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llvm-mirror/lib/Transforms/Scalar/MergedLoadStoreMotion.cpp
Chandler Carruth 531a8d8a72 [PM] Introduce an analysis set used to preserve all analyses over 
a function's CFG when that CFG is unchanged.

This allows transformation passes to simply claim they preserve the CFG
and analysis passes to check for the CFG being preserved to remove the
fanout of all analyses being listed in all passes.

I've gone through and removed or cleaned up as many of the comments
reminding us to do this as I could.

Differential Revision: https://reviews.llvm.org/D28627

llvm-svn: 292054
2017-01-15 06:32:49 +00:00

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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"
namespace {
//===----------------------------------------------------------------------===//
// 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);
};
} // end anonymous namespace
///
/// \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->andIRFlags(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->andIRFlags(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:
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, FunctionAnalysisManager &AM) {
MergedLoadStoreMotion Impl;
auto *MD = AM.getCachedResult<MemoryDependenceAnalysis>(F);
auto &AA = AM.getResult<AAManager>(F);
if (!Impl.run(F, MD, AA))
return PreservedAnalyses::all();
PreservedAnalyses PA;
PA.preserveSet<CFGAnalyses>();
PA.preserve<GlobalsAA>();
PA.preserve<MemoryDependenceAnalysis>();
return PA;
}
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