xref: /llvm-project/llvm/lib/Transforms/Utils/LCSSA.cpp (revision cdf4788401afff02e12279fc1fded94d6180639c)
1 //===-- LCSSA.cpp - Convert loops into loop-closed SSA form ---------------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This pass transforms loops by placing phi nodes at the end of the loops for
11 // all values that are live across the loop boundary.  For example, it turns
12 // the left into the right code:
13 //
14 // for (...)                for (...)
15 //   if (c)                   if (c)
16 //     X1 = ...                 X1 = ...
17 //   else                     else
18 //     X2 = ...                 X2 = ...
19 //   X3 = phi(X1, X2)         X3 = phi(X1, X2)
20 // ... = X3 + 4             X4 = phi(X3)
21 //                          ... = X4 + 4
22 //
23 // This is still valid LLVM; the extra phi nodes are purely redundant, and will
24 // be trivially eliminated by InstCombine.  The major benefit of this
25 // transformation is that it makes many other loop optimizations, such as
26 // LoopUnswitching, simpler.
27 //
28 //===----------------------------------------------------------------------===//
29 
30 #define DEBUG_TYPE "lcssa"
31 #include "llvm/Transforms/Scalar.h"
32 #include "llvm/ADT/STLExtras.h"
33 #include "llvm/ADT/Statistic.h"
34 #include "llvm/Analysis/AliasAnalysis.h"
35 #include "llvm/Analysis/LoopPass.h"
36 #include "llvm/Analysis/ScalarEvolution.h"
37 #include "llvm/IR/Constants.h"
38 #include "llvm/IR/Dominators.h"
39 #include "llvm/IR/Function.h"
40 #include "llvm/IR/Instructions.h"
41 #include "llvm/IR/PredIteratorCache.h"
42 #include "llvm/Pass.h"
43 #include "llvm/Transforms/Utils/LoopUtils.h"
44 #include "llvm/Transforms/Utils/SSAUpdater.h"
45 using namespace llvm;
46 
47 STATISTIC(NumLCSSA, "Number of live out of a loop variables");
48 
49 /// Return true if the specified block is in the list.
50 static bool isExitBlock(BasicBlock *BB,
51                         const SmallVectorImpl<BasicBlock *> &ExitBlocks) {
52   for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
53     if (ExitBlocks[i] == BB)
54       return true;
55   return false;
56 }
57 
58 /// Given an instruction in the loop, check to see if it has any uses that are
59 /// outside the current loop.  If so, insert LCSSA PHI nodes and rewrite the
60 /// uses.
61 static bool processInstruction(Loop &L, Instruction &Inst, DominatorTree &DT,
62                                const SmallVectorImpl<BasicBlock *> &ExitBlocks,
63                                PredIteratorCache &PredCache) {
64   SmallVector<Use *, 16> UsesToRewrite;
65 
66   BasicBlock *InstBB = Inst.getParent();
67 
68   for (Use &U : Inst.uses()) {
69     Instruction *User = cast<Instruction>(U.getUser());
70     BasicBlock *UserBB = User->getParent();
71     if (PHINode *PN = dyn_cast<PHINode>(User))
72       UserBB = PN->getIncomingBlock(U);
73 
74     if (InstBB != UserBB && !L.contains(UserBB))
75       UsesToRewrite.push_back(&U);
76   }
77 
78   // If there are no uses outside the loop, exit with no change.
79   if (UsesToRewrite.empty())
80     return false;
81 
82   ++NumLCSSA; // We are applying the transformation
83 
84   // Invoke instructions are special in that their result value is not available
85   // along their unwind edge. The code below tests to see whether DomBB
86   // dominates
87   // the value, so adjust DomBB to the normal destination block, which is
88   // effectively where the value is first usable.
89   BasicBlock *DomBB = Inst.getParent();
90   if (InvokeInst *Inv = dyn_cast<InvokeInst>(&Inst))
91     DomBB = Inv->getNormalDest();
92 
93   DomTreeNode *DomNode = DT.getNode(DomBB);
94 
95   SmallVector<PHINode *, 16> AddedPHIs;
96 
97   SSAUpdater SSAUpdate;
98   SSAUpdate.Initialize(Inst.getType(), Inst.getName());
99 
100   // Insert the LCSSA phi's into all of the exit blocks dominated by the
101   // value, and add them to the Phi's map.
102   for (SmallVectorImpl<BasicBlock *>::const_iterator BBI = ExitBlocks.begin(),
103                                                      BBE = ExitBlocks.end();
104        BBI != BBE; ++BBI) {
105     BasicBlock *ExitBB = *BBI;
106     if (!DT.dominates(DomNode, DT.getNode(ExitBB)))
107       continue;
108 
109     // If we already inserted something for this BB, don't reprocess it.
110     if (SSAUpdate.HasValueForBlock(ExitBB))
111       continue;
112 
113     PHINode *PN = PHINode::Create(Inst.getType(), PredCache.GetNumPreds(ExitBB),
114                                   Inst.getName() + ".lcssa", ExitBB->begin());
115 
116     // Add inputs from inside the loop for this PHI.
117     for (BasicBlock **PI = PredCache.GetPreds(ExitBB); *PI; ++PI) {
118       PN->addIncoming(&Inst, *PI);
119 
120       // If the exit block has a predecessor not within the loop, arrange for
121       // the incoming value use corresponding to that predecessor to be
122       // rewritten in terms of a different LCSSA PHI.
123       if (!L.contains(*PI))
124         UsesToRewrite.push_back(
125             &PN->getOperandUse(PN->getOperandNumForIncomingValue(
126                  PN->getNumIncomingValues() - 1)));
127     }
128 
129     AddedPHIs.push_back(PN);
130 
131     // Remember that this phi makes the value alive in this block.
132     SSAUpdate.AddAvailableValue(ExitBB, PN);
133   }
134 
135   // Rewrite all uses outside the loop in terms of the new PHIs we just
136   // inserted.
137   for (unsigned i = 0, e = UsesToRewrite.size(); i != e; ++i) {
138     // If this use is in an exit block, rewrite to use the newly inserted PHI.
139     // This is required for correctness because SSAUpdate doesn't handle uses in
140     // the same block.  It assumes the PHI we inserted is at the end of the
141     // block.
142     Instruction *User = cast<Instruction>(UsesToRewrite[i]->getUser());
143     BasicBlock *UserBB = User->getParent();
144     if (PHINode *PN = dyn_cast<PHINode>(User))
145       UserBB = PN->getIncomingBlock(*UsesToRewrite[i]);
146 
147     if (isa<PHINode>(UserBB->begin()) && isExitBlock(UserBB, ExitBlocks)) {
148       // Tell the VHs that the uses changed. This updates SCEV's caches.
149       if (UsesToRewrite[i]->get()->hasValueHandle())
150         ValueHandleBase::ValueIsRAUWd(*UsesToRewrite[i], UserBB->begin());
151       UsesToRewrite[i]->set(UserBB->begin());
152       continue;
153     }
154 
155     // Otherwise, do full PHI insertion.
156     SSAUpdate.RewriteUse(*UsesToRewrite[i]);
157   }
158 
159   // Remove PHI nodes that did not have any uses rewritten.
160   for (unsigned i = 0, e = AddedPHIs.size(); i != e; ++i) {
161     if (AddedPHIs[i]->use_empty())
162       AddedPHIs[i]->eraseFromParent();
163   }
164 
165   return true;
166 }
167 
168 /// Return true if the specified block dominates at least
169 /// one of the blocks in the specified list.
170 static bool
171 blockDominatesAnExit(BasicBlock *BB,
172                      DominatorTree &DT,
173                      const SmallVectorImpl<BasicBlock *> &ExitBlocks) {
174   DomTreeNode *DomNode = DT.getNode(BB);
175   for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
176     if (DT.dominates(DomNode, DT.getNode(ExitBlocks[i])))
177       return true;
178 
179   return false;
180 }
181 
182 bool llvm::formLCSSA(Loop &L, DominatorTree &DT, ScalarEvolution *SE) {
183   bool Changed = false;
184 
185   // Get the set of exiting blocks.
186   SmallVector<BasicBlock *, 8> ExitBlocks;
187   L.getExitBlocks(ExitBlocks);
188 
189   if (ExitBlocks.empty())
190     return false;
191 
192   PredIteratorCache PredCache;
193 
194   // Look at all the instructions in the loop, checking to see if they have uses
195   // outside the loop.  If so, rewrite those uses.
196   for (Loop::block_iterator BBI = L.block_begin(), BBE = L.block_end();
197        BBI != BBE; ++BBI) {
198     BasicBlock *BB = *BBI;
199 
200     // For large loops, avoid use-scanning by using dominance information:  In
201     // particular, if a block does not dominate any of the loop exits, then none
202     // of the values defined in the block could be used outside the loop.
203     if (!blockDominatesAnExit(BB, DT, ExitBlocks))
204       continue;
205 
206     for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
207       // Reject two common cases fast: instructions with no uses (like stores)
208       // and instructions with one use that is in the same block as this.
209       if (I->use_empty() ||
210           (I->hasOneUse() && I->user_back()->getParent() == BB &&
211            !isa<PHINode>(I->user_back())))
212         continue;
213 
214       Changed |= processInstruction(L, *I, DT, ExitBlocks, PredCache);
215     }
216   }
217 
218   // If we modified the code, remove any caches about the loop from SCEV to
219   // avoid dangling entries.
220   // FIXME: This is a big hammer, can we clear the cache more selectively?
221   if (SE && Changed)
222     SE->forgetLoop(&L);
223 
224   assert(L.isLCSSAForm(DT));
225 
226   return Changed;
227 }
228 
229 /// Process a loop nest depth first.
230 bool llvm::formLCSSARecursively(Loop &L, DominatorTree &DT,
231                                 ScalarEvolution *SE) {
232   bool Changed = false;
233 
234   // Recurse depth-first through inner loops.
235   for (Loop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
236     Changed |= formLCSSARecursively(**LI, DT, SE);
237 
238   Changed |= formLCSSA(L, DT, SE);
239   return Changed;
240 }
241 
242 namespace {
243 struct LCSSA : public FunctionPass {
244   static char ID; // Pass identification, replacement for typeid
245   LCSSA() : FunctionPass(ID) {
246     initializeLCSSAPass(*PassRegistry::getPassRegistry());
247   }
248 
249   // Cached analysis information for the current function.
250   DominatorTree *DT;
251   LoopInfo *LI;
252   ScalarEvolution *SE;
253 
254   bool runOnFunction(Function &F) override;
255 
256   /// This transformation requires natural loop information & requires that
257   /// loop preheaders be inserted into the CFG.  It maintains both of these,
258   /// as well as the CFG.  It also requires dominator information.
259   void getAnalysisUsage(AnalysisUsage &AU) const override {
260     AU.setPreservesCFG();
261 
262     AU.addRequired<DominatorTreeWrapperPass>();
263     AU.addRequired<LoopInfo>();
264     AU.addPreservedID(LoopSimplifyID);
265     AU.addPreserved<AliasAnalysis>();
266     AU.addPreserved<ScalarEvolution>();
267   }
268 
269 private:
270   bool processLoop(Loop &L);
271 
272   void verifyAnalysis() const override;
273 };
274 }
275 
276 char LCSSA::ID = 0;
277 INITIALIZE_PASS_BEGIN(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false)
278 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
279 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
280 INITIALIZE_PASS_END(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false)
281 
282 Pass *llvm::createLCSSAPass() { return new LCSSA(); }
283 char &llvm::LCSSAID = LCSSA::ID;
284 
285 
286 /// Process all loops in the function, inner-most out.
287 bool LCSSA::runOnFunction(Function &F) {
288   bool Changed = false;
289   LI = &getAnalysis<LoopInfo>();
290   DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
291   SE = getAnalysisIfAvailable<ScalarEvolution>();
292 
293   // Simplify each loop nest in the function.
294   for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
295     Changed |= formLCSSARecursively(**I, *DT, SE);
296 
297   return Changed;
298 }
299 
300 static void verifyLoop(Loop &L, DominatorTree &DT) {
301   // Recurse depth-first through inner loops.
302   for (Loop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
303     verifyLoop(**LI, DT);
304 
305   // Check the special guarantees that LCSSA makes.
306   //assert(L.isLCSSAForm(DT) && "LCSSA form not preserved!");
307 }
308 
309 void LCSSA::verifyAnalysis() const {
310   // Verify each loop nest in the function, assuming LI still points at that
311   // function's loop info.
312   for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
313     verifyLoop(**I, *DT);
314 }
315