Simple unswitch loops", false, false)
2984 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
2985 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
2986 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
2987 INITIALIZE_PASS_DEPENDENCY(LoopPass)
2988 INITIALIZE_PASS_DEPENDENCY(MemorySSAWrapperPass)
2989 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
2990 INITIALIZE_PASS_END(SimpleLoopUnswitchLegacyPass, "simple-loop-unswitch",
2991                     "Simple unswitch loops", false, false)
2992 
2993 Pass *llvm::createSimpleLoopUnswitchLegacyPass(bool NonTrivial) {
2994   return new SimpleLoopUnswitchLegacyPass(NonTrivial);
2995 }
2996