1 //===-- Sink.cpp - Code Sinking -------------------------------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
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.
13 //===----------------------------------------------------------------------===//
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/IR/Module.h"
25 #include "llvm/Support/Debug.h"
26 #include "llvm/Support/raw_ostream.h"
29 #define DEBUG_TYPE "sink"
31 STATISTIC(NumSunk, "Number of instructions sunk");
32 STATISTIC(NumSinkIter, "Number of sinking iterations");
35 class Sinking : public FunctionPass {
41 static char ID; // Pass identification
42 Sinking() : FunctionPass(ID) {
43 initializeSinkingPass(*PassRegistry::getPassRegistry());
46 bool runOnFunction(Function &F) override;
48 void getAnalysisUsage(AnalysisUsage &AU) const override {
50 FunctionPass::getAnalysisUsage(AU);
51 AU.addRequired<AAResultsWrapperPass>();
52 AU.addRequired<DominatorTreeWrapperPass>();
53 AU.addRequired<LoopInfoWrapperPass>();
54 AU.addPreserved<DominatorTreeWrapperPass>();
55 AU.addPreserved<LoopInfoWrapperPass>();
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;
63 } // end anonymous namespace
66 INITIALIZE_PASS_BEGIN(Sinking, "sink", "Code sinking", false, false)
67 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
68 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
69 INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
70 INITIALIZE_PASS_END(Sinking, "sink", "Code sinking", false, false)
72 FunctionPass *llvm::createSinkingPass() { return new Sinking(); }
74 /// AllUsesDominatedByBlock - Return true if all uses of the specified value
75 /// occur in blocks dominated by the specified block.
76 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
89 unsigned Num = PHINode::getIncomingValueNumForOperand(U.getOperandNo());
90 UseBlock = PN->getIncomingBlock(Num);
92 // Check that it dominates.
93 if (!DT->dominates(BB, UseBlock))
99 bool Sinking::runOnFunction(Function &F) {
100 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
101 LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
102 AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
104 bool MadeChange, EverMadeChange = false;
108 DEBUG(dbgs() << "Sinking iteration " << NumSinkIter << "\n");
109 // Process all basic blocks.
110 for (Function::iterator I = F.begin(), E = F.end();
112 MadeChange |= ProcessBlock(*I);
113 EverMadeChange |= MadeChange;
115 } while (MadeChange);
117 return EverMadeChange;
120 bool Sinking::ProcessBlock(BasicBlock &BB) {
121 // Can't sink anything out of a block that has less than two successors.
122 if (BB.getTerminator()->getNumSuccessors() <= 1 || BB.empty()) return false;
124 // Don't bother sinking code out of unreachable blocks. In addition to being
125 // unprofitable, it can also lead to infinite looping, because in an
126 // unreachable loop there may be nowhere to stop.
127 if (!DT->isReachableFromEntry(&BB)) return false;
129 bool MadeChange = false;
131 // Walk the basic block bottom-up. Remember if we saw a store.
132 BasicBlock::iterator I = BB.end();
134 bool ProcessedBegin = false;
135 SmallPtrSet<Instruction *, 8> Stores;
137 Instruction *Inst = &*I; // The instruction to sink.
139 // Predecrement I (if it's not begin) so that it isn't invalidated by
141 ProcessedBegin = I == BB.begin();
145 if (isa<DbgInfoIntrinsic>(Inst))
148 if (SinkInstruction(Inst, Stores))
149 ++NumSunk, MadeChange = true;
151 // If we just processed the first instruction in the block, we're done.
152 } while (!ProcessedBegin);
157 static bool isSafeToMove(Instruction *Inst, AliasAnalysis *AA,
158 SmallPtrSetImpl<Instruction *> &Stores) {
160 if (Inst->mayWriteToMemory()) {
165 if (LoadInst *L = dyn_cast<LoadInst>(Inst)) {
166 MemoryLocation Loc = MemoryLocation::get(L);
167 for (Instruction *S : Stores)
168 if (AA->getModRefInfo(S, Loc) & MRI_Mod)
172 if (isa<TerminatorInst>(Inst) || isa<PHINode>(Inst))
175 // Convergent operations cannot be made control-dependent on additional
177 if (auto CS = CallSite(Inst)) {
178 if (CS.hasFnAttr(Attribute::Convergent))
185 /// IsAcceptableTarget - Return true if it is possible to sink the instruction
186 /// in the specified basic block.
187 bool Sinking::IsAcceptableTarget(Instruction *Inst,
188 BasicBlock *SuccToSinkTo) const {
189 assert(Inst && "Instruction to be sunk is null");
190 assert(SuccToSinkTo && "Candidate sink target is null");
192 // It is not possible to sink an instruction into its own block. This can
193 // happen with loops.
194 if (Inst->getParent() == SuccToSinkTo)
197 // If the block has multiple predecessors, this would introduce computation
198 // on different code paths. We could split the critical edge, but for now we
200 // FIXME: Split critical edges if not backedges.
201 if (SuccToSinkTo->getUniquePredecessor() != Inst->getParent()) {
202 // We cannot sink a load across a critical edge - there may be stores in
204 if (!isSafeToSpeculativelyExecute(Inst))
207 // We don't want to sink across a critical edge if we don't dominate the
208 // successor. We could be introducing calculations to new code paths.
209 if (!DT->dominates(Inst->getParent(), SuccToSinkTo))
212 // Don't sink instructions into a loop.
213 Loop *succ = LI->getLoopFor(SuccToSinkTo);
214 Loop *cur = LI->getLoopFor(Inst->getParent());
215 if (succ != nullptr && succ != cur)
219 // Finally, check that all the uses of the instruction are actually
220 // dominated by the candidate
221 return AllUsesDominatedByBlock(Inst, SuccToSinkTo);
224 /// SinkInstruction - Determine whether it is safe to sink the specified machine
225 /// instruction out of its current block into a successor.
226 bool Sinking::SinkInstruction(Instruction *Inst,
227 SmallPtrSetImpl<Instruction *> &Stores) {
229 // Don't sink static alloca instructions. CodeGen assumes allocas outside the
230 // entry block are dynamically sized stack objects.
231 if (AllocaInst *AI = dyn_cast<AllocaInst>(Inst))
232 if (AI->isStaticAlloca())
235 // Check if it's safe to move the instruction.
236 if (!isSafeToMove(Inst, AA, Stores))
239 // FIXME: This should include support for sinking instructions within the
240 // block they are currently in to shorten the live ranges. We often get
241 // instructions sunk into the top of a large block, but it would be better to
242 // also sink them down before their first use in the block. This xform has to
243 // be careful not to *increase* register pressure though, e.g. sinking
244 // "x = y + z" down if it kills y and z would increase the live ranges of y
245 // and z and only shrink the live range of x.
247 // SuccToSinkTo - This is the successor to sink this instruction to, once we
249 BasicBlock *SuccToSinkTo = nullptr;
251 // Instructions can only be sunk if all their uses are in blocks
252 // dominated by one of the successors.
253 // Look at all the postdominators and see if we can sink it in one.
254 DomTreeNode *DTN = DT->getNode(Inst->getParent());
255 for (DomTreeNode::iterator I = DTN->begin(), E = DTN->end();
256 I != E && SuccToSinkTo == nullptr; ++I) {
257 BasicBlock *Candidate = (*I)->getBlock();
258 if ((*I)->getIDom()->getBlock() == Inst->getParent() &&
259 IsAcceptableTarget(Inst, Candidate))
260 SuccToSinkTo = Candidate;
263 // If no suitable postdominator was found, look at all the successors and
264 // decide which one we should sink to, if any.
265 for (succ_iterator I = succ_begin(Inst->getParent()),
266 E = succ_end(Inst->getParent()); I != E && !SuccToSinkTo; ++I) {
267 if (IsAcceptableTarget(Inst, *I))
271 // If we couldn't find a block to sink to, ignore this instruction.
275 DEBUG(dbgs() << "Sink" << *Inst << " (";
276 Inst->getParent()->printAsOperand(dbgs(), false);
278 SuccToSinkTo->printAsOperand(dbgs(), false);
281 // Move the instruction.
282 Inst->moveBefore(&*SuccToSinkTo->getFirstInsertionPt());