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

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//===- MergedLoadStoreMotion.cpp - merge and hoist/sink load/stores -------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
//! \file
//! 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. New tail/footer block may be
// insterted if the tail/footer block has more predecessors (not only the two
// predecessors that are forming the diamond). 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.
//
// NOTE: This code no longer performs load hoisting, it is subsumed by GVNHoist.
//
//===----------------------------------------------------------------------===//
//
//
// 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/ValueTracking.h"
#include "llvm/IR/Metadata.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
using namespace llvm;
#define DEBUG_TYPE "mldst-motion"
namespace {
//===----------------------------------------------------------------------===//
// MergedLoadStoreMotion Pass
//===----------------------------------------------------------------------===//
class MergedLoadStoreMotion {
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;
const bool SplitFooterBB;
public:
MergedLoadStoreMotion(bool SplitFooterBB) : SplitFooterBB(SplitFooterBB) {}
bool run(Function &F, AliasAnalysis &AA);
private:
BasicBlock *getDiamondTail(BasicBlock *BB);
bool isDiamondHead(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 canSinkStoresAndGEPs(StoreInst *S0, StoreInst *S1) const;
void sinkStoresAndGEPs(BasicBlock *BB, StoreInst *SinkCand,
StoreInst *ElseInst);
bool mergeStores(BasicBlock *BB);
};
} // end anonymous namespace
///
/// 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();
}
///
/// 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;
}
///
/// 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, ModRefInfo::ModRef);
}
///
/// 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) {
LLVM_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;
}
///
/// 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->applyMergedLocation(S0->getDebugLoc(), S1->getDebugLoc());
NewPN->addIncoming(Opd1, S0->getParent());
NewPN->addIncoming(Opd2, S1->getParent());
return NewPN;
}
///
/// Check if 2 stores can be sunk together with corresponding GEPs
///
bool MergedLoadStoreMotion::canSinkStoresAndGEPs(StoreInst *S0,
StoreInst *S1) const {
auto *A0 = dyn_cast<Instruction>(S0->getPointerOperand());
auto *A1 = dyn_cast<Instruction>(S1->getPointerOperand());
return A0 && A1 && A0->isIdenticalTo(A1) && A0->hasOneUse() &&
(A0->getParent() == S0->getParent()) && A1->hasOneUse() &&
(A1->getParent() == S1->getParent()) && isa<GetElementPtrInst>(A0);
}
///
/// Merge two stores to same address and sink into \p BB
///
/// Also sinks GEP instruction computing the store address
///
void MergedLoadStoreMotion::sinkStoresAndGEPs(BasicBlock *BB, StoreInst *S0,
StoreInst *S1) {
// Only one definition?
auto *A0 = dyn_cast<Instruction>(S0->getPointerOperand());
auto *A1 = dyn_cast<Instruction>(S1->getPointerOperand());
LLVM_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);
S0->eraseFromParent();
S1->eraseFromParent();
A0->replaceAllUsesWith(ANew);
A0->eraseFromParent();
A1->replaceAllUsesWith(ANew);
A1->eraseFromParent();
}
///
/// True when two stores are equivalent and can sink into the footer
///
/// Starting from a diamond head block, iterate over the instructions in one
/// successor block and try to match a store in the second successor.
///
bool MergedLoadStoreMotion::mergeStores(BasicBlock *HeadBB) {
bool MergedStores = false;
BasicBlock *TailBB = getDiamondTail(HeadBB);
BasicBlock *SinkBB = TailBB;
assert(SinkBB && "Footer of a diamond cannot be empty");
succ_iterator SI = succ_begin(HeadBB);
assert(SI != succ_end(HeadBB) && "Diamond head cannot have zero successors");
BasicBlock *Pred0 = *SI;
++SI;
assert(SI != succ_end(HeadBB) && "Diamond head cannot have single successor");
BasicBlock *Pred1 = *SI;
// tail block of a diamond/hammock?
if (Pred0 == Pred1)
return false; // No.
// bail out early if we can not merge into the footer BB
if (!SplitFooterBB && TailBB->hasNPredecessorsOrMore(3))
return false;
// #Instructions in Pred1 for Compile Time Control
auto InstsNoDbg = Pred1->instructionsWithoutDebug();
int Size1 = std::distance(InstsNoDbg.begin(), InstsNoDbg.end());
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)) {
if (!canSinkStoresAndGEPs(S0, S1))
// 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.
break;
if (SinkBB == TailBB && TailBB->hasNPredecessorsOrMore(3)) {
// We have more than 2 predecessors. Insert a new block
// postdominating 2 predecessors we're going to sink from.
SinkBB = SplitBlockPredecessors(TailBB, {Pred0, Pred1}, ".sink.split");
if (!SinkBB)
break;
}
MergedStores = true;
sinkStoresAndGEPs(SinkBB, S0, S1);
RBI = Pred0->rbegin();
RBE = Pred0->rend();
LLVM_DEBUG(dbgs() << "Search again\n"; Instruction *I = &*RBI; I->dump());
}
}
return MergedStores;
}
bool MergedLoadStoreMotion::run(Function &F, AliasAnalysis &AA) {
this->AA = &AA;
bool Changed = false;
LLVM_DEBUG(dbgs() << "Instruction Merger\n");
// Merge unconditional branches, allowing PRE to catch more
// optimization opportunities.
// This loop doesn't care about newly inserted/split blocks
// since they never will be diamond heads.
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 |= mergeStores(BB);
}
}
return Changed;
}
namespace {
class MergedLoadStoreMotionLegacyPass : public FunctionPass {
const bool SplitFooterBB;
public:
static char ID; // Pass identification, replacement for typeid
MergedLoadStoreMotionLegacyPass(bool SplitFooterBB = false)
: FunctionPass(ID), SplitFooterBB(SplitFooterBB) {
initializeMergedLoadStoreMotionLegacyPassPass(
*PassRegistry::getPassRegistry());
}
///
/// Run the transformation for each function
///
bool runOnFunction(Function &F) override {
if (skipFunction(F))
return false;
MergedLoadStoreMotion Impl(SplitFooterBB);
return Impl.run(F, getAnalysis<AAResultsWrapperPass>().getAAResults());
}
private:
void getAnalysisUsage(AnalysisUsage &AU) const override {
if (!SplitFooterBB)
AU.setPreservesCFG();
AU.addRequired<AAResultsWrapperPass>();
AU.addPreserved<GlobalsAAWrapperPass>();
}
};
char MergedLoadStoreMotionLegacyPass::ID = 0;
} // anonymous namespace
///
/// createMergedLoadStoreMotionPass - The public interface to this file.
///
FunctionPass *llvm::createMergedLoadStoreMotionPass(bool SplitFooterBB) {
return new MergedLoadStoreMotionLegacyPass(SplitFooterBB);
}
INITIALIZE_PASS_BEGIN(MergedLoadStoreMotionLegacyPass, "mldst-motion",
"MergedLoadStoreMotion", false, false)
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
INITIALIZE_PASS_END(MergedLoadStoreMotionLegacyPass, "mldst-motion",
"MergedLoadStoreMotion", false, false)
PreservedAnalyses
MergedLoadStoreMotionPass::run(Function &F, FunctionAnalysisManager &AM) {
MergedLoadStoreMotion Impl(Options.SplitFooterBB);
auto &AA = AM.getResult<AAManager>(F);
if (!Impl.run(F, AA))
return PreservedAnalyses::all();
PreservedAnalyses PA;
if (!Options.SplitFooterBB)
PA.preserveSet<CFGAnalyses>();
PA.preserve<GlobalsAA>();
return PA;
}
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