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

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//===----------------------- AlignmentFromAssumptions.cpp -----------------===//
// Set Load/Store Alignments From Assumptions
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements a ScalarEvolution-based transformation to set
// the alignments of load, stores and memory intrinsics based on the truth
// expressions of assume intrinsics. The primary motivation is to handle
// complex alignment assumptions that apply to vector loads and stores that
// appear after vectorization and unrolling.
//
//===----------------------------------------------------------------------===//
#define AA_NAME "alignment-from-assumptions"
#define DEBUG_TYPE AA_NAME
#include "llvm/Transforms/Scalar.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/Module.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
STATISTIC(NumLoadAlignChanged,
"Number of loads changed by alignment assumptions");
STATISTIC(NumStoreAlignChanged,
"Number of stores changed by alignment assumptions");
STATISTIC(NumMemIntAlignChanged,
"Number of memory intrinsics changed by alignment assumptions");
namespace {
struct AlignmentFromAssumptions : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
AlignmentFromAssumptions() : FunctionPass(ID) {
initializeAlignmentFromAssumptionsPass(*PassRegistry::getPassRegistry());
}
bool runOnFunction(Function &F) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<AssumptionCacheTracker>();
AU.addRequired<ScalarEvolutionWrapperPass>();
AU.addRequired<DominatorTreeWrapperPass>();
AU.setPreservesCFG();
AU.addPreserved<LoopInfoWrapperPass>();
AU.addPreserved<DominatorTreeWrapperPass>();
AU.addPreserved<ScalarEvolutionWrapperPass>();
}
// For memory transfers, we need a common alignment for both the source and
// destination. If we have a new alignment for only one operand of a transfer
// instruction, save it in these maps. If we reach the other operand through
// another assumption later, then we may change the alignment at that point.
DenseMap<MemTransferInst *, unsigned> NewDestAlignments, NewSrcAlignments;
ScalarEvolution *SE;
DominatorTree *DT;
bool extractAlignmentInfo(CallInst *I, Value *&AAPtr, const SCEV *&AlignSCEV,
const SCEV *&OffSCEV);
bool processAssumption(CallInst *I);
};
}
char AlignmentFromAssumptions::ID = 0;
static const char aip_name[] = "Alignment from assumptions";
INITIALIZE_PASS_BEGIN(AlignmentFromAssumptions, AA_NAME,
aip_name, false, false)
INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
INITIALIZE_PASS_END(AlignmentFromAssumptions, AA_NAME,
aip_name, false, false)
FunctionPass *llvm::createAlignmentFromAssumptionsPass() {
return new AlignmentFromAssumptions();
}
// Given an expression for the (constant) alignment, AlignSCEV, and an
// expression for the displacement between a pointer and the aligned address,
// DiffSCEV, compute the alignment of the displaced pointer if it can be reduced
// to a constant. Using SCEV to compute alignment handles the case where
// DiffSCEV is a recurrence with constant start such that the aligned offset
// is constant. e.g. {16,+,32} % 32 -> 16.
static unsigned getNewAlignmentDiff(const SCEV *DiffSCEV,
const SCEV *AlignSCEV,
ScalarEvolution *SE) {
// DiffUnits = Diff % int64_t(Alignment)
const SCEV *DiffAlignDiv = SE->getUDivExpr(DiffSCEV, AlignSCEV);
const SCEV *DiffAlign = SE->getMulExpr(DiffAlignDiv, AlignSCEV);
const SCEV *DiffUnitsSCEV = SE->getMinusSCEV(DiffAlign, DiffSCEV);
DEBUG(dbgs() << "\talignment relative to " << *AlignSCEV << " is " <<
*DiffUnitsSCEV << " (diff: " << *DiffSCEV << ")\n");
if (const SCEVConstant *ConstDUSCEV =
dyn_cast<SCEVConstant>(DiffUnitsSCEV)) {
int64_t DiffUnits = ConstDUSCEV->getValue()->getSExtValue();
// If the displacement is an exact multiple of the alignment, then the
// displaced pointer has the same alignment as the aligned pointer, so
// return the alignment value.
if (!DiffUnits)
return (unsigned)
cast<SCEVConstant>(AlignSCEV)->getValue()->getSExtValue();
// If the displacement is not an exact multiple, but the remainder is a
// constant, then return this remainder (but only if it is a power of 2).
uint64_t DiffUnitsAbs = std::abs(DiffUnits);
if (isPowerOf2_64(DiffUnitsAbs))
return (unsigned) DiffUnitsAbs;
}
return 0;
}
// There is an address given by an offset OffSCEV from AASCEV which has an
// alignment AlignSCEV. Use that information, if possible, to compute a new
// alignment for Ptr.
static unsigned getNewAlignment(const SCEV *AASCEV, const SCEV *AlignSCEV,
const SCEV *OffSCEV, Value *Ptr,
ScalarEvolution *SE) {
const SCEV *PtrSCEV = SE->getSCEV(Ptr);
const SCEV *DiffSCEV = SE->getMinusSCEV(PtrSCEV, AASCEV);
// On 32-bit platforms, DiffSCEV might now have type i32 -- we've always
// sign-extended OffSCEV to i64, so make sure they agree again.
DiffSCEV = SE->getNoopOrSignExtend(DiffSCEV, OffSCEV->getType());
// What we really want to know is the overall offset to the aligned
// address. This address is displaced by the provided offset.
DiffSCEV = SE->getMinusSCEV(DiffSCEV, OffSCEV);
DEBUG(dbgs() << "AFI: alignment of " << *Ptr << " relative to " <<
*AlignSCEV << " and offset " << *OffSCEV <<
" using diff " << *DiffSCEV << "\n");
unsigned NewAlignment = getNewAlignmentDiff(DiffSCEV, AlignSCEV, SE);
DEBUG(dbgs() << "\tnew alignment: " << NewAlignment << "\n");
if (NewAlignment) {
return NewAlignment;
} else if (const SCEVAddRecExpr *DiffARSCEV =
dyn_cast<SCEVAddRecExpr>(DiffSCEV)) {
// The relative offset to the alignment assumption did not yield a constant,
// but we should try harder: if we assume that a is 32-byte aligned, then in
// for (i = 0; i < 1024; i += 4) r += a[i]; not all of the loads from a are
// 32-byte aligned, but instead alternate between 32 and 16-byte alignment.
// As a result, the new alignment will not be a constant, but can still
// be improved over the default (of 4) to 16.
const SCEV *DiffStartSCEV = DiffARSCEV->getStart();
const SCEV *DiffIncSCEV = DiffARSCEV->getStepRecurrence(*SE);
DEBUG(dbgs() << "\ttrying start/inc alignment using start " <<
*DiffStartSCEV << " and inc " << *DiffIncSCEV << "\n");
// Now compute the new alignment using the displacement to the value in the
// first iteration, and also the alignment using the per-iteration delta.
// If these are the same, then use that answer. Otherwise, use the smaller
// one, but only if it divides the larger one.
NewAlignment = getNewAlignmentDiff(DiffStartSCEV, AlignSCEV, SE);
unsigned NewIncAlignment = getNewAlignmentDiff(DiffIncSCEV, AlignSCEV, SE);
DEBUG(dbgs() << "\tnew start alignment: " << NewAlignment << "\n");
DEBUG(dbgs() << "\tnew inc alignment: " << NewIncAlignment << "\n");
if (!NewAlignment || !NewIncAlignment) {
return 0;
} else if (NewAlignment > NewIncAlignment) {
if (NewAlignment % NewIncAlignment == 0) {
DEBUG(dbgs() << "\tnew start/inc alignment: " <<
NewIncAlignment << "\n");
return NewIncAlignment;
}
} else if (NewIncAlignment > NewAlignment) {
if (NewIncAlignment % NewAlignment == 0) {
DEBUG(dbgs() << "\tnew start/inc alignment: " <<
NewAlignment << "\n");
return NewAlignment;
}
} else if (NewIncAlignment == NewAlignment) {
DEBUG(dbgs() << "\tnew start/inc alignment: " <<
NewAlignment << "\n");
return NewAlignment;
}
}
return 0;
}
bool AlignmentFromAssumptions::extractAlignmentInfo(CallInst *I,
Value *&AAPtr, const SCEV *&AlignSCEV,
const SCEV *&OffSCEV) {
// An alignment assume must be a statement about the least-significant
// bits of the pointer being zero, possibly with some offset.
ICmpInst *ICI = dyn_cast<ICmpInst>(I->getArgOperand(0));
if (!ICI)
return false;
// This must be an expression of the form: x & m == 0.
if (ICI->getPredicate() != ICmpInst::ICMP_EQ)
return false;
// Swap things around so that the RHS is 0.
Value *CmpLHS = ICI->getOperand(0);
Value *CmpRHS = ICI->getOperand(1);
const SCEV *CmpLHSSCEV = SE->getSCEV(CmpLHS);
const SCEV *CmpRHSSCEV = SE->getSCEV(CmpRHS);
if (CmpLHSSCEV->isZero())
std::swap(CmpLHS, CmpRHS);
else if (!CmpRHSSCEV->isZero())
return false;
BinaryOperator *CmpBO = dyn_cast<BinaryOperator>(CmpLHS);
if (!CmpBO || CmpBO->getOpcode() != Instruction::And)
return false;
// Swap things around so that the right operand of the and is a constant
// (the mask); we cannot deal with variable masks.
Value *AndLHS = CmpBO->getOperand(0);
Value *AndRHS = CmpBO->getOperand(1);
const SCEV *AndLHSSCEV = SE->getSCEV(AndLHS);
const SCEV *AndRHSSCEV = SE->getSCEV(AndRHS);
if (isa<SCEVConstant>(AndLHSSCEV)) {
std::swap(AndLHS, AndRHS);
std::swap(AndLHSSCEV, AndRHSSCEV);
}
const SCEVConstant *MaskSCEV = dyn_cast<SCEVConstant>(AndRHSSCEV);
if (!MaskSCEV)
return false;
// The mask must have some trailing ones (otherwise the condition is
// trivial and tells us nothing about the alignment of the left operand).
unsigned TrailingOnes =
MaskSCEV->getValue()->getValue().countTrailingOnes();
if (!TrailingOnes)
return false;
// Cap the alignment at the maximum with which LLVM can deal (and make sure
// we don't overflow the shift).
uint64_t Alignment;
TrailingOnes = std::min(TrailingOnes,
unsigned(sizeof(unsigned) * CHAR_BIT - 1));
Alignment = std::min(1u << TrailingOnes, +Value::MaximumAlignment);
Type *Int64Ty = Type::getInt64Ty(I->getParent()->getParent()->getContext());
AlignSCEV = SE->getConstant(Int64Ty, Alignment);
// The LHS might be a ptrtoint instruction, or it might be the pointer
// with an offset.
AAPtr = nullptr;
OffSCEV = nullptr;
if (PtrToIntInst *PToI = dyn_cast<PtrToIntInst>(AndLHS)) {
AAPtr = PToI->getPointerOperand();
OffSCEV = SE->getConstant(Int64Ty, 0);
} else if (const SCEVAddExpr* AndLHSAddSCEV =
dyn_cast<SCEVAddExpr>(AndLHSSCEV)) {
// Try to find the ptrtoint; subtract it and the rest is the offset.
for (SCEVAddExpr::op_iterator J = AndLHSAddSCEV->op_begin(),
JE = AndLHSAddSCEV->op_end(); J != JE; ++J)
if (const SCEVUnknown *OpUnk = dyn_cast<SCEVUnknown>(*J))
if (PtrToIntInst *PToI = dyn_cast<PtrToIntInst>(OpUnk->getValue())) {
AAPtr = PToI->getPointerOperand();
OffSCEV = SE->getMinusSCEV(AndLHSAddSCEV, *J);
break;
}
}
if (!AAPtr)
return false;
// Sign extend the offset to 64 bits (so that it is like all of the other
// expressions).
unsigned OffSCEVBits = OffSCEV->getType()->getPrimitiveSizeInBits();
if (OffSCEVBits < 64)
OffSCEV = SE->getSignExtendExpr(OffSCEV, Int64Ty);
else if (OffSCEVBits > 64)
return false;
AAPtr = AAPtr->stripPointerCasts();
return true;
}
bool AlignmentFromAssumptions::processAssumption(CallInst *ACall) {
Value *AAPtr;
const SCEV *AlignSCEV, *OffSCEV;
if (!extractAlignmentInfo(ACall, AAPtr, AlignSCEV, OffSCEV))
return false;
const SCEV *AASCEV = SE->getSCEV(AAPtr);
// Apply the assumption to all other users of the specified pointer.
SmallPtrSet<Instruction *, 32> Visited;
SmallVector<Instruction*, 16> WorkList;
for (User *J : AAPtr->users()) {
if (J == ACall)
continue;
if (Instruction *K = dyn_cast<Instruction>(J))
if (isValidAssumeForContext(ACall, K, DT))
WorkList.push_back(K);
}
while (!WorkList.empty()) {
Instruction *J = WorkList.pop_back_val();
if (LoadInst *LI = dyn_cast<LoadInst>(J)) {
unsigned NewAlignment = getNewAlignment(AASCEV, AlignSCEV, OffSCEV,
LI->getPointerOperand(), SE);
if (NewAlignment > LI->getAlignment()) {
LI->setAlignment(NewAlignment);
++NumLoadAlignChanged;
}
} else if (StoreInst *SI = dyn_cast<StoreInst>(J)) {
unsigned NewAlignment = getNewAlignment(AASCEV, AlignSCEV, OffSCEV,
SI->getPointerOperand(), SE);
if (NewAlignment > SI->getAlignment()) {
SI->setAlignment(NewAlignment);
++NumStoreAlignChanged;
}
} else if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(J)) {
unsigned NewDestAlignment = getNewAlignment(AASCEV, AlignSCEV, OffSCEV,
MI->getDest(), SE);
// For memory transfers, we need a common alignment for both the
// source and destination. If we have a new alignment for this
// instruction, but only for one operand, save it. If we reach the
// other operand through another assumption later, then we may
// change the alignment at that point.
if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(MI)) {
unsigned NewSrcAlignment = getNewAlignment(AASCEV, AlignSCEV, OffSCEV,
MTI->getSource(), SE);
DenseMap<MemTransferInst *, unsigned>::iterator DI =
NewDestAlignments.find(MTI);
unsigned AltDestAlignment = (DI == NewDestAlignments.end()) ?
0 : DI->second;
DenseMap<MemTransferInst *, unsigned>::iterator SI =
NewSrcAlignments.find(MTI);
unsigned AltSrcAlignment = (SI == NewSrcAlignments.end()) ?
0 : SI->second;
DEBUG(dbgs() << "\tmem trans: " << NewDestAlignment << " " <<
AltDestAlignment << " " << NewSrcAlignment <<
" " << AltSrcAlignment << "\n");
// Of these four alignments, pick the largest possible...
unsigned NewAlignment = 0;
if (NewDestAlignment <= std::max(NewSrcAlignment, AltSrcAlignment))
NewAlignment = std::max(NewAlignment, NewDestAlignment);
if (AltDestAlignment <= std::max(NewSrcAlignment, AltSrcAlignment))
NewAlignment = std::max(NewAlignment, AltDestAlignment);
if (NewSrcAlignment <= std::max(NewDestAlignment, AltDestAlignment))
NewAlignment = std::max(NewAlignment, NewSrcAlignment);
if (AltSrcAlignment <= std::max(NewDestAlignment, AltDestAlignment))
NewAlignment = std::max(NewAlignment, AltSrcAlignment);
if (NewAlignment > MI->getAlignment()) {
MI->setAlignment(ConstantInt::get(Type::getInt32Ty(
MI->getParent()->getContext()), NewAlignment));
++NumMemIntAlignChanged;
}
NewDestAlignments.insert(std::make_pair(MTI, NewDestAlignment));
NewSrcAlignments.insert(std::make_pair(MTI, NewSrcAlignment));
} else if (NewDestAlignment > MI->getAlignment()) {
assert((!isa<MemIntrinsic>(MI) || isa<MemSetInst>(MI)) &&
"Unknown memory intrinsic");
MI->setAlignment(ConstantInt::get(Type::getInt32Ty(
MI->getParent()->getContext()), NewDestAlignment));
++NumMemIntAlignChanged;
}
}
// Now that we've updated that use of the pointer, look for other uses of
// the pointer to update.
Visited.insert(J);
for (User *UJ : J->users()) {
Instruction *K = cast<Instruction>(UJ);
if (!Visited.count(K) && isValidAssumeForContext(ACall, K, DT))
WorkList.push_back(K);
}
}
return true;
}
bool AlignmentFromAssumptions::runOnFunction(Function &F) {
bool Changed = false;
auto &AC = getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
NewDestAlignments.clear();
NewSrcAlignments.clear();
for (auto &AssumeVH : AC.assumptions())
if (AssumeVH)
Changed |= processAssumption(cast<CallInst>(AssumeVH));
return Changed;
}
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