//===-- Sink.cpp - Code Sinking -------------------------------------------===// // // The LLVM Compiler Infrastructure // // This file is distributed under the University of Illinois Open Source // License. See LICENSE.TXT for details. // //===----------------------------------------------------------------------===// // // 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.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/Support/Debug.h" #include "llvm/Support/raw_ostream.h" using namespace llvm; #define DEBUG_TYPE "sink" STATISTIC(NumSunk, "Number of instructions sunk"); STATISTIC(NumSinkIter, "Number of sinking iterations"); namespace { class Sinking : public FunctionPass { DominatorTree *DT; LoopInfo *LI; AliasAnalysis *AA; public: static char ID; // Pass identification Sinking() : FunctionPass(ID) { initializeSinkingPass(*PassRegistry::getPassRegistry()); } bool runOnFunction(Function &F) override; 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>(); } private: bool ProcessBlock(BasicBlock &BB); bool SinkInstruction(Instruction *I, SmallPtrSetImpl<Instruction*> &Stores); bool AllUsesDominatedByBlock(Instruction *Inst, BasicBlock *BB) const; bool IsAcceptableTarget(Instruction *Inst, BasicBlock *SuccToSinkTo) const; }; } // end anonymous namespace char Sinking::ID = 0; INITIALIZE_PASS_BEGIN(Sinking, "sink", "Code sinking", false, false) INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass) INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass) INITIALIZE_PASS_END(Sinking, "sink", "Code sinking", false, false) FunctionPass *llvm::createSinkingPass() { return new Sinking(); } /// AllUsesDominatedByBlock - Return true if all uses of the specified value /// occur in blocks dominated by the specified block. bool Sinking::AllUsesDominatedByBlock(Instruction *Inst, BasicBlock *BB) const { // Ignoring debug uses is necessary so debug info doesn't affect the code. // This may leave a referencing dbg_value in the original block, before // the definition of the vreg. Dwarf generator handles this although the // user might not get the right info at runtime. for (Use &U : Inst->uses()) { // Determine the block of the use. Instruction *UseInst = cast<Instruction>(U.getUser()); BasicBlock *UseBlock = UseInst->getParent(); 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); } // Check that it dominates. if (!DT->dominates(BB, UseBlock)) return false; } return true; } bool Sinking::runOnFunction(Function &F) { DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree(); LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo(); AA = &getAnalysis<AAResultsWrapperPass>().getAAResults(); bool MadeChange, EverMadeChange = false; do { MadeChange = false; DEBUG(dbgs() << "Sinking iteration " << NumSinkIter << "\n"); // Process all basic blocks. for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) MadeChange |= ProcessBlock(*I); EverMadeChange |= MadeChange; NumSinkIter++; } while (MadeChange); return EverMadeChange; } bool Sinking::ProcessBlock(BasicBlock &BB) { // 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 (isa<DbgInfoIntrinsic>(Inst)) continue; if (SinkInstruction(Inst, Stores)) ++NumSunk, MadeChange = true; // If we just processed the first instruction in the block, we're done. } while (!ProcessedBegin); return MadeChange; } 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 (AA->getModRefInfo(S, Loc) & MRI_Mod) return false; } if (isa<TerminatorInst>(Inst) || isa<PHINode>(Inst) || Inst->isEHPad() || Inst->mayThrow()) return false; // Convergent operations cannot be made control-dependent on additional // values. if (auto CS = CallSite(Inst)) { if (CS.hasFnAttr(Attribute::Convergent)) return false; } return true; } /// IsAcceptableTarget - Return true if it is possible to sink the instruction /// in the specified basic block. bool Sinking::IsAcceptableTarget(Instruction *Inst, BasicBlock *SuccToSinkTo) const { assert(Inst && "Instruction to be sunk is null"); assert(SuccToSinkTo && "Candidate sink target is null"); // It is not possible to sink an instruction into its own block. This can // happen with loops. if (Inst->getParent() == SuccToSinkTo) return false; // It's never legal to sink an instruction into a block which terminates in an // EH-pad. if (SuccToSinkTo->getTerminator()->isExceptional()) 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 (!isSafeToSpeculativelyExecute(Inst)) 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; } // Finally, check that all the uses of the instruction are actually // dominated by the candidate return AllUsesDominatedByBlock(Inst, SuccToSinkTo); } /// SinkInstruction - Determine whether it is safe to sink the specified machine /// instruction out of its current block into a successor. bool Sinking::SinkInstruction(Instruction *Inst, SmallPtrSetImpl<Instruction *> &Stores) { // 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; // Instructions can only be sunk if all their uses are in blocks // dominated by one of the successors. // Look at all the postdominators and see if we can sink it in one. DomTreeNode *DTN = DT->getNode(Inst->getParent()); for (DomTreeNode::iterator I = DTN->begin(), E = DTN->end(); I != E && SuccToSinkTo == nullptr; ++I) { BasicBlock *Candidate = (*I)->getBlock(); if ((*I)->getIDom()->getBlock() == Inst->getParent() && IsAcceptableTarget(Inst, Candidate)) SuccToSinkTo = Candidate; } // If no suitable postdominator was found, look at all the successors and // decide which one we should sink to, if any. for (succ_iterator I = succ_begin(Inst->getParent()), E = succ_end(Inst->getParent()); I != E && !SuccToSinkTo; ++I) { if (IsAcceptableTarget(Inst, *I)) SuccToSinkTo = *I; } // If we couldn't find a block to sink to, ignore this instruction. if (!SuccToSinkTo) return false; 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; }