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//===-- 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;
static char ID; // Pass identification
Sinking() : FunctionPass(ID) {
bool runOnFunction(Function &F) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
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_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<AliasAnalysis>();
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;
} 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 || BB.empty()) 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();
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 (isa<DbgInfoIntrinsic>(Inst))
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()) {
return false;
if (LoadInst *L = dyn_cast<LoadInst>(Inst)) {
MemoryLocation Loc = MemoryLocation::get(L);
for (Instruction *S : Stores)
if (AA->getModRefInfo(S, Loc) & AliasAnalysis::Mod)
return false;
if (isa<TerminatorInst>(Inst) || isa<PHINode>(Inst))
return false;
// Convergent operations can only be moved to control equivalent blocks.
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;
// 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.
return true;