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llvm-project/llvm/lib/Transforms/Scalar/MergedLoadStoreMotion.cpp

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//===- MergedLoadStoreMotion.cpp - merge and hoist/sink load/stores -------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
//! \file
//! \brief This pass performs merges of loads and stores on both sides of a
// diamond (hammock). It hoists the loads and sinks the stores.
//
// The algorithm iteratively hoists two loads to the same address out of a
// diamond (hammock) and merges them into a single load in the header. Similar
// it sinks and merges two stores to the tail block (footer). The algorithm
// iterates over the instructions of one side of the diamond and attempts to
// find a matching load/store on the other side. It hoists / sinks when it
// thinks it safe to do so. This optimization helps with eg. hiding load
// latencies, triggering if-conversion, and reducing static code size.
//
//===----------------------------------------------------------------------===//
//
//
// Example:
// Diamond shaped code before merge:
//
// header:
// br %cond, label %if.then, label %if.else
// + +
// + +
// + +
// if.then: if.else:
// %lt = load %addr_l %le = load %addr_l
// <use %lt> <use %le>
// <...> <...>
// store %st, %addr_s store %se, %addr_s
// br label %if.end br label %if.end
// + +
// + +
// + +
// if.end ("footer"):
// <...>
//
// Diamond shaped code after merge:
//
// header:
// %l = load %addr_l
// br %cond, label %if.then, label %if.else
// + +
// + +
// + +
// if.then: if.else:
// <use %l> <use %l>
// <...> <...>
// br label %if.end br label %if.end
// + +
// + +
// + +
// if.end ("footer"):
// %s.sink = phi [%st, if.then], [%se, if.else]
// <...>
// store %s.sink, %addr_s
// <...>
//
//
//===----------------------- TODO -----------------------------------------===//
//
// 1) Generalize to regions other than diamonds
// 2) Be more aggressive merging memory operations
// Note that both changes require register pressure control
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar/MergedLoadStoreMotion.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/CFG.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/Loads.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/MemoryDependenceAnalysis.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
using namespace llvm;
#define DEBUG_TYPE "mldst-motion"
//===----------------------------------------------------------------------===//
// MergedLoadStoreMotion Pass
//===----------------------------------------------------------------------===//
class MergedLoadStoreMotion {
MemoryDependenceResults *MD = nullptr;
AliasAnalysis *AA = nullptr;
// The mergeLoad/Store algorithms could have Size0 * Size1 complexity,
// where Size0 and Size1 are the #instructions on the two sides of
// the diamond. The constant chosen here is arbitrary. Compiler Time
// Control is enforced by the check Size0 * Size1 < MagicCompileTimeControl.
const int MagicCompileTimeControl = 250;
public:
bool run(Function &F, MemoryDependenceResults *MD, AliasAnalysis &AA);
private:
///
/// \brief Remove instruction from parent and update memory dependence
/// analysis.
///
void removeInstruction(Instruction *Inst);
BasicBlock *getDiamondTail(BasicBlock *BB);
bool isDiamondHead(BasicBlock *BB);
// Routines for hoisting loads
bool isLoadHoistBarrierInRange(const Instruction &Start,
const Instruction &End, LoadInst *LI,
bool SafeToLoadUnconditionally);
LoadInst *canHoistFromBlock(BasicBlock *BB, LoadInst *LI);
void hoistInstruction(BasicBlock *BB, Instruction *HoistCand,
Instruction *ElseInst);
bool isSafeToHoist(Instruction *I) const;
bool hoistLoad(BasicBlock *BB, LoadInst *HoistCand, LoadInst *ElseInst);
bool mergeLoads(BasicBlock *BB);
// Routines for sinking stores
StoreInst *canSinkFromBlock(BasicBlock *BB, StoreInst *SI);
PHINode *getPHIOperand(BasicBlock *BB, StoreInst *S0, StoreInst *S1);
bool isStoreSinkBarrierInRange(const Instruction &Start,
const Instruction &End, MemoryLocation Loc);
bool sinkStore(BasicBlock *BB, StoreInst *SinkCand, StoreInst *ElseInst);
bool mergeStores(BasicBlock *BB);
};
///
/// \brief Remove instruction from parent and update memory dependence analysis.
///
void MergedLoadStoreMotion::removeInstruction(Instruction *Inst) {
// Notify the memory dependence analysis.
if (MD) {
MD->removeInstruction(Inst);
if (auto *LI = dyn_cast<LoadInst>(Inst))
MD->invalidateCachedPointerInfo(LI->getPointerOperand());
if (Inst->getType()->isPtrOrPtrVectorTy()) {
MD->invalidateCachedPointerInfo(Inst);
}
}
Inst->eraseFromParent();
}
///
/// \brief Return tail block of a diamond.
///
BasicBlock *MergedLoadStoreMotion::getDiamondTail(BasicBlock *BB) {
assert(isDiamondHead(BB) && "Basic block is not head of a diamond");
return BB->getTerminator()->getSuccessor(0)->getSingleSuccessor();
}
///
/// \brief True when BB is the head of a diamond (hammock)
///
bool MergedLoadStoreMotion::isDiamondHead(BasicBlock *BB) {
if (!BB)
return false;
auto *BI = dyn_cast<BranchInst>(BB->getTerminator());
if (!BI || !BI->isConditional())
return false;
BasicBlock *Succ0 = BI->getSuccessor(0);
BasicBlock *Succ1 = BI->getSuccessor(1);
if (!Succ0->getSinglePredecessor())
return false;
if (!Succ1->getSinglePredecessor())
return false;
BasicBlock *Succ0Succ = Succ0->getSingleSuccessor();
BasicBlock *Succ1Succ = Succ1->getSingleSuccessor();
// Ignore triangles.
if (!Succ0Succ || !Succ1Succ || Succ0Succ != Succ1Succ)
return false;
return true;
}
///
/// \brief True when instruction is a hoist barrier for a load
///
/// Whenever an instruction could possibly modify the value
/// being loaded or protect against the load from happening
/// it is considered a hoist barrier.
///
bool MergedLoadStoreMotion::isLoadHoistBarrierInRange(
const Instruction &Start, const Instruction &End, LoadInst *LI,
bool SafeToLoadUnconditionally) {
if (!SafeToLoadUnconditionally)
for (const Instruction &Inst :
make_range(Start.getIterator(), End.getIterator()))
if (!isGuaranteedToTransferExecutionToSuccessor(&Inst))
return true;
MemoryLocation Loc = MemoryLocation::get(LI);
return AA->canInstructionRangeModRef(Start, End, Loc, MRI_Mod);
}
///
/// \brief Decide if a load can be hoisted
///
/// When there is a load in \p BB to the same address as \p LI
/// and it can be hoisted from \p BB, return that load.
/// Otherwise return Null.
///
LoadInst *MergedLoadStoreMotion::canHoistFromBlock(BasicBlock *BB1,
LoadInst *Load0) {
BasicBlock *BB0 = Load0->getParent();
BasicBlock *Head = BB0->getSinglePredecessor();
bool SafeToLoadUnconditionally = isSafeToLoadUnconditionally(
Load0->getPointerOperand(), Load0->getAlignment(),
Load0->getModule()->getDataLayout(),
/*ScanFrom=*/Head->getTerminator());
for (BasicBlock::iterator BBI = BB1->begin(), BBE = BB1->end(); BBI != BBE;
++BBI) {
Instruction *Inst = &*BBI;
// Only merge and hoist loads when their result in used only in BB
auto *Load1 = dyn_cast<LoadInst>(Inst);
if (!Load1 || Inst->isUsedOutsideOfBlock(BB1))
continue;
MemoryLocation Loc0 = MemoryLocation::get(Load0);
MemoryLocation Loc1 = MemoryLocation::get(Load1);
if (Load0->isSameOperationAs(Load1) && AA->isMustAlias(Loc0, Loc1) &&
!isLoadHoistBarrierInRange(BB1->front(), *Load1, Load1,
SafeToLoadUnconditionally) &&
!isLoadHoistBarrierInRange(BB0->front(), *Load0, Load0,
SafeToLoadUnconditionally)) {
return Load1;
}
}
return nullptr;
}
///
/// \brief Merge two equivalent instructions \p HoistCand and \p ElseInst into
/// \p BB
///
/// BB is the head of a diamond
///
void MergedLoadStoreMotion::hoistInstruction(BasicBlock *BB,
Instruction *HoistCand,
Instruction *ElseInst) {
DEBUG(dbgs() << " Hoist Instruction into BB \n"; BB->dump();
dbgs() << "Instruction Left\n"; HoistCand->dump(); dbgs() << "\n";
dbgs() << "Instruction Right\n"; ElseInst->dump(); dbgs() << "\n");
// Hoist the instruction.
assert(HoistCand->getParent() != BB);
// Intersect optional metadata.
HoistCand->intersectOptionalDataWith(ElseInst);
HoistCand->dropUnknownNonDebugMetadata();
// Prepend point for instruction insert
Instruction *HoistPt = BB->getTerminator();
// Merged instruction
Instruction *HoistedInst = HoistCand->clone();
// Hoist instruction.
HoistedInst->insertBefore(HoistPt);
HoistCand->replaceAllUsesWith(HoistedInst);
removeInstruction(HoistCand);
// Replace the else block instruction.
ElseInst->replaceAllUsesWith(HoistedInst);
removeInstruction(ElseInst);
}
///
/// \brief Return true if no operand of \p I is defined in I's parent block
///
bool MergedLoadStoreMotion::isSafeToHoist(Instruction *I) const {
BasicBlock *Parent = I->getParent();
for (Use &U : I->operands())
if (auto *Instr = dyn_cast<Instruction>(&U))
if (Instr->getParent() == Parent)
return false;
return true;
}
///
/// \brief Merge two equivalent loads and GEPs and hoist into diamond head
///
bool MergedLoadStoreMotion::hoistLoad(BasicBlock *BB, LoadInst *L0,
LoadInst *L1) {
// Only one definition?
auto *A0 = dyn_cast<Instruction>(L0->getPointerOperand());
auto *A1 = dyn_cast<Instruction>(L1->getPointerOperand());
if (A0 && A1 && A0->isIdenticalTo(A1) && isSafeToHoist(A0) &&
A0->hasOneUse() && (A0->getParent() == L0->getParent()) &&
A1->hasOneUse() && (A1->getParent() == L1->getParent()) &&
isa<GetElementPtrInst>(A0)) {
DEBUG(dbgs() << "Hoist Instruction into BB \n"; BB->dump();
dbgs() << "Instruction Left\n"; L0->dump(); dbgs() << "\n";
dbgs() << "Instruction Right\n"; L1->dump(); dbgs() << "\n");
hoistInstruction(BB, A0, A1);
hoistInstruction(BB, L0, L1);
return true;
}
return false;
}
///
/// \brief Try to hoist two loads to same address into diamond header
///
/// Starting from a diamond head block, iterate over the instructions in one
/// successor block and try to match a load in the second successor.
///
bool MergedLoadStoreMotion::mergeLoads(BasicBlock *BB) {
bool MergedLoads = false;
assert(isDiamondHead(BB));
BranchInst *BI = cast<BranchInst>(BB->getTerminator());
BasicBlock *Succ0 = BI->getSuccessor(0);
BasicBlock *Succ1 = BI->getSuccessor(1);
// #Instructions in Succ1 for Compile Time Control
int Size1 = Succ1->size();
int NLoads = 0;
for (BasicBlock::iterator BBI = Succ0->begin(), BBE = Succ0->end();
BBI != BBE;) {
Instruction *I = &*BBI;
++BBI;
// Don't move non-simple (atomic, volatile) loads.
auto *L0 = dyn_cast<LoadInst>(I);
if (!L0 || !L0->isSimple() || L0->isUsedOutsideOfBlock(Succ0))
continue;
++NLoads;
if (NLoads * Size1 >= MagicCompileTimeControl)
break;
if (LoadInst *L1 = canHoistFromBlock(Succ1, L0)) {
bool Res = hoistLoad(BB, L0, L1);
MergedLoads |= Res;
// Don't attempt to hoist above loads that had not been hoisted.
if (!Res)
break;
}
}
return MergedLoads;
}
///
/// \brief True when instruction is a sink barrier for a store
/// located in Loc
///
/// Whenever an instruction could possibly read or modify the
/// value being stored or protect against the store from
/// happening it is considered a sink barrier.
///
bool MergedLoadStoreMotion::isStoreSinkBarrierInRange(const Instruction &Start,
const Instruction &End,
MemoryLocation Loc) {
for (const Instruction &Inst :
make_range(Start.getIterator(), End.getIterator()))
if (Inst.mayThrow())
return true;
return AA->canInstructionRangeModRef(Start, End, Loc, MRI_ModRef);
}
///
/// \brief Check if \p BB contains a store to the same address as \p SI
///
/// \return The store in \p when it is safe to sink. Otherwise return Null.
///
StoreInst *MergedLoadStoreMotion::canSinkFromBlock(BasicBlock *BB1,
StoreInst *Store0) {
DEBUG(dbgs() << "can Sink? : "; Store0->dump(); dbgs() << "\n");
BasicBlock *BB0 = Store0->getParent();
for (Instruction &Inst : reverse(*BB1)) {
auto *Store1 = dyn_cast<StoreInst>(&Inst);
if (!Store1)
continue;
MemoryLocation Loc0 = MemoryLocation::get(Store0);
MemoryLocation Loc1 = MemoryLocation::get(Store1);
if (AA->isMustAlias(Loc0, Loc1) && Store0->isSameOperationAs(Store1) &&
!isStoreSinkBarrierInRange(*Store1->getNextNode(), BB1->back(), Loc1) &&
!isStoreSinkBarrierInRange(*Store0->getNextNode(), BB0->back(), Loc0)) {
return Store1;
}
}
return nullptr;
}
///
/// \brief Create a PHI node in BB for the operands of S0 and S1
///
PHINode *MergedLoadStoreMotion::getPHIOperand(BasicBlock *BB, StoreInst *S0,
StoreInst *S1) {
// Create a phi if the values mismatch.
Value *Opd1 = S0->getValueOperand();
Value *Opd2 = S1->getValueOperand();
if (Opd1 == Opd2)
return nullptr;
auto *NewPN = PHINode::Create(Opd1->getType(), 2, Opd2->getName() + ".sink",
&BB->front());
NewPN->addIncoming(Opd1, S0->getParent());
NewPN->addIncoming(Opd2, S1->getParent());
if (MD && NewPN->getType()->getScalarType()->isPointerTy())
MD->invalidateCachedPointerInfo(NewPN);
return NewPN;
}
///
/// \brief Merge two stores to same address and sink into \p BB
///
/// Also sinks GEP instruction computing the store address
///
bool MergedLoadStoreMotion::sinkStore(BasicBlock *BB, StoreInst *S0,
StoreInst *S1) {
// Only one definition?
auto *A0 = dyn_cast<Instruction>(S0->getPointerOperand());
auto *A1 = dyn_cast<Instruction>(S1->getPointerOperand());
if (A0 && A1 && A0->isIdenticalTo(A1) && A0->hasOneUse() &&
(A0->getParent() == S0->getParent()) && A1->hasOneUse() &&
(A1->getParent() == S1->getParent()) && isa<GetElementPtrInst>(A0)) {
DEBUG(dbgs() << "Sink Instruction into BB \n"; BB->dump();
dbgs() << "Instruction Left\n"; S0->dump(); dbgs() << "\n";
dbgs() << "Instruction Right\n"; S1->dump(); dbgs() << "\n");
// Hoist the instruction.
BasicBlock::iterator InsertPt = BB->getFirstInsertionPt();
// Intersect optional metadata.
S0->intersectOptionalDataWith(S1);
S0->dropUnknownNonDebugMetadata();
// Create the new store to be inserted at the join point.
StoreInst *SNew = cast<StoreInst>(S0->clone());
Instruction *ANew = A0->clone();
SNew->insertBefore(&*InsertPt);
ANew->insertBefore(SNew);
assert(S0->getParent() == A0->getParent());
assert(S1->getParent() == A1->getParent());
// New PHI operand? Use it.
if (PHINode *NewPN = getPHIOperand(BB, S0, S1))
SNew->setOperand(0, NewPN);
removeInstruction(S0);
removeInstruction(S1);
A0->replaceAllUsesWith(ANew);
removeInstruction(A0);
A1->replaceAllUsesWith(ANew);
removeInstruction(A1);
return true;
}
return false;
}
///
/// \brief True when two stores are equivalent and can sink into the footer
///
/// Starting from a diamond tail block, iterate over the instructions in one
/// predecessor block and try to match a store in the second predecessor.
///
bool MergedLoadStoreMotion::mergeStores(BasicBlock *T) {
bool MergedStores = false;
assert(T && "Footer of a diamond cannot be empty");
pred_iterator PI = pred_begin(T), E = pred_end(T);
assert(PI != E);
BasicBlock *Pred0 = *PI;
++PI;
BasicBlock *Pred1 = *PI;
++PI;
// tail block of a diamond/hammock?
if (Pred0 == Pred1)
return false; // No.
if (PI != E)
return false; // No. More than 2 predecessors.
// #Instructions in Succ1 for Compile Time Control
int Size1 = Pred1->size();
int NStores = 0;
for (BasicBlock::reverse_iterator RBI = Pred0->rbegin(), RBE = Pred0->rend();
RBI != RBE;) {
Instruction *I = &*RBI;
++RBI;
// Don't sink non-simple (atomic, volatile) stores.
auto *S0 = dyn_cast<StoreInst>(I);
if (!S0 || !S0->isSimple())
continue;
++NStores;
if (NStores * Size1 >= MagicCompileTimeControl)
break;
if (StoreInst *S1 = canSinkFromBlock(Pred1, S0)) {
bool Res = sinkStore(T, S0, S1);
MergedStores |= Res;
// Don't attempt to sink below stores that had to stick around
// But after removal of a store and some of its feeding
// instruction search again from the beginning since the iterator
// is likely stale at this point.
if (!Res)
break;
RBI = Pred0->rbegin();
RBE = Pred0->rend();
DEBUG(dbgs() << "Search again\n"; Instruction *I = &*RBI; I->dump());
}
}
return MergedStores;
}
bool MergedLoadStoreMotion::run(Function &F, MemoryDependenceResults *MD,
AliasAnalysis &AA) {
this->MD = MD;
this->AA = &AA;
bool Changed = false;
DEBUG(dbgs() << "Instruction Merger\n");
// Merge unconditional branches, allowing PRE to catch more
// optimization opportunities.
for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE;) {
BasicBlock *BB = &*FI++;
// Hoist equivalent loads and sink stores
// outside diamonds when possible
if (isDiamondHead(BB)) {
Changed |= mergeLoads(BB);
Changed |= mergeStores(getDiamondTail(BB));
}
}
return Changed;
}
namespace {
class MergedLoadStoreMotionLegacyPass : public FunctionPass {
public:
static char ID; // Pass identification, replacement for typeid
MergedLoadStoreMotionLegacyPass() : FunctionPass(ID) {
initializeMergedLoadStoreMotionLegacyPassPass(
*PassRegistry::getPassRegistry());
}
///
/// \brief Run the transformation for each function
///
bool runOnFunction(Function &F) override {
if (skipFunction(F))
return false;
MergedLoadStoreMotion Impl;
auto *MDWP = getAnalysisIfAvailable<MemoryDependenceWrapperPass>();
return Impl.run(F, MDWP ? &MDWP->getMemDep() : nullptr,
getAnalysis<AAResultsWrapperPass>().getAAResults());
}
private:
// This transformation requires dominator postdominator info
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
AU.addRequired<AAResultsWrapperPass>();
AU.addPreserved<GlobalsAAWrapperPass>();
AU.addPreserved<MemoryDependenceWrapperPass>();
}
};
char MergedLoadStoreMotionLegacyPass::ID = 0;
} // anonymous namespace
///
/// \brief createMergedLoadStoreMotionPass - The public interface to this file.
///
FunctionPass *llvm::createMergedLoadStoreMotionPass() {
return new MergedLoadStoreMotionLegacyPass();
}
INITIALIZE_PASS_BEGIN(MergedLoadStoreMotionLegacyPass, "mldst-motion",
"MergedLoadStoreMotion", false, false)
INITIALIZE_PASS_DEPENDENCY(MemoryDependenceWrapperPass)
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
INITIALIZE_PASS_END(MergedLoadStoreMotionLegacyPass, "mldst-motion",
"MergedLoadStoreMotion", false, false)
PreservedAnalyses
MergedLoadStoreMotionPass::run(Function &F, AnalysisManager<Function> &AM) {
MergedLoadStoreMotion Impl;
auto *MD = AM.getCachedResult<MemoryDependenceAnalysis>(F);
auto &AA = AM.getResult<AAManager>(F);
if (!Impl.run(F, MD, AA))
return PreservedAnalyses::all();
// FIXME: This pass should also 'preserve the CFG'.
// The new pass manager has currently no way to do it.
PreservedAnalyses PA;
PA.preserve<GlobalsAA>();
PA.preserve<MemoryDependenceAnalysis>();
return PA;
}
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