| //===-- Sink.cpp - Code Sinking -------------------------------------------===// |
| // |
| // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. |
| // See https://llvm.org/LICENSE.txt for license information. |
| // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This pass moves instructions into successor blocks, when possible, so that |
| // they aren't executed on paths where their results aren't needed. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "llvm/Transforms/Scalar/Sink.h" |
| #include "llvm/ADT/Statistic.h" |
| #include "llvm/Analysis/AliasAnalysis.h" |
| #include "llvm/Analysis/LoopInfo.h" |
| #include "llvm/Analysis/ValueTracking.h" |
| #include "llvm/IR/CFG.h" |
| #include "llvm/IR/DataLayout.h" |
| #include "llvm/IR/Dominators.h" |
| #include "llvm/IR/IntrinsicInst.h" |
| #include "llvm/IR/Module.h" |
| #include "llvm/InitializePasses.h" |
| #include "llvm/Support/Debug.h" |
| #include "llvm/Support/raw_ostream.h" |
| #include "llvm/Transforms/Scalar.h" |
| using namespace llvm; |
| |
| #define DEBUG_TYPE "sink" |
| |
| STATISTIC(NumSunk, "Number of instructions sunk"); |
| STATISTIC(NumSinkIter, "Number of sinking iterations"); |
| |
| static bool isSafeToMove(Instruction *Inst, AliasAnalysis &AA, |
| SmallPtrSetImpl<Instruction *> &Stores) { |
| |
| if (Inst->mayWriteToMemory()) { |
| Stores.insert(Inst); |
| return false; |
| } |
| |
| if (LoadInst *L = dyn_cast<LoadInst>(Inst)) { |
| MemoryLocation Loc = MemoryLocation::get(L); |
| for (Instruction *S : Stores) |
| if (isModSet(AA.getModRefInfo(S, Loc))) |
| return false; |
| } |
| |
| if (Inst->isTerminator() || isa<PHINode>(Inst) || Inst->isEHPad() || |
| Inst->mayThrow()) |
| return false; |
| |
| if (auto *Call = dyn_cast<CallBase>(Inst)) { |
| // Convergent operations cannot be made control-dependent on additional |
| // values. |
| if (Call->isConvergent()) |
| return false; |
| |
| for (Instruction *S : Stores) |
| if (isModSet(AA.getModRefInfo(S, Call))) |
| return false; |
| } |
| |
| return true; |
| } |
| |
| /// IsAcceptableTarget - Return true if it is possible to sink the instruction |
| /// in the specified basic block. |
| static bool IsAcceptableTarget(Instruction *Inst, BasicBlock *SuccToSinkTo, |
| DominatorTree &DT, LoopInfo &LI) { |
| assert(Inst && "Instruction to be sunk is null"); |
| assert(SuccToSinkTo && "Candidate sink target is null"); |
| |
| // It's never legal to sink an instruction into a block which terminates in an |
| // EH-pad. |
| if (SuccToSinkTo->getTerminator()->isExceptionalTerminator()) |
| return false; |
| |
| // If the block has multiple predecessors, this would introduce computation |
| // on different code paths. We could split the critical edge, but for now we |
| // just punt. |
| // FIXME: Split critical edges if not backedges. |
| if (SuccToSinkTo->getUniquePredecessor() != Inst->getParent()) { |
| // We cannot sink a load across a critical edge - there may be stores in |
| // other code paths. |
| if (Inst->mayReadFromMemory()) |
| return false; |
| |
| // We don't want to sink across a critical edge if we don't dominate the |
| // successor. We could be introducing calculations to new code paths. |
| if (!DT.dominates(Inst->getParent(), SuccToSinkTo)) |
| return false; |
| |
| // Don't sink instructions into a loop. |
| Loop *succ = LI.getLoopFor(SuccToSinkTo); |
| Loop *cur = LI.getLoopFor(Inst->getParent()); |
| if (succ != nullptr && succ != cur) |
| return false; |
| } |
| |
| return true; |
| } |
| |
| /// SinkInstruction - Determine whether it is safe to sink the specified machine |
| /// instruction out of its current block into a successor. |
| static bool SinkInstruction(Instruction *Inst, |
| SmallPtrSetImpl<Instruction *> &Stores, |
| DominatorTree &DT, LoopInfo &LI, AAResults &AA) { |
| |
| // Don't sink static alloca instructions. CodeGen assumes allocas outside the |
| // entry block are dynamically sized stack objects. |
| if (AllocaInst *AI = dyn_cast<AllocaInst>(Inst)) |
| if (AI->isStaticAlloca()) |
| return false; |
| |
| // Check if it's safe to move the instruction. |
| if (!isSafeToMove(Inst, AA, Stores)) |
| return false; |
| |
| // FIXME: This should include support for sinking instructions within the |
| // block they are currently in to shorten the live ranges. We often get |
| // instructions sunk into the top of a large block, but it would be better to |
| // also sink them down before their first use in the block. This xform has to |
| // be careful not to *increase* register pressure though, e.g. sinking |
| // "x = y + z" down if it kills y and z would increase the live ranges of y |
| // and z and only shrink the live range of x. |
| |
| // SuccToSinkTo - This is the successor to sink this instruction to, once we |
| // decide. |
| BasicBlock *SuccToSinkTo = nullptr; |
| |
| // Find the nearest common dominator of all users as the candidate. |
| BasicBlock *BB = Inst->getParent(); |
| for (Use &U : Inst->uses()) { |
| Instruction *UseInst = cast<Instruction>(U.getUser()); |
| BasicBlock *UseBlock = UseInst->getParent(); |
| // Don't worry about dead users. |
| if (!DT.isReachableFromEntry(UseBlock)) |
| continue; |
| if (PHINode *PN = dyn_cast<PHINode>(UseInst)) { |
| // PHI nodes use the operand in the predecessor block, not the block with |
| // the PHI. |
| unsigned Num = PHINode::getIncomingValueNumForOperand(U.getOperandNo()); |
| UseBlock = PN->getIncomingBlock(Num); |
| } |
| if (SuccToSinkTo) |
| SuccToSinkTo = DT.findNearestCommonDominator(SuccToSinkTo, UseBlock); |
| else |
| SuccToSinkTo = UseBlock; |
| // The current basic block needs to dominate the candidate. |
| if (!DT.dominates(BB, SuccToSinkTo)) |
| return false; |
| } |
| |
| if (SuccToSinkTo) { |
| // The nearest common dominator may be in a parent loop of BB, which may not |
| // be beneficial. Find an ancestor. |
| while (SuccToSinkTo != BB && |
| !IsAcceptableTarget(Inst, SuccToSinkTo, DT, LI)) |
| SuccToSinkTo = DT.getNode(SuccToSinkTo)->getIDom()->getBlock(); |
| if (SuccToSinkTo == BB) |
| SuccToSinkTo = nullptr; |
| } |
| |
| // If we couldn't find a block to sink to, ignore this instruction. |
| if (!SuccToSinkTo) |
| return false; |
| |
| LLVM_DEBUG(dbgs() << "Sink" << *Inst << " ("; |
| Inst->getParent()->printAsOperand(dbgs(), false); dbgs() << " -> "; |
| SuccToSinkTo->printAsOperand(dbgs(), false); dbgs() << ")\n"); |
| |
| // Move the instruction. |
| Inst->moveBefore(&*SuccToSinkTo->getFirstInsertionPt()); |
| return true; |
| } |
| |
| static bool ProcessBlock(BasicBlock &BB, DominatorTree &DT, LoopInfo &LI, |
| AAResults &AA) { |
| // Can't sink anything out of a block that has less than two successors. |
| if (BB.getTerminator()->getNumSuccessors() <= 1) return false; |
| |
| // Don't bother sinking code out of unreachable blocks. In addition to being |
| // unprofitable, it can also lead to infinite looping, because in an |
| // unreachable loop there may be nowhere to stop. |
| if (!DT.isReachableFromEntry(&BB)) return false; |
| |
| bool MadeChange = false; |
| |
| // Walk the basic block bottom-up. Remember if we saw a store. |
| BasicBlock::iterator I = BB.end(); |
| --I; |
| bool ProcessedBegin = false; |
| SmallPtrSet<Instruction *, 8> Stores; |
| do { |
| Instruction *Inst = &*I; // The instruction to sink. |
| |
| // Predecrement I (if it's not begin) so that it isn't invalidated by |
| // sinking. |
| ProcessedBegin = I == BB.begin(); |
| if (!ProcessedBegin) |
| --I; |
| |
| if (Inst->isDebugOrPseudoInst()) |
| continue; |
| |
| if (SinkInstruction(Inst, Stores, DT, LI, AA)) { |
| ++NumSunk; |
| MadeChange = true; |
| } |
| |
| // If we just processed the first instruction in the block, we're done. |
| } while (!ProcessedBegin); |
| |
| return MadeChange; |
| } |
| |
| static bool iterativelySinkInstructions(Function &F, DominatorTree &DT, |
| LoopInfo &LI, AAResults &AA) { |
| bool MadeChange, EverMadeChange = false; |
| |
| do { |
| MadeChange = false; |
| LLVM_DEBUG(dbgs() << "Sinking iteration " << NumSinkIter << "\n"); |
| // Process all basic blocks. |
| for (BasicBlock &I : F) |
| MadeChange |= ProcessBlock(I, DT, LI, AA); |
| EverMadeChange |= MadeChange; |
| NumSinkIter++; |
| } while (MadeChange); |
| |
| return EverMadeChange; |
| } |
| |
| PreservedAnalyses SinkingPass::run(Function &F, FunctionAnalysisManager &AM) { |
| auto &DT = AM.getResult<DominatorTreeAnalysis>(F); |
| auto &LI = AM.getResult<LoopAnalysis>(F); |
| auto &AA = AM.getResult<AAManager>(F); |
| |
| if (!iterativelySinkInstructions(F, DT, LI, AA)) |
| return PreservedAnalyses::all(); |
| |
| PreservedAnalyses PA; |
| PA.preserveSet<CFGAnalyses>(); |
| return PA; |
| } |
| |
| namespace { |
| class SinkingLegacyPass : public FunctionPass { |
| public: |
| static char ID; // Pass identification |
| SinkingLegacyPass() : FunctionPass(ID) { |
| initializeSinkingLegacyPassPass(*PassRegistry::getPassRegistry()); |
| } |
| |
| bool runOnFunction(Function &F) override { |
| auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree(); |
| auto &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo(); |
| auto &AA = getAnalysis<AAResultsWrapperPass>().getAAResults(); |
| |
| return iterativelySinkInstructions(F, DT, LI, AA); |
| } |
| |
| void getAnalysisUsage(AnalysisUsage &AU) const override { |
| AU.setPreservesCFG(); |
| FunctionPass::getAnalysisUsage(AU); |
| AU.addRequired<AAResultsWrapperPass>(); |
| AU.addRequired<DominatorTreeWrapperPass>(); |
| AU.addRequired<LoopInfoWrapperPass>(); |
| AU.addPreserved<DominatorTreeWrapperPass>(); |
| AU.addPreserved<LoopInfoWrapperPass>(); |
| } |
| }; |
| } // end anonymous namespace |
| |
| char SinkingLegacyPass::ID = 0; |
| INITIALIZE_PASS_BEGIN(SinkingLegacyPass, "sink", "Code sinking", false, false) |
| INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass) |
| INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) |
| INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass) |
| INITIALIZE_PASS_END(SinkingLegacyPass, "sink", "Code sinking", false, false) |
| |
| FunctionPass *llvm::createSinkingPass() { return new SinkingLegacyPass(); } |