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llvm-mirror/lib/Transforms/Scalar/MergedLoadStoreMotion.cpp
Arthur Eubanks 7a1762f190 [NewPM] Don't mark AA analyses as preserved 
Currently all AA analyses marked as preserved are stateless, not taking
into account their dependent analyses. So there's no need to mark them
as preserved, they won't be invalidated unless their analyses are.

SCEVAAResults was the one exception to this, it was treated like a
typical analysis result. Make it like the others and don't invalidate
unless SCEV is invalidated.

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D102032
2021-05-18 13:49:03 -07:00

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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/InitializePasses.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 (BasicBlock &BB : make_early_inc_range(F))
// 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>();
return PA;
}
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