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llvm-project/llvm/lib/CodeGen/GlobalISel/LoadStoreOpt.cpp
Amara Emerson 19f4d68252
[GlobalISel] Fix store merging incorrectly classifying an unknown index expr as 0. (#90375) 
During analysis, we incorrectly leave the offset part of an address info
struct
as zero, when in actual fact we failed to decompose it into base +
offset.
This results in incorrectly assuming that the address is adjacent to
another store
addr. To fix this we wrap the offset in an optional<> so we can
distinguish between
real zero and unknown.

Fixes issue #90242
2024-04-30 14:42:14 -07:00

993 lines
35 KiB
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//===- LoadStoreOpt.cpp ----------- Generic memory optimizations -*- C++ -*-==//
//
// 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 file implements the LoadStoreOpt optimization pass.
//===----------------------------------------------------------------------===//
#include "llvm/CodeGen/GlobalISel/LoadStoreOpt.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/MemoryLocation.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/CodeGen/GlobalISel/GenericMachineInstrs.h"
#include "llvm/CodeGen/GlobalISel/LegalizerInfo.h"
#include "llvm/CodeGen/GlobalISel/MIPatternMatch.h"
#include "llvm/CodeGen/GlobalISel/Utils.h"
#include "llvm/CodeGen/LowLevelTypeUtils.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineOptimizationRemarkEmitter.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/Register.h"
#include "llvm/CodeGen/TargetLowering.h"
#include "llvm/CodeGen/TargetOpcodes.h"
#include "llvm/IR/DebugInfoMetadata.h"
#include "llvm/InitializePasses.h"
#include "llvm/Support/AtomicOrdering.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/MathExtras.h"
#include <algorithm>
#define DEBUG_TYPE "loadstore-opt"
using namespace llvm;
using namespace ore;
using namespace MIPatternMatch;
STATISTIC(NumStoresMerged, "Number of stores merged");
const unsigned MaxStoreSizeToForm = 128;
char LoadStoreOpt::ID = 0;
INITIALIZE_PASS_BEGIN(LoadStoreOpt, DEBUG_TYPE, "Generic memory optimizations",
false, false)
INITIALIZE_PASS_END(LoadStoreOpt, DEBUG_TYPE, "Generic memory optimizations",
false, false)
LoadStoreOpt::LoadStoreOpt(std::function<bool(const MachineFunction &)> F)
: MachineFunctionPass(ID), DoNotRunPass(F) {}
LoadStoreOpt::LoadStoreOpt()
: LoadStoreOpt([](const MachineFunction &) { return false; }) {}
void LoadStoreOpt::init(MachineFunction &MF) {
this->MF = &MF;
MRI = &MF.getRegInfo();
AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
TLI = MF.getSubtarget().getTargetLowering();
LI = MF.getSubtarget().getLegalizerInfo();
Builder.setMF(MF);
IsPreLegalizer = !MF.getProperties().hasProperty(
MachineFunctionProperties::Property::Legalized);
InstsToErase.clear();
}
void LoadStoreOpt::getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<AAResultsWrapperPass>();
AU.setPreservesAll();
getSelectionDAGFallbackAnalysisUsage(AU);
MachineFunctionPass::getAnalysisUsage(AU);
}
BaseIndexOffset GISelAddressing::getPointerInfo(Register Ptr,
MachineRegisterInfo &MRI) {
BaseIndexOffset Info;
Register PtrAddRHS;
Register BaseReg;
if (!mi_match(Ptr, MRI, m_GPtrAdd(m_Reg(BaseReg), m_Reg(PtrAddRHS)))) {
Info.setBase(Ptr);
Info.setOffset(0);
return Info;
}
Info.setBase(BaseReg);
auto RHSCst = getIConstantVRegValWithLookThrough(PtrAddRHS, MRI);
if (RHSCst)
Info.setOffset(RHSCst->Value.getSExtValue());
// Just recognize a simple case for now. In future we'll need to match
// indexing patterns for base + index + constant.
Info.setIndex(PtrAddRHS);
return Info;
}
bool GISelAddressing::aliasIsKnownForLoadStore(const MachineInstr &MI1,
const MachineInstr &MI2,
bool &IsAlias,
MachineRegisterInfo &MRI) {
auto *LdSt1 = dyn_cast<GLoadStore>(&MI1);
auto *LdSt2 = dyn_cast<GLoadStore>(&MI2);
if (!LdSt1 || !LdSt2)
return false;
BaseIndexOffset BasePtr0 = getPointerInfo(LdSt1->getPointerReg(), MRI);
BaseIndexOffset BasePtr1 = getPointerInfo(LdSt2->getPointerReg(), MRI);
if (!BasePtr0.getBase().isValid() || !BasePtr1.getBase().isValid())
return false;
LocationSize Size1 = LdSt1->getMemSize();
LocationSize Size2 = LdSt2->getMemSize();
int64_t PtrDiff;
if (BasePtr0.getBase() == BasePtr1.getBase() && BasePtr0.hasValidOffset() &&
BasePtr1.hasValidOffset()) {
PtrDiff = BasePtr1.getOffset() - BasePtr0.getOffset();
// If the size of memory access is unknown, do not use it to do analysis.
// One example of unknown size memory access is to load/store scalable
// vector objects on the stack.
// BasePtr1 is PtrDiff away from BasePtr0. They alias if none of the
// following situations arise:
if (PtrDiff >= 0 && Size1.hasValue() && !Size1.isScalable()) {
// [----BasePtr0----]
// [---BasePtr1--]
// ========PtrDiff========>
IsAlias = !((int64_t)Size1.getValue() <= PtrDiff);
return true;
}
if (PtrDiff < 0 && Size2.hasValue() && !Size2.isScalable()) {
// [----BasePtr0----]
// [---BasePtr1--]
// =====(-PtrDiff)====>
IsAlias = !((PtrDiff + (int64_t)Size2.getValue()) <= 0);
return true;
}
return false;
}
// If both BasePtr0 and BasePtr1 are FrameIndexes, we will not be
// able to calculate their relative offset if at least one arises
// from an alloca. However, these allocas cannot overlap and we
// can infer there is no alias.
auto *Base0Def = getDefIgnoringCopies(BasePtr0.getBase(), MRI);
auto *Base1Def = getDefIgnoringCopies(BasePtr1.getBase(), MRI);
if (!Base0Def || !Base1Def)
return false; // Couldn't tell anything.
if (Base0Def->getOpcode() != Base1Def->getOpcode())
return false;
if (Base0Def->getOpcode() == TargetOpcode::G_FRAME_INDEX) {
MachineFrameInfo &MFI = Base0Def->getMF()->getFrameInfo();
// If the bases have the same frame index but we couldn't find a
// constant offset, (indices are different) be conservative.
if (Base0Def != Base1Def &&
(!MFI.isFixedObjectIndex(Base0Def->getOperand(1).getIndex()) ||
!MFI.isFixedObjectIndex(Base1Def->getOperand(1).getIndex()))) {
IsAlias = false;
return true;
}
}
// This implementation is a lot more primitive than the SDAG one for now.
// FIXME: what about constant pools?
if (Base0Def->getOpcode() == TargetOpcode::G_GLOBAL_VALUE) {
auto GV0 = Base0Def->getOperand(1).getGlobal();
auto GV1 = Base1Def->getOperand(1).getGlobal();
if (GV0 != GV1) {
IsAlias = false;
return true;
}
}
// Can't tell anything about aliasing.
return false;
}
bool GISelAddressing::instMayAlias(const MachineInstr &MI,
const MachineInstr &Other,
MachineRegisterInfo &MRI,
AliasAnalysis *AA) {
struct MemUseCharacteristics {
bool IsVolatile;
bool IsAtomic;
Register BasePtr;
int64_t Offset;
LocationSize NumBytes;
MachineMemOperand *MMO;
};
auto getCharacteristics =
[&](const MachineInstr *MI) -> MemUseCharacteristics {
if (const auto *LS = dyn_cast<GLoadStore>(MI)) {
Register BaseReg;
int64_t Offset = 0;
// No pre/post-inc addressing modes are considered here, unlike in SDAG.
if (!mi_match(LS->getPointerReg(), MRI,
m_GPtrAdd(m_Reg(BaseReg), m_ICst(Offset)))) {
BaseReg = LS->getPointerReg();
Offset = 0;
}
LocationSize Size = LS->getMMO().getSize();
return {LS->isVolatile(), LS->isAtomic(), BaseReg,
Offset /*base offset*/, Size, &LS->getMMO()};
}
// FIXME: support recognizing lifetime instructions.
// Default.
return {false /*isvolatile*/,
/*isAtomic*/ false,
Register(),
(int64_t)0 /*offset*/,
LocationSize::beforeOrAfterPointer() /*size*/,
(MachineMemOperand *)nullptr};
};
MemUseCharacteristics MUC0 = getCharacteristics(&MI),
MUC1 = getCharacteristics(&Other);
// If they are to the same address, then they must be aliases.
if (MUC0.BasePtr.isValid() && MUC0.BasePtr == MUC1.BasePtr &&
MUC0.Offset == MUC1.Offset)
return true;
// If they are both volatile then they cannot be reordered.
if (MUC0.IsVolatile && MUC1.IsVolatile)
return true;
// Be conservative about atomics for the moment
// TODO: This is way overconservative for unordered atomics (see D66309)
if (MUC0.IsAtomic && MUC1.IsAtomic)
return true;
// If one operation reads from invariant memory, and the other may store, they
// cannot alias.
if (MUC0.MMO && MUC1.MMO) {
if ((MUC0.MMO->isInvariant() && MUC1.MMO->isStore()) ||
(MUC1.MMO->isInvariant() && MUC0.MMO->isStore()))
return false;
}
// If NumBytes is scalable and offset is not 0, conservatively return may
// alias
if ((MUC0.NumBytes.isScalable() && MUC0.Offset != 0) ||
(MUC1.NumBytes.isScalable() && MUC1.Offset != 0))
return true;
const bool BothNotScalable =
!MUC0.NumBytes.isScalable() && !MUC1.NumBytes.isScalable();
// Try to prove that there is aliasing, or that there is no aliasing. Either
// way, we can return now. If nothing can be proved, proceed with more tests.
bool IsAlias;
if (BothNotScalable &&
GISelAddressing::aliasIsKnownForLoadStore(MI, Other, IsAlias, MRI))
return IsAlias;
// The following all rely on MMO0 and MMO1 being valid.
if (!MUC0.MMO || !MUC1.MMO)
return true;
// FIXME: port the alignment based alias analysis from SDAG's isAlias().
int64_t SrcValOffset0 = MUC0.MMO->getOffset();
int64_t SrcValOffset1 = MUC1.MMO->getOffset();
LocationSize Size0 = MUC0.NumBytes;
LocationSize Size1 = MUC1.NumBytes;
if (AA && MUC0.MMO->getValue() && MUC1.MMO->getValue() && Size0.hasValue() &&
Size1.hasValue()) {
// Use alias analysis information.
int64_t MinOffset = std::min(SrcValOffset0, SrcValOffset1);
int64_t Overlap0 =
Size0.getValue().getKnownMinValue() + SrcValOffset0 - MinOffset;
int64_t Overlap1 =
Size1.getValue().getKnownMinValue() + SrcValOffset1 - MinOffset;
LocationSize Loc0 =
Size0.isScalable() ? Size0 : LocationSize::precise(Overlap0);
LocationSize Loc1 =
Size1.isScalable() ? Size1 : LocationSize::precise(Overlap1);
if (AA->isNoAlias(
MemoryLocation(MUC0.MMO->getValue(), Loc0, MUC0.MMO->getAAInfo()),
MemoryLocation(MUC1.MMO->getValue(), Loc1, MUC1.MMO->getAAInfo())))
return false;
}
// Otherwise we have to assume they alias.
return true;
}
/// Returns true if the instruction creates an unavoidable hazard that
/// forces a boundary between store merge candidates.
static bool isInstHardMergeHazard(MachineInstr &MI) {
return MI.hasUnmodeledSideEffects() || MI.hasOrderedMemoryRef();
}
bool LoadStoreOpt::mergeStores(SmallVectorImpl<GStore *> &StoresToMerge) {
// Try to merge all the stores in the vector, splitting into separate segments
// as necessary.
assert(StoresToMerge.size() > 1 && "Expected multiple stores to merge");
LLT OrigTy = MRI->getType(StoresToMerge[0]->getValueReg());
LLT PtrTy = MRI->getType(StoresToMerge[0]->getPointerReg());
unsigned AS = PtrTy.getAddressSpace();
// Ensure the legal store info is computed for this address space.
initializeStoreMergeTargetInfo(AS);
const auto &LegalSizes = LegalStoreSizes[AS];
#ifndef NDEBUG
for (auto *StoreMI : StoresToMerge)
assert(MRI->getType(StoreMI->getValueReg()) == OrigTy);
#endif
const auto &DL = MF->getFunction().getParent()->getDataLayout();
bool AnyMerged = false;
do {
unsigned NumPow2 = llvm::bit_floor(StoresToMerge.size());
unsigned MaxSizeBits = NumPow2 * OrigTy.getSizeInBits().getFixedValue();
// Compute the biggest store we can generate to handle the number of stores.
unsigned MergeSizeBits;
for (MergeSizeBits = MaxSizeBits; MergeSizeBits > 1; MergeSizeBits /= 2) {
LLT StoreTy = LLT::scalar(MergeSizeBits);
EVT StoreEVT =
getApproximateEVTForLLT(StoreTy, DL, MF->getFunction().getContext());
if (LegalSizes.size() > MergeSizeBits && LegalSizes[MergeSizeBits] &&
TLI->canMergeStoresTo(AS, StoreEVT, *MF) &&
(TLI->isTypeLegal(StoreEVT)))
break; // We can generate a MergeSize bits store.
}
if (MergeSizeBits <= OrigTy.getSizeInBits())
return AnyMerged; // No greater merge.
unsigned NumStoresToMerge = MergeSizeBits / OrigTy.getSizeInBits();
// Perform the actual merging.
SmallVector<GStore *, 8> SingleMergeStores(
StoresToMerge.begin(), StoresToMerge.begin() + NumStoresToMerge);
AnyMerged |= doSingleStoreMerge(SingleMergeStores);
StoresToMerge.erase(StoresToMerge.begin(),
StoresToMerge.begin() + NumStoresToMerge);
} while (StoresToMerge.size() > 1);
return AnyMerged;
}
bool LoadStoreOpt::isLegalOrBeforeLegalizer(const LegalityQuery &Query,
MachineFunction &MF) const {
auto Action = LI->getAction(Query).Action;
// If the instruction is unsupported, it can't be legalized at all.
if (Action == LegalizeActions::Unsupported)
return false;
return IsPreLegalizer || Action == LegalizeAction::Legal;
}
bool LoadStoreOpt::doSingleStoreMerge(SmallVectorImpl<GStore *> &Stores) {
assert(Stores.size() > 1);
// We know that all the stores are consecutive and there are no aliasing
// operations in the range. However, the values that are being stored may be
// generated anywhere before each store. To ensure we have the values
// available, we materialize the wide value and new store at the place of the
// final store in the merge sequence.
GStore *FirstStore = Stores[0];
const unsigned NumStores = Stores.size();
LLT SmallTy = MRI->getType(FirstStore->getValueReg());
LLT WideValueTy =
LLT::scalar(NumStores * SmallTy.getSizeInBits().getFixedValue());
// For each store, compute pairwise merged debug locs.
DebugLoc MergedLoc = Stores.front()->getDebugLoc();
for (auto *Store : drop_begin(Stores))
MergedLoc = DILocation::getMergedLocation(MergedLoc, Store->getDebugLoc());
Builder.setInstr(*Stores.back());
Builder.setDebugLoc(MergedLoc);
// If all of the store values are constants, then create a wide constant
// directly. Otherwise, we need to generate some instructions to merge the
// existing values together into a wider type.
SmallVector<APInt, 8> ConstantVals;
for (auto *Store : Stores) {
auto MaybeCst =
getIConstantVRegValWithLookThrough(Store->getValueReg(), *MRI);
if (!MaybeCst) {
ConstantVals.clear();
break;
}
ConstantVals.emplace_back(MaybeCst->Value);
}
Register WideReg;
auto *WideMMO =
MF->getMachineMemOperand(&FirstStore->getMMO(), 0, WideValueTy);
if (ConstantVals.empty()) {
// Mimic the SDAG behaviour here and don't try to do anything for unknown
// values. In future, we should also support the cases of loads and
// extracted vector elements.
return false;
}
assert(ConstantVals.size() == NumStores);
// Check if our wide constant is legal.
if (!isLegalOrBeforeLegalizer({TargetOpcode::G_CONSTANT, {WideValueTy}}, *MF))
return false;
APInt WideConst(WideValueTy.getSizeInBits(), 0);
for (unsigned Idx = 0; Idx < ConstantVals.size(); ++Idx) {
// Insert the smaller constant into the corresponding position in the
// wider one.
WideConst.insertBits(ConstantVals[Idx], Idx * SmallTy.getSizeInBits());
}
WideReg = Builder.buildConstant(WideValueTy, WideConst).getReg(0);
auto NewStore =
Builder.buildStore(WideReg, FirstStore->getPointerReg(), *WideMMO);
(void) NewStore;
LLVM_DEBUG(dbgs() << "Merged " << Stores.size()
<< " stores into merged store: " << *NewStore);
LLVM_DEBUG(for (auto *MI : Stores) dbgs() << " " << *MI;);
NumStoresMerged += Stores.size();
MachineOptimizationRemarkEmitter MORE(*MF, nullptr);
MORE.emit([&]() {
MachineOptimizationRemark R(DEBUG_TYPE, "MergedStore",
FirstStore->getDebugLoc(),
FirstStore->getParent());
R << "Merged " << NV("NumMerged", Stores.size()) << " stores of "
<< NV("OrigWidth", SmallTy.getSizeInBytes())
<< " bytes into a single store of "
<< NV("NewWidth", WideValueTy.getSizeInBytes()) << " bytes";
return R;
});
for (auto *MI : Stores)
InstsToErase.insert(MI);
return true;
}
bool LoadStoreOpt::processMergeCandidate(StoreMergeCandidate &C) {
if (C.Stores.size() < 2) {
C.reset();
return false;
}
LLVM_DEBUG(dbgs() << "Checking store merge candidate with " << C.Stores.size()
<< " stores, starting with " << *C.Stores[0]);
// We know that the stores in the candidate are adjacent.
// Now we need to check if any potential aliasing instructions recorded
// during the search alias with load/stores added to the candidate after.
// For example, if we have the candidate:
// C.Stores = [ST1, ST2, ST3, ST4]
// and after seeing ST2 we saw a load LD1, which did not alias with ST1 or
// ST2, then we would have recorded it into the PotentialAliases structure
// with the associated index value of "1". Then we see ST3 and ST4 and add
// them to the candidate group. We know that LD1 does not alias with ST1 or
// ST2, since we already did that check. However we don't yet know if it
// may alias ST3 and ST4, so we perform those checks now.
SmallVector<GStore *> StoresToMerge;
auto DoesStoreAliasWithPotential = [&](unsigned Idx, GStore &CheckStore) {
for (auto AliasInfo : reverse(C.PotentialAliases)) {
MachineInstr *PotentialAliasOp = AliasInfo.first;
unsigned PreCheckedIdx = AliasInfo.second;
if (static_cast<unsigned>(Idx) < PreCheckedIdx) {
// Once our store index is lower than the index associated with the
// potential alias, we know that we've already checked for this alias
// and all of the earlier potential aliases too.
return false;
}
// Need to check this alias.
if (GISelAddressing::instMayAlias(CheckStore, *PotentialAliasOp, *MRI,
AA)) {
LLVM_DEBUG(dbgs() << "Potential alias " << *PotentialAliasOp
<< " detected\n");
return true;
}
}
return false;
};
// Start from the last store in the group, and check if it aliases with any
// of the potential aliasing operations in the list.
for (int StoreIdx = C.Stores.size() - 1; StoreIdx >= 0; --StoreIdx) {
auto *CheckStore = C.Stores[StoreIdx];
if (DoesStoreAliasWithPotential(StoreIdx, *CheckStore))
continue;
StoresToMerge.emplace_back(CheckStore);
}
LLVM_DEBUG(dbgs() << StoresToMerge.size()
<< " stores remaining after alias checks. Merging...\n");
// Now we've checked for aliasing hazards, merge any stores left.
C.reset();
if (StoresToMerge.size() < 2)
return false;
return mergeStores(StoresToMerge);
}
bool LoadStoreOpt::operationAliasesWithCandidate(MachineInstr &MI,
StoreMergeCandidate &C) {
if (C.Stores.empty())
return false;
return llvm::any_of(C.Stores, [&](MachineInstr *OtherMI) {
return instMayAlias(MI, *OtherMI, *MRI, AA);
});
}
void LoadStoreOpt::StoreMergeCandidate::addPotentialAlias(MachineInstr &MI) {
PotentialAliases.emplace_back(std::make_pair(&MI, Stores.size() - 1));
}
bool LoadStoreOpt::addStoreToCandidate(GStore &StoreMI,
StoreMergeCandidate &C) {
// Check if the given store writes to an adjacent address, and other
// requirements.
LLT ValueTy = MRI->getType(StoreMI.getValueReg());
LLT PtrTy = MRI->getType(StoreMI.getPointerReg());
// Only handle scalars.
if (!ValueTy.isScalar())
return false;
// Don't allow truncating stores for now.
if (StoreMI.getMemSizeInBits() != ValueTy.getSizeInBits())
return false;
// Avoid adding volatile or ordered stores to the candidate. We already have a
// check for this in instMayAlias() but that only get's called later between
// potential aliasing hazards.
if (!StoreMI.isSimple())
return false;
Register StoreAddr = StoreMI.getPointerReg();
auto BIO = getPointerInfo(StoreAddr, *MRI);
Register StoreBase = BIO.getBase();
if (C.Stores.empty()) {
C.BasePtr = StoreBase;
if (!BIO.hasValidOffset()) {
C.CurrentLowestOffset = 0;
} else {
C.CurrentLowestOffset = BIO.getOffset();
}
// This is the first store of the candidate.
// If the offset can't possibly allow for a lower addressed store with the
// same base, don't bother adding it.
if (BIO.hasValidOffset() &&
BIO.getOffset() < static_cast<int64_t>(ValueTy.getSizeInBytes()))
return false;
C.Stores.emplace_back(&StoreMI);
LLVM_DEBUG(dbgs() << "Starting a new merge candidate group with: "
<< StoreMI);
return true;
}
// Check the store is the same size as the existing ones in the candidate.
if (MRI->getType(C.Stores[0]->getValueReg()).getSizeInBits() !=
ValueTy.getSizeInBits())
return false;
if (MRI->getType(C.Stores[0]->getPointerReg()).getAddressSpace() !=
PtrTy.getAddressSpace())
return false;
// There are other stores in the candidate. Check that the store address
// writes to the next lowest adjacent address.
if (C.BasePtr != StoreBase)
return false;
// If we don't have a valid offset, we can't guarantee to be an adjacent
// offset.
if (!BIO.hasValidOffset())
return false;
if ((C.CurrentLowestOffset -
static_cast<int64_t>(ValueTy.getSizeInBytes())) != BIO.getOffset())
return false;
// This writes to an adjacent address. Allow it.
C.Stores.emplace_back(&StoreMI);
C.CurrentLowestOffset = C.CurrentLowestOffset - ValueTy.getSizeInBytes();
LLVM_DEBUG(dbgs() << "Candidate added store: " << StoreMI);
return true;
}
bool LoadStoreOpt::mergeBlockStores(MachineBasicBlock &MBB) {
bool Changed = false;
// Walk through the block bottom-up, looking for merging candidates.
StoreMergeCandidate Candidate;
for (MachineInstr &MI : llvm::reverse(MBB)) {
if (InstsToErase.contains(&MI))
continue;
if (auto *StoreMI = dyn_cast<GStore>(&MI)) {
// We have a G_STORE. Add it to the candidate if it writes to an adjacent
// address.
if (!addStoreToCandidate(*StoreMI, Candidate)) {
// Store wasn't eligible to be added. May need to record it as a
// potential alias.
if (operationAliasesWithCandidate(*StoreMI, Candidate)) {
Changed |= processMergeCandidate(Candidate);
continue;
}
Candidate.addPotentialAlias(*StoreMI);
}
continue;
}
// If we don't have any stores yet, this instruction can't pose a problem.
if (Candidate.Stores.empty())
continue;
// We're dealing with some other kind of instruction.
if (isInstHardMergeHazard(MI)) {
Changed |= processMergeCandidate(Candidate);
Candidate.Stores.clear();
continue;
}
if (!MI.mayLoadOrStore())
continue;
if (operationAliasesWithCandidate(MI, Candidate)) {
// We have a potential alias, so process the current candidate if we can
// and then continue looking for a new candidate.
Changed |= processMergeCandidate(Candidate);
continue;
}
// Record this instruction as a potential alias for future stores that are
// added to the candidate.
Candidate.addPotentialAlias(MI);
}
// Process any candidate left after finishing searching the entire block.
Changed |= processMergeCandidate(Candidate);
// Erase instructions now that we're no longer iterating over the block.
for (auto *MI : InstsToErase)
MI->eraseFromParent();
InstsToErase.clear();
return Changed;
}
/// Check if the store \p Store is a truncstore that can be merged. That is,
/// it's a store of a shifted value of \p SrcVal. If \p SrcVal is an empty
/// Register then it does not need to match and SrcVal is set to the source
/// value found.
/// On match, returns the start byte offset of the \p SrcVal that is being
/// stored.
static std::optional<int64_t>
getTruncStoreByteOffset(GStore &Store, Register &SrcVal,
MachineRegisterInfo &MRI) {
Register TruncVal;
if (!mi_match(Store.getValueReg(), MRI, m_GTrunc(m_Reg(TruncVal))))
return std::nullopt;
// The shift amount must be a constant multiple of the narrow type.
// It is translated to the offset address in the wide source value "y".
//
// x = G_LSHR y, ShiftAmtC
// s8 z = G_TRUNC x
// store z, ...
Register FoundSrcVal;
int64_t ShiftAmt;
if (!mi_match(TruncVal, MRI,
m_any_of(m_GLShr(m_Reg(FoundSrcVal), m_ICst(ShiftAmt)),
m_GAShr(m_Reg(FoundSrcVal), m_ICst(ShiftAmt))))) {
if (!SrcVal.isValid() || TruncVal == SrcVal) {
if (!SrcVal.isValid())
SrcVal = TruncVal;
return 0; // If it's the lowest index store.
}
return std::nullopt;
}
unsigned NarrowBits = Store.getMMO().getMemoryType().getScalarSizeInBits();
if (ShiftAmt % NarrowBits != 0)
return std::nullopt;
const unsigned Offset = ShiftAmt / NarrowBits;
if (SrcVal.isValid() && FoundSrcVal != SrcVal)
return std::nullopt;
if (!SrcVal.isValid())
SrcVal = FoundSrcVal;
else if (MRI.getType(SrcVal) != MRI.getType(FoundSrcVal))
return std::nullopt;
return Offset;
}
/// Match a pattern where a wide type scalar value is stored by several narrow
/// stores. Fold it into a single store or a BSWAP and a store if the targets
/// supports it.
///
/// Assuming little endian target:
/// i8 *p = ...
/// i32 val = ...
/// p[0] = (val >> 0) & 0xFF;
/// p[1] = (val >> 8) & 0xFF;
/// p[2] = (val >> 16) & 0xFF;
/// p[3] = (val >> 24) & 0xFF;
/// =>
/// *((i32)p) = val;
///
/// i8 *p = ...
/// i32 val = ...
/// p[0] = (val >> 24) & 0xFF;
/// p[1] = (val >> 16) & 0xFF;
/// p[2] = (val >> 8) & 0xFF;
/// p[3] = (val >> 0) & 0xFF;
/// =>
/// *((i32)p) = BSWAP(val);
bool LoadStoreOpt::mergeTruncStore(GStore &StoreMI,
SmallPtrSetImpl<GStore *> &DeletedStores) {
LLT MemTy = StoreMI.getMMO().getMemoryType();
// We only handle merging simple stores of 1-4 bytes.
if (!MemTy.isScalar())
return false;
switch (MemTy.getSizeInBits()) {
case 8:
case 16:
case 32:
break;
default:
return false;
}
if (!StoreMI.isSimple())
return false;
// We do a simple search for mergeable stores prior to this one.
// Any potential alias hazard along the way terminates the search.
SmallVector<GStore *> FoundStores;
// We're looking for:
// 1) a (store(trunc(...)))
// 2) of an LSHR/ASHR of a single wide value, by the appropriate shift to get
// the partial value stored.
// 3) where the offsets form either a little or big-endian sequence.
auto &LastStore = StoreMI;
// The single base pointer that all stores must use.
Register BaseReg;
int64_t LastOffset;
if (!mi_match(LastStore.getPointerReg(), *MRI,
m_GPtrAdd(m_Reg(BaseReg), m_ICst(LastOffset)))) {
BaseReg = LastStore.getPointerReg();
LastOffset = 0;
}
GStore *LowestIdxStore = &LastStore;
int64_t LowestIdxOffset = LastOffset;
Register WideSrcVal;
auto LowestShiftAmt = getTruncStoreByteOffset(LastStore, WideSrcVal, *MRI);
if (!LowestShiftAmt)
return false; // Didn't match a trunc.
assert(WideSrcVal.isValid());
LLT WideStoreTy = MRI->getType(WideSrcVal);
// The wide type might not be a multiple of the memory type, e.g. s48 and s32.
if (WideStoreTy.getSizeInBits() % MemTy.getSizeInBits() != 0)
return false;
const unsigned NumStoresRequired =
WideStoreTy.getSizeInBits() / MemTy.getSizeInBits();
SmallVector<int64_t, 8> OffsetMap(NumStoresRequired, INT64_MAX);
OffsetMap[*LowestShiftAmt] = LastOffset;
FoundStores.emplace_back(&LastStore);
const int MaxInstsToCheck = 10;
int NumInstsChecked = 0;
for (auto II = ++LastStore.getReverseIterator();
II != LastStore.getParent()->rend() && NumInstsChecked < MaxInstsToCheck;
++II) {
NumInstsChecked++;
GStore *NewStore;
if ((NewStore = dyn_cast<GStore>(&*II))) {
if (NewStore->getMMO().getMemoryType() != MemTy || !NewStore->isSimple())
break;
} else if (II->isLoadFoldBarrier() || II->mayLoad()) {
break;
} else {
continue; // This is a safe instruction we can look past.
}
Register NewBaseReg;
int64_t MemOffset;
// Check we're storing to the same base + some offset.
if (!mi_match(NewStore->getPointerReg(), *MRI,
m_GPtrAdd(m_Reg(NewBaseReg), m_ICst(MemOffset)))) {
NewBaseReg = NewStore->getPointerReg();
MemOffset = 0;
}
if (BaseReg != NewBaseReg)
break;
auto ShiftByteOffset = getTruncStoreByteOffset(*NewStore, WideSrcVal, *MRI);
if (!ShiftByteOffset)
break;
if (MemOffset < LowestIdxOffset) {
LowestIdxOffset = MemOffset;
LowestIdxStore = NewStore;
}
// Map the offset in the store and the offset in the combined value, and
// early return if it has been set before.
if (*ShiftByteOffset < 0 || *ShiftByteOffset >= NumStoresRequired ||
OffsetMap[*ShiftByteOffset] != INT64_MAX)
break;
OffsetMap[*ShiftByteOffset] = MemOffset;
FoundStores.emplace_back(NewStore);
// Reset counter since we've found a matching inst.
NumInstsChecked = 0;
if (FoundStores.size() == NumStoresRequired)
break;
}
if (FoundStores.size() != NumStoresRequired) {
if (FoundStores.size() == 1)
return false;
// We didn't find enough stores to merge into the size of the original
// source value, but we may be able to generate a smaller store if we
// truncate the source value.
WideStoreTy = LLT::scalar(FoundStores.size() * MemTy.getScalarSizeInBits());
}
unsigned NumStoresFound = FoundStores.size();
const auto &DL = LastStore.getMF()->getDataLayout();
auto &C = LastStore.getMF()->getFunction().getContext();
// Check that a store of the wide type is both allowed and fast on the target
unsigned Fast = 0;
bool Allowed = TLI->allowsMemoryAccess(
C, DL, WideStoreTy, LowestIdxStore->getMMO(), &Fast);
if (!Allowed || !Fast)
return false;
// Check if the pieces of the value are going to the expected places in memory
// to merge the stores.
unsigned NarrowBits = MemTy.getScalarSizeInBits();
auto checkOffsets = [&](bool MatchLittleEndian) {
if (MatchLittleEndian) {
for (unsigned i = 0; i != NumStoresFound; ++i)
if (OffsetMap[i] != i * (NarrowBits / 8) + LowestIdxOffset)
return false;
} else { // MatchBigEndian by reversing loop counter.
for (unsigned i = 0, j = NumStoresFound - 1; i != NumStoresFound;
++i, --j)
if (OffsetMap[j] != i * (NarrowBits / 8) + LowestIdxOffset)
return false;
}
return true;
};
// Check if the offsets line up for the native data layout of this target.
bool NeedBswap = false;
bool NeedRotate = false;
if (!checkOffsets(DL.isLittleEndian())) {
// Special-case: check if byte offsets line up for the opposite endian.
if (NarrowBits == 8 && checkOffsets(DL.isBigEndian()))
NeedBswap = true;
else if (NumStoresFound == 2 && checkOffsets(DL.isBigEndian()))
NeedRotate = true;
else
return false;
}
if (NeedBswap &&
!isLegalOrBeforeLegalizer({TargetOpcode::G_BSWAP, {WideStoreTy}}, *MF))
return false;
if (NeedRotate &&
!isLegalOrBeforeLegalizer(
{TargetOpcode::G_ROTR, {WideStoreTy, WideStoreTy}}, *MF))
return false;
Builder.setInstrAndDebugLoc(StoreMI);
if (WideStoreTy != MRI->getType(WideSrcVal))
WideSrcVal = Builder.buildTrunc(WideStoreTy, WideSrcVal).getReg(0);
if (NeedBswap) {
WideSrcVal = Builder.buildBSwap(WideStoreTy, WideSrcVal).getReg(0);
} else if (NeedRotate) {
assert(WideStoreTy.getSizeInBits() % 2 == 0 &&
"Unexpected type for rotate");
auto RotAmt =
Builder.buildConstant(WideStoreTy, WideStoreTy.getSizeInBits() / 2);
WideSrcVal =
Builder.buildRotateRight(WideStoreTy, WideSrcVal, RotAmt).getReg(0);
}
Builder.buildStore(WideSrcVal, LowestIdxStore->getPointerReg(),
LowestIdxStore->getMMO().getPointerInfo(),
LowestIdxStore->getMMO().getAlign());
// Erase the old stores.
for (auto *ST : FoundStores) {
ST->eraseFromParent();
DeletedStores.insert(ST);
}
return true;
}
bool LoadStoreOpt::mergeTruncStoresBlock(MachineBasicBlock &BB) {
bool Changed = false;
SmallVector<GStore *, 16> Stores;
SmallPtrSet<GStore *, 8> DeletedStores;
// Walk up the block so we can see the most eligible stores.
for (MachineInstr &MI : llvm::reverse(BB))
if (auto *StoreMI = dyn_cast<GStore>(&MI))
Stores.emplace_back(StoreMI);
for (auto *StoreMI : Stores) {
if (DeletedStores.count(StoreMI))
continue;
if (mergeTruncStore(*StoreMI, DeletedStores))
Changed = true;
}
return Changed;
}
bool LoadStoreOpt::mergeFunctionStores(MachineFunction &MF) {
bool Changed = false;
for (auto &BB : MF){
Changed |= mergeBlockStores(BB);
Changed |= mergeTruncStoresBlock(BB);
}
// Erase all dead instructions left over by the merging.
if (Changed) {
for (auto &BB : MF) {
for (auto &I : make_early_inc_range(make_range(BB.rbegin(), BB.rend()))) {
if (isTriviallyDead(I, *MRI))
I.eraseFromParent();
}
}
}
return Changed;
}
void LoadStoreOpt::initializeStoreMergeTargetInfo(unsigned AddrSpace) {
// Query the legalizer info to record what store types are legal.
// We record this because we don't want to bother trying to merge stores into
// illegal ones, which would just result in being split again.
if (LegalStoreSizes.count(AddrSpace)) {
assert(LegalStoreSizes[AddrSpace].any());
return; // Already cached sizes for this address space.
}
// Need to reserve at least MaxStoreSizeToForm + 1 bits.
BitVector LegalSizes(MaxStoreSizeToForm * 2);
const auto &LI = *MF->getSubtarget().getLegalizerInfo();
const auto &DL = MF->getFunction().getParent()->getDataLayout();
Type *IRPtrTy = PointerType::get(MF->getFunction().getContext(), AddrSpace);
LLT PtrTy = getLLTForType(*IRPtrTy, DL);
// We assume that we're not going to be generating any stores wider than
// MaxStoreSizeToForm bits for now.
for (unsigned Size = 2; Size <= MaxStoreSizeToForm; Size *= 2) {
LLT Ty = LLT::scalar(Size);
SmallVector<LegalityQuery::MemDesc, 2> MemDescrs(
{{Ty, Ty.getSizeInBits(), AtomicOrdering::NotAtomic}});
SmallVector<LLT> StoreTys({Ty, PtrTy});
LegalityQuery Q(TargetOpcode::G_STORE, StoreTys, MemDescrs);
LegalizeActionStep ActionStep = LI.getAction(Q);
if (ActionStep.Action == LegalizeActions::Legal)
LegalSizes.set(Size);
}
assert(LegalSizes.any() && "Expected some store sizes to be legal!");
LegalStoreSizes[AddrSpace] = LegalSizes;
}
bool LoadStoreOpt::runOnMachineFunction(MachineFunction &MF) {
// If the ISel pipeline failed, do not bother running that pass.
if (MF.getProperties().hasProperty(
MachineFunctionProperties::Property::FailedISel))
return false;
LLVM_DEBUG(dbgs() << "Begin memory optimizations for: " << MF.getName()
<< '\n');
init(MF);
bool Changed = false;
Changed |= mergeFunctionStores(MF);
LegalStoreSizes.clear();
return Changed;
}
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