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//===-- Sink.cpp - Code Sinking -------------------------------------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See 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/IR/Dominators.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()) {
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() || !Inst->willReturn())
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))
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);
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.
return true;
static bool ProcessBlock(BasicBlock &BB, DominatorTree &DT, LoopInfo &LI,
AAResults &AA) {
// 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();
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)
if (Inst->isDebugOrPseudoInst())
if (SinkInstruction(Inst, Stores, DT, LI, AA)) {
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;
} 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;
return PA;
namespace {
class SinkingLegacyPass : public FunctionPass {
static char ID; // Pass identification
SinkingLegacyPass() : FunctionPass(ID) {
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 {
} // end anonymous namespace
char SinkingLegacyPass::ID = 0;
INITIALIZE_PASS_BEGIN(SinkingLegacyPass, "sink", "Code sinking", false, false)
INITIALIZE_PASS_END(SinkingLegacyPass, "sink", "Code sinking", false, false)
FunctionPass *llvm::createSinkingPass() { return new SinkingLegacyPass(); }