1 //===-- Sink.cpp - Code Sinking -------------------------------------------===//
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 moves instructions into successor blocks, when possible, so that
11 // they aren't executed on paths where their results aren't needed.
12 //
13 //===----------------------------------------------------------------------===//
14
15 #include "llvm/Transforms/Scalar.h"
16 #include "llvm/ADT/Statistic.h"
17 #include "llvm/Analysis/AliasAnalysis.h"
18 #include "llvm/Analysis/LoopInfo.h"
19 #include "llvm/Analysis/ValueTracking.h"
20 #include "llvm/IR/CFG.h"
21 #include "llvm/IR/DataLayout.h"
22 #include "llvm/IR/Dominators.h"
23 #include "llvm/IR/IntrinsicInst.h"
24 #include "llvm/Support/Debug.h"
25 #include "llvm/Support/raw_ostream.h"
26 using namespace llvm;
27
28 #define DEBUG_TYPE "sink"
29
30 STATISTIC(NumSunk, "Number of instructions sunk");
31 STATISTIC(NumSinkIter, "Number of sinking iterations");
32
33 namespace {
34 class Sinking : public FunctionPass {
35 DominatorTree *DT;
36 LoopInfo *LI;
37 AliasAnalysis *AA;
38 const DataLayout *DL;
39
40 public:
41 static char ID; // Pass identification
Sinking()42 Sinking() : FunctionPass(ID) {
43 initializeSinkingPass(*PassRegistry::getPassRegistry());
44 }
45
46 bool runOnFunction(Function &F) override;
47
getAnalysisUsage(AnalysisUsage & AU) const48 void getAnalysisUsage(AnalysisUsage &AU) const override {
49 AU.setPreservesCFG();
50 FunctionPass::getAnalysisUsage(AU);
51 AU.addRequired<AliasAnalysis>();
52 AU.addRequired<DominatorTreeWrapperPass>();
53 AU.addRequired<LoopInfo>();
54 AU.addPreserved<DominatorTreeWrapperPass>();
55 AU.addPreserved<LoopInfo>();
56 }
57 private:
58 bool ProcessBlock(BasicBlock &BB);
59 bool SinkInstruction(Instruction *I, SmallPtrSetImpl<Instruction*> &Stores);
60 bool AllUsesDominatedByBlock(Instruction *Inst, BasicBlock *BB) const;
61 bool IsAcceptableTarget(Instruction *Inst, BasicBlock *SuccToSinkTo) const;
62 };
63 } // end anonymous namespace
64
65 char Sinking::ID = 0;
66 INITIALIZE_PASS_BEGIN(Sinking, "sink", "Code sinking", false, false)
INITIALIZE_PASS_DEPENDENCY(LoopInfo)67 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
68 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
69 INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
70 INITIALIZE_PASS_END(Sinking, "sink", "Code sinking", false, false)
71
72 FunctionPass *llvm::createSinkingPass() { return new Sinking(); }
73
74 /// AllUsesDominatedByBlock - Return true if all uses of the specified value
75 /// occur in blocks dominated by the specified block.
AllUsesDominatedByBlock(Instruction * Inst,BasicBlock * BB) const76 bool Sinking::AllUsesDominatedByBlock(Instruction *Inst,
77 BasicBlock *BB) const {
78 // Ignoring debug uses is necessary so debug info doesn't affect the code.
79 // This may leave a referencing dbg_value in the original block, before
80 // the definition of the vreg. Dwarf generator handles this although the
81 // user might not get the right info at runtime.
82 for (Use &U : Inst->uses()) {
83 // Determine the block of the use.
84 Instruction *UseInst = cast<Instruction>(U.getUser());
85 BasicBlock *UseBlock = UseInst->getParent();
86 if (PHINode *PN = dyn_cast<PHINode>(UseInst)) {
87 // PHI nodes use the operand in the predecessor block, not the block with
88 // the PHI.
89 unsigned Num = PHINode::getIncomingValueNumForOperand(U.getOperandNo());
90 UseBlock = PN->getIncomingBlock(Num);
91 }
92 // Check that it dominates.
93 if (!DT->dominates(BB, UseBlock))
94 return false;
95 }
96 return true;
97 }
98
runOnFunction(Function & F)99 bool Sinking::runOnFunction(Function &F) {
100 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
101 LI = &getAnalysis<LoopInfo>();
102 AA = &getAnalysis<AliasAnalysis>();
103 DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
104 DL = DLP ? &DLP->getDataLayout() : nullptr;
105
106 bool MadeChange, EverMadeChange = false;
107
108 do {
109 MadeChange = false;
110 DEBUG(dbgs() << "Sinking iteration " << NumSinkIter << "\n");
111 // Process all basic blocks.
112 for (Function::iterator I = F.begin(), E = F.end();
113 I != E; ++I)
114 MadeChange |= ProcessBlock(*I);
115 EverMadeChange |= MadeChange;
116 NumSinkIter++;
117 } while (MadeChange);
118
119 return EverMadeChange;
120 }
121
ProcessBlock(BasicBlock & BB)122 bool Sinking::ProcessBlock(BasicBlock &BB) {
123 // Can't sink anything out of a block that has less than two successors.
124 if (BB.getTerminator()->getNumSuccessors() <= 1 || BB.empty()) return false;
125
126 // Don't bother sinking code out of unreachable blocks. In addition to being
127 // unprofitable, it can also lead to infinite looping, because in an
128 // unreachable loop there may be nowhere to stop.
129 if (!DT->isReachableFromEntry(&BB)) return false;
130
131 bool MadeChange = false;
132
133 // Walk the basic block bottom-up. Remember if we saw a store.
134 BasicBlock::iterator I = BB.end();
135 --I;
136 bool ProcessedBegin = false;
137 SmallPtrSet<Instruction *, 8> Stores;
138 do {
139 Instruction *Inst = I; // The instruction to sink.
140
141 // Predecrement I (if it's not begin) so that it isn't invalidated by
142 // sinking.
143 ProcessedBegin = I == BB.begin();
144 if (!ProcessedBegin)
145 --I;
146
147 if (isa<DbgInfoIntrinsic>(Inst))
148 continue;
149
150 if (SinkInstruction(Inst, Stores))
151 ++NumSunk, MadeChange = true;
152
153 // If we just processed the first instruction in the block, we're done.
154 } while (!ProcessedBegin);
155
156 return MadeChange;
157 }
158
isSafeToMove(Instruction * Inst,AliasAnalysis * AA,SmallPtrSetImpl<Instruction * > & Stores)159 static bool isSafeToMove(Instruction *Inst, AliasAnalysis *AA,
160 SmallPtrSetImpl<Instruction *> &Stores) {
161
162 if (Inst->mayWriteToMemory()) {
163 Stores.insert(Inst);
164 return false;
165 }
166
167 if (LoadInst *L = dyn_cast<LoadInst>(Inst)) {
168 AliasAnalysis::Location Loc = AA->getLocation(L);
169 for (Instruction *S : Stores)
170 if (AA->getModRefInfo(S, Loc) & AliasAnalysis::Mod)
171 return false;
172 }
173
174 if (isa<TerminatorInst>(Inst) || isa<PHINode>(Inst))
175 return false;
176
177 return true;
178 }
179
180 /// IsAcceptableTarget - Return true if it is possible to sink the instruction
181 /// in the specified basic block.
IsAcceptableTarget(Instruction * Inst,BasicBlock * SuccToSinkTo) const182 bool Sinking::IsAcceptableTarget(Instruction *Inst,
183 BasicBlock *SuccToSinkTo) const {
184 assert(Inst && "Instruction to be sunk is null");
185 assert(SuccToSinkTo && "Candidate sink target is null");
186
187 // It is not possible to sink an instruction into its own block. This can
188 // happen with loops.
189 if (Inst->getParent() == SuccToSinkTo)
190 return false;
191
192 // If the block has multiple predecessors, this would introduce computation
193 // on different code paths. We could split the critical edge, but for now we
194 // just punt.
195 // FIXME: Split critical edges if not backedges.
196 if (SuccToSinkTo->getUniquePredecessor() != Inst->getParent()) {
197 // We cannot sink a load across a critical edge - there may be stores in
198 // other code paths.
199 if (!isSafeToSpeculativelyExecute(Inst, DL))
200 return false;
201
202 // We don't want to sink across a critical edge if we don't dominate the
203 // successor. We could be introducing calculations to new code paths.
204 if (!DT->dominates(Inst->getParent(), SuccToSinkTo))
205 return false;
206
207 // Don't sink instructions into a loop.
208 Loop *succ = LI->getLoopFor(SuccToSinkTo);
209 Loop *cur = LI->getLoopFor(Inst->getParent());
210 if (succ != nullptr && succ != cur)
211 return false;
212 }
213
214 // Finally, check that all the uses of the instruction are actually
215 // dominated by the candidate
216 return AllUsesDominatedByBlock(Inst, SuccToSinkTo);
217 }
218
219 /// SinkInstruction - Determine whether it is safe to sink the specified machine
220 /// instruction out of its current block into a successor.
SinkInstruction(Instruction * Inst,SmallPtrSetImpl<Instruction * > & Stores)221 bool Sinking::SinkInstruction(Instruction *Inst,
222 SmallPtrSetImpl<Instruction *> &Stores) {
223
224 // Don't sink static alloca instructions. CodeGen assumes allocas outside the
225 // entry block are dynamically sized stack objects.
226 if (AllocaInst *AI = dyn_cast<AllocaInst>(Inst))
227 if (AI->isStaticAlloca())
228 return false;
229
230 // Check if it's safe to move the instruction.
231 if (!isSafeToMove(Inst, AA, Stores))
232 return false;
233
234 // FIXME: This should include support for sinking instructions within the
235 // block they are currently in to shorten the live ranges. We often get
236 // instructions sunk into the top of a large block, but it would be better to
237 // also sink them down before their first use in the block. This xform has to
238 // be careful not to *increase* register pressure though, e.g. sinking
239 // "x = y + z" down if it kills y and z would increase the live ranges of y
240 // and z and only shrink the live range of x.
241
242 // SuccToSinkTo - This is the successor to sink this instruction to, once we
243 // decide.
244 BasicBlock *SuccToSinkTo = nullptr;
245
246 // Instructions can only be sunk if all their uses are in blocks
247 // dominated by one of the successors.
248 // Look at all the postdominators and see if we can sink it in one.
249 DomTreeNode *DTN = DT->getNode(Inst->getParent());
250 for (DomTreeNode::iterator I = DTN->begin(), E = DTN->end();
251 I != E && SuccToSinkTo == nullptr; ++I) {
252 BasicBlock *Candidate = (*I)->getBlock();
253 if ((*I)->getIDom()->getBlock() == Inst->getParent() &&
254 IsAcceptableTarget(Inst, Candidate))
255 SuccToSinkTo = Candidate;
256 }
257
258 // If no suitable postdominator was found, look at all the successors and
259 // decide which one we should sink to, if any.
260 for (succ_iterator I = succ_begin(Inst->getParent()),
261 E = succ_end(Inst->getParent()); I != E && !SuccToSinkTo; ++I) {
262 if (IsAcceptableTarget(Inst, *I))
263 SuccToSinkTo = *I;
264 }
265
266 // If we couldn't find a block to sink to, ignore this instruction.
267 if (!SuccToSinkTo)
268 return false;
269
270 DEBUG(dbgs() << "Sink" << *Inst << " (";
271 Inst->getParent()->printAsOperand(dbgs(), false);
272 dbgs() << " -> ";
273 SuccToSinkTo->printAsOperand(dbgs(), false);
274 dbgs() << ")\n");
275
276 // Move the instruction.
277 Inst->moveBefore(SuccToSinkTo->getFirstInsertionPt());
278 return true;
279 }
280