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

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//===- StraightLineStrengthReduce.cpp - -----------------------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements straight-line strength reduction (SLSR). Unlike loop
// strength reduction, this algorithm is designed to reduce arithmetic
// redundancy in straight-line code instead of loops. It has proven to be
// effective in simplifying arithmetic statements derived from an unrolled loop.
// It can also simplify the logic of SeparateConstOffsetFromGEP.
//
// There are many optimizations we can perform in the domain of SLSR. This file
// for now contains only an initial step. Specifically, we look for strength
// reduction candidates in the following forms:
//
// Form 1: B + i * S
// Form 2: (B + i) * S
// Form 3: &B[i * S]
//
// where S is an integer variable, and i is a constant integer. If we found two
// candidates S1 and S2 in the same form and S1 dominates S2, we may rewrite S2
// in a simpler way with respect to S1. For example,
//
// S1: X = B + i * S
// S2: Y = B + i' * S => X + (i' - i) * S
//
// S1: X = (B + i) * S
// S2: Y = (B + i') * S => X + (i' - i) * S
//
// S1: X = &B[i * S]
// S2: Y = &B[i' * S] => &X[(i' - i) * S]
//
// Note: (i' - i) * S is folded to the extent possible.
//
// This rewriting is in general a good idea. The code patterns we focus on
// usually come from loop unrolling, so (i' - i) * S is likely the same
// across iterations and can be reused. When that happens, the optimized form
// takes only one add starting from the second iteration.
//
// When such rewriting is possible, we call S1 a "basis" of S2. When S2 has
// multiple bases, we choose to rewrite S2 with respect to its "immediate"
// basis, the basis that is the closest ancestor in the dominator tree.
//
// TODO:
//
// - Floating point arithmetics when fast math is enabled.
//
// - SLSR may decrease ILP at the architecture level. Targets that are very
// sensitive to ILP may want to disable it. Having SLSR to consider ILP is
// left as future work.
//
// - When (i' - i) is constant but i and i' are not, we could still perform
// SLSR.
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/GetElementPtrTypeIterator.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Operator.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Value.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Transforms/Scalar.h"
#include <cassert>
#include <cstdint>
#include <limits>
#include <list>
#include <vector>
using namespace llvm;
using namespace PatternMatch;
static const unsigned UnknownAddressSpace =
std::numeric_limits<unsigned>::max();
namespace {
class StraightLineStrengthReduce : public FunctionPass {
public:
// SLSR candidate. Such a candidate must be in one of the forms described in
// the header comments.
struct Candidate {
enum Kind {
Invalid, // reserved for the default constructor
Add, // B + i * S
Mul, // (B + i) * S
GEP, // &B[..][i * S][..]
};
Candidate() = default;
Candidate(Kind CT, const SCEV *B, ConstantInt *Idx, Value *S,
Instruction *I)
: CandidateKind(CT), Base(B), Index(Idx), Stride(S), Ins(I) {}
Kind CandidateKind = Invalid;
const SCEV *Base = nullptr;
// Note that Index and Stride of a GEP candidate do not necessarily have the
// same integer type. In that case, during rewriting, Stride will be
// sign-extended or truncated to Index's type.
ConstantInt *Index = nullptr;
Value *Stride = nullptr;
// The instruction this candidate corresponds to. It helps us to rewrite a
// candidate with respect to its immediate basis. Note that one instruction
// can correspond to multiple candidates depending on how you associate the
// expression. For instance,
//
// (a + 1) * (b + 2)
//
// can be treated as
//
// <Base: a, Index: 1, Stride: b + 2>
//
// or
//
// <Base: b, Index: 2, Stride: a + 1>
Instruction *Ins = nullptr;
// Points to the immediate basis of this candidate, or nullptr if we cannot
// find any basis for this candidate.
Candidate *Basis = nullptr;
};
static char ID;
StraightLineStrengthReduce() : FunctionPass(ID) {
initializeStraightLineStrengthReducePass(*PassRegistry::getPassRegistry());
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<DominatorTreeWrapperPass>();
AU.addRequired<ScalarEvolutionWrapperPass>();
AU.addRequired<TargetTransformInfoWrapperPass>();
// We do not modify the shape of the CFG.
AU.setPreservesCFG();
}
bool doInitialization(Module &M) override {
DL = &M.getDataLayout();
return false;
}
bool runOnFunction(Function &F) override;
private:
// Returns true if Basis is a basis for C, i.e., Basis dominates C and they
// share the same base and stride.
bool isBasisFor(const Candidate &Basis, const Candidate &C);
// Returns whether the candidate can be folded into an addressing mode.
bool isFoldable(const Candidate &C, TargetTransformInfo *TTI,
const DataLayout *DL);
// Returns true if C is already in a simplest form and not worth being
// rewritten.
bool isSimplestForm(const Candidate &C);
// Checks whether I is in a candidate form. If so, adds all the matching forms
// to Candidates, and tries to find the immediate basis for each of them.
void allocateCandidatesAndFindBasis(Instruction *I);
// Allocate candidates and find bases for Add instructions.
void allocateCandidatesAndFindBasisForAdd(Instruction *I);
// Given I = LHS + RHS, factors RHS into i * S and makes (LHS + i * S) a
// candidate.
void allocateCandidatesAndFindBasisForAdd(Value *LHS, Value *RHS,
Instruction *I);
// Allocate candidates and find bases for Mul instructions.
void allocateCandidatesAndFindBasisForMul(Instruction *I);
// Splits LHS into Base + Index and, if succeeds, calls
// allocateCandidatesAndFindBasis.
void allocateCandidatesAndFindBasisForMul(Value *LHS, Value *RHS,
Instruction *I);
// Allocate candidates and find bases for GetElementPtr instructions.
void allocateCandidatesAndFindBasisForGEP(GetElementPtrInst *GEP);
// A helper function that scales Idx with ElementSize before invoking
// allocateCandidatesAndFindBasis.
void allocateCandidatesAndFindBasisForGEP(const SCEV *B, ConstantInt *Idx,
Value *S, uint64_t ElementSize,
Instruction *I);
// Adds the given form <CT, B, Idx, S> to Candidates, and finds its immediate
// basis.
void allocateCandidatesAndFindBasis(Candidate::Kind CT, const SCEV *B,
ConstantInt *Idx, Value *S,
Instruction *I);
// Rewrites candidate C with respect to Basis.
void rewriteCandidateWithBasis(const Candidate &C, const Candidate &Basis);
// A helper function that factors ArrayIdx to a product of a stride and a
// constant index, and invokes allocateCandidatesAndFindBasis with the
// factorings.
void factorArrayIndex(Value *ArrayIdx, const SCEV *Base, uint64_t ElementSize,
GetElementPtrInst *GEP);
// Emit code that computes the "bump" from Basis to C. If the candidate is a
// GEP and the bump is not divisible by the element size of the GEP, this
// function sets the BumpWithUglyGEP flag to notify its caller to bump the
// basis using an ugly GEP.
static Value *emitBump(const Candidate &Basis, const Candidate &C,
IRBuilder<> &Builder, const DataLayout *DL,
bool &BumpWithUglyGEP);
const DataLayout *DL = nullptr;
DominatorTree *DT = nullptr;
ScalarEvolution *SE;
TargetTransformInfo *TTI = nullptr;
std::list<Candidate> Candidates;
// Temporarily holds all instructions that are unlinked (but not deleted) by
// rewriteCandidateWithBasis. These instructions will be actually removed
// after all rewriting finishes.
std::vector<Instruction *> UnlinkedInstructions;
};
} // end anonymous namespace
char StraightLineStrengthReduce::ID = 0;
INITIALIZE_PASS_BEGIN(StraightLineStrengthReduce, "slsr",
"Straight line strength reduction", false, false)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
INITIALIZE_PASS_END(StraightLineStrengthReduce, "slsr",
"Straight line strength reduction", false, false)
FunctionPass *llvm::createStraightLineStrengthReducePass() {
return new StraightLineStrengthReduce();
}
bool StraightLineStrengthReduce::isBasisFor(const Candidate &Basis,
const Candidate &C) {
return (Basis.Ins != C.Ins && // skip the same instruction
// They must have the same type too. Basis.Base == C.Base doesn't
// guarantee their types are the same (PR23975).
Basis.Ins->getType() == C.Ins->getType() &&
// Basis must dominate C in order to rewrite C with respect to Basis.
DT->dominates(Basis.Ins->getParent(), C.Ins->getParent()) &&
// They share the same base, stride, and candidate kind.
Basis.Base == C.Base && Basis.Stride == C.Stride &&
Basis.CandidateKind == C.CandidateKind);
}
static bool isGEPFoldable(GetElementPtrInst *GEP,
const TargetTransformInfo *TTI) {
SmallVector<const Value*, 4> Indices;
for (auto I = GEP->idx_begin(); I != GEP->idx_end(); ++I)
Indices.push_back(*I);
return TTI->getGEPCost(GEP->getSourceElementType(), GEP->getPointerOperand(),
Indices) == TargetTransformInfo::TCC_Free;
}
// Returns whether (Base + Index * Stride) can be folded to an addressing mode.
static bool isAddFoldable(const SCEV *Base, ConstantInt *Index, Value *Stride,
TargetTransformInfo *TTI) {
// Index->getSExtValue() may crash if Index is wider than 64-bit.
return Index->getBitWidth() <= 64 &&
TTI->isLegalAddressingMode(Base->getType(), nullptr, 0, true,
Index->getSExtValue(), UnknownAddressSpace);
}
bool StraightLineStrengthReduce::isFoldable(const Candidate &C,
TargetTransformInfo *TTI,
const DataLayout *DL) {
if (C.CandidateKind == Candidate::Add)
return isAddFoldable(C.Base, C.Index, C.Stride, TTI);
if (C.CandidateKind == Candidate::GEP)
return isGEPFoldable(cast<GetElementPtrInst>(C.Ins), TTI);
return false;
}
// Returns true if GEP has zero or one non-zero index.
static bool hasOnlyOneNonZeroIndex(GetElementPtrInst *GEP) {
unsigned NumNonZeroIndices = 0;
for (auto I = GEP->idx_begin(); I != GEP->idx_end(); ++I) {
ConstantInt *ConstIdx = dyn_cast<ConstantInt>(*I);
if (ConstIdx == nullptr || !ConstIdx->isZero())
++NumNonZeroIndices;
}
return NumNonZeroIndices <= 1;
}
bool StraightLineStrengthReduce::isSimplestForm(const Candidate &C) {
if (C.CandidateKind == Candidate::Add) {
// B + 1 * S or B + (-1) * S
return C.Index->isOne() || C.Index->isMinusOne();
}
if (C.CandidateKind == Candidate::Mul) {
// (B + 0) * S
return C.Index->isZero();
}
if (C.CandidateKind == Candidate::GEP) {
// (char*)B + S or (char*)B - S
return ((C.Index->isOne() || C.Index->isMinusOne()) &&
hasOnlyOneNonZeroIndex(cast<GetElementPtrInst>(C.Ins)));
}
return false;
}
// TODO: We currently implement an algorithm whose time complexity is linear in
// the number of existing candidates. However, we could do better by using
// ScopedHashTable. Specifically, while traversing the dominator tree, we could
// maintain all the candidates that dominate the basic block being traversed in
// a ScopedHashTable. This hash table is indexed by the base and the stride of
// a candidate. Therefore, finding the immediate basis of a candidate boils down
// to one hash-table look up.
void StraightLineStrengthReduce::allocateCandidatesAndFindBasis(
Candidate::Kind CT, const SCEV *B, ConstantInt *Idx, Value *S,
Instruction *I) {
Candidate C(CT, B, Idx, S, I);
// SLSR can complicate an instruction in two cases:
//
// 1. If we can fold I into an addressing mode, computing I is likely free or
// takes only one instruction.
//
// 2. I is already in a simplest form. For example, when
// X = B + 8 * S
// Y = B + S,
// rewriting Y to X - 7 * S is probably a bad idea.
//
// In the above cases, we still add I to the candidate list so that I can be
// the basis of other candidates, but we leave I's basis blank so that I
// won't be rewritten.
if (!isFoldable(C, TTI, DL) && !isSimplestForm(C)) {
// Try to compute the immediate basis of C.
unsigned NumIterations = 0;
// Limit the scan radius to avoid running in quadratice time.
static const unsigned MaxNumIterations = 50;
for (auto Basis = Candidates.rbegin();
Basis != Candidates.rend() && NumIterations < MaxNumIterations;
++Basis, ++NumIterations) {
if (isBasisFor(*Basis, C)) {
C.Basis = &(*Basis);
break;
}
}
}
// Regardless of whether we find a basis for C, we need to push C to the
// candidate list so that it can be the basis of other candidates.
Candidates.push_back(C);
}
void StraightLineStrengthReduce::allocateCandidatesAndFindBasis(
Instruction *I) {
switch (I->getOpcode()) {
case Instruction::Add:
allocateCandidatesAndFindBasisForAdd(I);
break;
case Instruction::Mul:
allocateCandidatesAndFindBasisForMul(I);
break;
case Instruction::GetElementPtr:
allocateCandidatesAndFindBasisForGEP(cast<GetElementPtrInst>(I));
break;
}
}
void StraightLineStrengthReduce::allocateCandidatesAndFindBasisForAdd(
Instruction *I) {
// Try matching B + i * S.
if (!isa<IntegerType>(I->getType()))
return;
assert(I->getNumOperands() == 2 && "isn't I an add?");
Value *LHS = I->getOperand(0), *RHS = I->getOperand(1);
allocateCandidatesAndFindBasisForAdd(LHS, RHS, I);
if (LHS != RHS)
allocateCandidatesAndFindBasisForAdd(RHS, LHS, I);
}
void StraightLineStrengthReduce::allocateCandidatesAndFindBasisForAdd(
Value *LHS, Value *RHS, Instruction *I) {
Value *S = nullptr;
ConstantInt *Idx = nullptr;
if (match(RHS, m_Mul(m_Value(S), m_ConstantInt(Idx)))) {
// I = LHS + RHS = LHS + Idx * S
allocateCandidatesAndFindBasis(Candidate::Add, SE->getSCEV(LHS), Idx, S, I);
} else if (match(RHS, m_Shl(m_Value(S), m_ConstantInt(Idx)))) {
// I = LHS + RHS = LHS + (S << Idx) = LHS + S * (1 << Idx)
APInt One(Idx->getBitWidth(), 1);
Idx = ConstantInt::get(Idx->getContext(), One << Idx->getValue());
allocateCandidatesAndFindBasis(Candidate::Add, SE->getSCEV(LHS), Idx, S, I);
} else {
// At least, I = LHS + 1 * RHS
ConstantInt *One = ConstantInt::get(cast<IntegerType>(I->getType()), 1);
allocateCandidatesAndFindBasis(Candidate::Add, SE->getSCEV(LHS), One, RHS,
I);
}
}
// Returns true if A matches B + C where C is constant.
static bool matchesAdd(Value *A, Value *&B, ConstantInt *&C) {
return (match(A, m_Add(m_Value(B), m_ConstantInt(C))) ||
match(A, m_Add(m_ConstantInt(C), m_Value(B))));
}
// Returns true if A matches B | C where C is constant.
static bool matchesOr(Value *A, Value *&B, ConstantInt *&C) {
return (match(A, m_Or(m_Value(B), m_ConstantInt(C))) ||
match(A, m_Or(m_ConstantInt(C), m_Value(B))));
}
void StraightLineStrengthReduce::allocateCandidatesAndFindBasisForMul(
Value *LHS, Value *RHS, Instruction *I) {
Value *B = nullptr;
ConstantInt *Idx = nullptr;
if (matchesAdd(LHS, B, Idx)) {
// If LHS is in the form of "Base + Index", then I is in the form of
// "(Base + Index) * RHS".
allocateCandidatesAndFindBasis(Candidate::Mul, SE->getSCEV(B), Idx, RHS, I);
} else if (matchesOr(LHS, B, Idx) && haveNoCommonBitsSet(B, Idx, *DL)) {
// If LHS is in the form of "Base | Index" and Base and Index have no common
// bits set, then
// Base | Index = Base + Index
// and I is thus in the form of "(Base + Index) * RHS".
allocateCandidatesAndFindBasis(Candidate::Mul, SE->getSCEV(B), Idx, RHS, I);
} else {
// Otherwise, at least try the form (LHS + 0) * RHS.
ConstantInt *Zero = ConstantInt::get(cast<IntegerType>(I->getType()), 0);
allocateCandidatesAndFindBasis(Candidate::Mul, SE->getSCEV(LHS), Zero, RHS,
I);
}
}
void StraightLineStrengthReduce::allocateCandidatesAndFindBasisForMul(
Instruction *I) {
// Try matching (B + i) * S.
// TODO: we could extend SLSR to float and vector types.
if (!isa<IntegerType>(I->getType()))
return;
assert(I->getNumOperands() == 2 && "isn't I a mul?");
Value *LHS = I->getOperand(0), *RHS = I->getOperand(1);
allocateCandidatesAndFindBasisForMul(LHS, RHS, I);
if (LHS != RHS) {
// Symmetrically, try to split RHS to Base + Index.
allocateCandidatesAndFindBasisForMul(RHS, LHS, I);
}
}
void StraightLineStrengthReduce::allocateCandidatesAndFindBasisForGEP(
const SCEV *B, ConstantInt *Idx, Value *S, uint64_t ElementSize,
Instruction *I) {
// I = B + sext(Idx *nsw S) * ElementSize
// = B + (sext(Idx) * sext(S)) * ElementSize
// = B + (sext(Idx) * ElementSize) * sext(S)
// Casting to IntegerType is safe because we skipped vector GEPs.
IntegerType *IntPtrTy = cast<IntegerType>(DL->getIntPtrType(I->getType()));
ConstantInt *ScaledIdx = ConstantInt::get(
IntPtrTy, Idx->getSExtValue() * (int64_t)ElementSize, true);
allocateCandidatesAndFindBasis(Candidate::GEP, B, ScaledIdx, S, I);
}
void StraightLineStrengthReduce::factorArrayIndex(Value *ArrayIdx,
const SCEV *Base,
uint64_t ElementSize,
GetElementPtrInst *GEP) {
// At least, ArrayIdx = ArrayIdx *nsw 1.
allocateCandidatesAndFindBasisForGEP(
Base, ConstantInt::get(cast<IntegerType>(ArrayIdx->getType()), 1),
ArrayIdx, ElementSize, GEP);
Value *LHS = nullptr;
ConstantInt *RHS = nullptr;
// One alternative is matching the SCEV of ArrayIdx instead of ArrayIdx
// itself. This would allow us to handle the shl case for free. However,
// matching SCEVs has two issues:
//
// 1. this would complicate rewriting because the rewriting procedure
// would have to translate SCEVs back to IR instructions. This translation
// is difficult when LHS is further evaluated to a composite SCEV.
//
// 2. ScalarEvolution is designed to be control-flow oblivious. It tends
// to strip nsw/nuw flags which are critical for SLSR to trace into
// sext'ed multiplication.
if (match(ArrayIdx, m_NSWMul(m_Value(LHS), m_ConstantInt(RHS)))) {
// SLSR is currently unsafe if i * S may overflow.
// GEP = Base + sext(LHS *nsw RHS) * ElementSize
allocateCandidatesAndFindBasisForGEP(Base, RHS, LHS, ElementSize, GEP);
} else if (match(ArrayIdx, m_NSWShl(m_Value(LHS), m_ConstantInt(RHS)))) {
// GEP = Base + sext(LHS <<nsw RHS) * ElementSize
// = Base + sext(LHS *nsw (1 << RHS)) * ElementSize
APInt One(RHS->getBitWidth(), 1);
ConstantInt *PowerOf2 =
ConstantInt::get(RHS->getContext(), One << RHS->getValue());
allocateCandidatesAndFindBasisForGEP(Base, PowerOf2, LHS, ElementSize, GEP);
}
}
void StraightLineStrengthReduce::allocateCandidatesAndFindBasisForGEP(
GetElementPtrInst *GEP) {
// TODO: handle vector GEPs
if (GEP->getType()->isVectorTy())
return;
SmallVector<const SCEV *, 4> IndexExprs;
for (auto I = GEP->idx_begin(); I != GEP->idx_end(); ++I)
IndexExprs.push_back(SE->getSCEV(*I));
gep_type_iterator GTI = gep_type_begin(GEP);
for (unsigned I = 1, E = GEP->getNumOperands(); I != E; ++I, ++GTI) {
if (GTI.isStruct())
continue;
const SCEV *OrigIndexExpr = IndexExprs[I - 1];
IndexExprs[I - 1] = SE->getZero(OrigIndexExpr->getType());
// The base of this candidate is GEP's base plus the offsets of all
// indices except this current one.
const SCEV *BaseExpr = SE->getGEPExpr(cast<GEPOperator>(GEP), IndexExprs);
Value *ArrayIdx = GEP->getOperand(I);
uint64_t ElementSize = DL->getTypeAllocSize(GTI.getIndexedType());
if (ArrayIdx->getType()->getIntegerBitWidth() <=
DL->getPointerSizeInBits(GEP->getAddressSpace())) {
// Skip factoring if ArrayIdx is wider than the pointer size, because
// ArrayIdx is implicitly truncated to the pointer size.
factorArrayIndex(ArrayIdx, BaseExpr, ElementSize, GEP);
}
// When ArrayIdx is the sext of a value, we try to factor that value as
// well. Handling this case is important because array indices are
// typically sign-extended to the pointer size.
Value *TruncatedArrayIdx = nullptr;
if (match(ArrayIdx, m_SExt(m_Value(TruncatedArrayIdx))) &&
TruncatedArrayIdx->getType()->getIntegerBitWidth() <=
DL->getPointerSizeInBits(GEP->getAddressSpace())) {
// Skip factoring if TruncatedArrayIdx is wider than the pointer size,
// because TruncatedArrayIdx is implicitly truncated to the pointer size.
factorArrayIndex(TruncatedArrayIdx, BaseExpr, ElementSize, GEP);
}
IndexExprs[I - 1] = OrigIndexExpr;
}
}
// A helper function that unifies the bitwidth of A and B.
static void unifyBitWidth(APInt &A, APInt &B) {
if (A.getBitWidth() < B.getBitWidth())
A = A.sext(B.getBitWidth());
else if (A.getBitWidth() > B.getBitWidth())
B = B.sext(A.getBitWidth());
}
Value *StraightLineStrengthReduce::emitBump(const Candidate &Basis,
const Candidate &C,
IRBuilder<> &Builder,
const DataLayout *DL,
bool &BumpWithUglyGEP) {
APInt Idx = C.Index->getValue(), BasisIdx = Basis.Index->getValue();
unifyBitWidth(Idx, BasisIdx);
APInt IndexOffset = Idx - BasisIdx;
BumpWithUglyGEP = false;
if (Basis.CandidateKind == Candidate::GEP) {
APInt ElementSize(
IndexOffset.getBitWidth(),
DL->getTypeAllocSize(
cast<GetElementPtrInst>(Basis.Ins)->getResultElementType()));
APInt Q, R;
APInt::sdivrem(IndexOffset, ElementSize, Q, R);
if (R == 0)
IndexOffset = Q;
else
BumpWithUglyGEP = true;
}
// Compute Bump = C - Basis = (i' - i) * S.
// Common case 1: if (i' - i) is 1, Bump = S.
if (IndexOffset == 1)
return C.Stride;
// Common case 2: if (i' - i) is -1, Bump = -S.
if (IndexOffset.isAllOnesValue())
return Builder.CreateNeg(C.Stride);
// Otherwise, Bump = (i' - i) * sext/trunc(S). Note that (i' - i) and S may
// have different bit widths.
IntegerType *DeltaType =
IntegerType::get(Basis.Ins->getContext(), IndexOffset.getBitWidth());
Value *ExtendedStride = Builder.CreateSExtOrTrunc(C.Stride, DeltaType);
if (IndexOffset.isPowerOf2()) {
// If (i' - i) is a power of 2, Bump = sext/trunc(S) << log(i' - i).
ConstantInt *Exponent = ConstantInt::get(DeltaType, IndexOffset.logBase2());
return Builder.CreateShl(ExtendedStride, Exponent);
}
if ((-IndexOffset).isPowerOf2()) {
// If (i - i') is a power of 2, Bump = -sext/trunc(S) << log(i' - i).
ConstantInt *Exponent =
ConstantInt::get(DeltaType, (-IndexOffset).logBase2());
return Builder.CreateNeg(Builder.CreateShl(ExtendedStride, Exponent));
}
Constant *Delta = ConstantInt::get(DeltaType, IndexOffset);
return Builder.CreateMul(ExtendedStride, Delta);
}
void StraightLineStrengthReduce::rewriteCandidateWithBasis(
const Candidate &C, const Candidate &Basis) {
assert(C.CandidateKind == Basis.CandidateKind && C.Base == Basis.Base &&
C.Stride == Basis.Stride);
// We run rewriteCandidateWithBasis on all candidates in a post-order, so the
// basis of a candidate cannot be unlinked before the candidate.
assert(Basis.Ins->getParent() != nullptr && "the basis is unlinked");
// An instruction can correspond to multiple candidates. Therefore, instead of
// simply deleting an instruction when we rewrite it, we mark its parent as
// nullptr (i.e. unlink it) so that we can skip the candidates whose
// instruction is already rewritten.
if (!C.Ins->getParent())
return;
IRBuilder<> Builder(C.Ins);
bool BumpWithUglyGEP;
Value *Bump = emitBump(Basis, C, Builder, DL, BumpWithUglyGEP);
Value *Reduced = nullptr; // equivalent to but weaker than C.Ins
switch (C.CandidateKind) {
case Candidate::Add:
case Candidate::Mul:
// C = Basis + Bump
if (BinaryOperator::isNeg(Bump)) {
// If Bump is a neg instruction, emit C = Basis - (-Bump).
Reduced =
Builder.CreateSub(Basis.Ins, BinaryOperator::getNegArgument(Bump));
// We only use the negative argument of Bump, and Bump itself may be
// trivially dead.
RecursivelyDeleteTriviallyDeadInstructions(Bump);
} else {
// It's tempting to preserve nsw on Bump and/or Reduced. However, it's
// usually unsound, e.g.,
//
// X = (-2 +nsw 1) *nsw INT_MAX
// Y = (-2 +nsw 3) *nsw INT_MAX
// =>
// Y = X + 2 * INT_MAX
//
// Neither + and * in the resultant expression are nsw.
Reduced = Builder.CreateAdd(Basis.Ins, Bump);
}
break;
case Candidate::GEP:
{
Type *IntPtrTy = DL->getIntPtrType(C.Ins->getType());
bool InBounds = cast<GetElementPtrInst>(C.Ins)->isInBounds();
if (BumpWithUglyGEP) {
// C = (char *)Basis + Bump
unsigned AS = Basis.Ins->getType()->getPointerAddressSpace();
Type *CharTy = Type::getInt8PtrTy(Basis.Ins->getContext(), AS);
Reduced = Builder.CreateBitCast(Basis.Ins, CharTy);
if (InBounds)
Reduced =
Builder.CreateInBoundsGEP(Builder.getInt8Ty(), Reduced, Bump);
else
Reduced = Builder.CreateGEP(Builder.getInt8Ty(), Reduced, Bump);
Reduced = Builder.CreateBitCast(Reduced, C.Ins->getType());
} else {
// C = gep Basis, Bump
// Canonicalize bump to pointer size.
Bump = Builder.CreateSExtOrTrunc(Bump, IntPtrTy);
if (InBounds)
Reduced = Builder.CreateInBoundsGEP(nullptr, Basis.Ins, Bump);
else
Reduced = Builder.CreateGEP(nullptr, Basis.Ins, Bump);
}
break;
}
default:
llvm_unreachable("C.CandidateKind is invalid");
};
Reduced->takeName(C.Ins);
C.Ins->replaceAllUsesWith(Reduced);
// Unlink C.Ins so that we can skip other candidates also corresponding to
// C.Ins. The actual deletion is postponed to the end of runOnFunction.
C.Ins->removeFromParent();
UnlinkedInstructions.push_back(C.Ins);
}
bool StraightLineStrengthReduce::runOnFunction(Function &F) {
if (skipFunction(F))
return false;
TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
// Traverse the dominator tree in the depth-first order. This order makes sure
// all bases of a candidate are in Candidates when we process it.
for (const auto Node : depth_first(DT))
for (auto &I : *(Node->getBlock()))
allocateCandidatesAndFindBasis(&I);
// Rewrite candidates in the reverse depth-first order. This order makes sure
// a candidate being rewritten is not a basis for any other candidate.
while (!Candidates.empty()) {
const Candidate &C = Candidates.back();
if (C.Basis != nullptr) {
rewriteCandidateWithBasis(C, *C.Basis);
}
Candidates.pop_back();
}
// Delete all unlink instructions.
for (auto *UnlinkedInst : UnlinkedInstructions) {
for (unsigned I = 0, E = UnlinkedInst->getNumOperands(); I != E; ++I) {
Value *Op = UnlinkedInst->getOperand(I);
UnlinkedInst->setOperand(I, nullptr);
RecursivelyDeleteTriviallyDeadInstructions(Op);
}
UnlinkedInst->deleteValue();
}
bool Ret = !UnlinkedInstructions.empty();
UnlinkedInstructions.clear();
return Ret;
}
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