//
// 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 candidate in the form of
+// reduction candidates in the following forms:
//
-// (B + i) * S
+// Form 1: B + i * S
+// Form 2: (B + i) * S
+// Form 3: &B[i * S]
//
-// where B and S are integer constants or variables, and i is a constant
-// integer. If we found two such candidates
+// 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
+// S1: X = B + i * S
+// S2: Y = B + i' * S => X + (i' - i) * S
//
-// and S1 dominates S2, we call S1 a basis of S2, and can replace S2 with
+// S1: X = (B + i) * S
+// S2: Y = (B + i') * S => X + (i' - i) * S
//
-// Y = X + (i' - i) * S
+// S1: X = &B[i * S]
+// S2: Y = &B[i' * S] => &X[(i' - i) * S]
//
-// where (i' - i) * S is folded to the extent possible. When S2 has multiple
-// bases, we pick the one that is closest to S2, or S2's "immediate" basis.
+// Note: (i' - i) * S is folded to the extent possible.
//
-// TODO:
+// 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.
//
-// - Handle candidates in the form of B + i * S
+// 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.
//
-// - Handle candidates in the form of pointer arithmetics. e.g., B[i * S]
+// 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 <vector>
#include "llvm/ADT/DenseSet.h"
+#include "llvm/ADT/FoldingSet.h"
+#include "llvm/Analysis/ScalarEvolution.h"
+#include "llvm/Analysis/TargetTransformInfo.h"
+#include "llvm/Analysis/ValueTracking.h"
+#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
+#include "llvm/Transforms/Utils/Local.h"
using namespace llvm;
using namespace PatternMatch;
namespace {
class StraightLineStrengthReduce : public FunctionPass {
- public:
- // SLSR candidate. Such a candidate must be in the form of
- // (Base + Index) * Stride
+public:
+ // SLSR candidate. Such a candidate must be in one of the forms described in
+ // the header comments.
struct Candidate : public ilist_node<Candidate> {
- Candidate(Value *B = nullptr, ConstantInt *Idx = nullptr,
- Value *S = nullptr, Instruction *I = nullptr)
- : Base(B), Index(Idx), Stride(S), Ins(I), Basis(nullptr) {}
- Value *Base;
+ enum Kind {
+ Invalid, // reserved for the default constructor
+ Add, // B + i * S
+ Mul, // (B + i) * S
+ GEP, // &B[..][i * S][..]
+ };
+
+ Candidate()
+ : CandidateKind(Invalid), Base(nullptr), Index(nullptr),
+ Stride(nullptr), Ins(nullptr), Basis(nullptr) {}
+ Candidate(Kind CT, const SCEV *B, ConstantInt *Idx, Value *S,
+ Instruction *I)
+ : CandidateKind(CT), Base(B), Index(Idx), Stride(S), Ins(I),
+ Basis(nullptr) {}
+ Kind CandidateKind;
+ const SCEV *Base;
+ // 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;
Value *Stride;
// The instruction this candidate corresponds to. It helps us to rewrite a
// candidate with respect to its immediate basis. Note that one instruction
- // can corresponds to multiple candidates depending on how you associate the
+ // can correspond to multiple candidates depending on how you associate the
// expression. For instance,
//
// (a + 1) * (b + 2)
static char ID;
- StraightLineStrengthReduce() : FunctionPass(ID), DT(nullptr) {
+ StraightLineStrengthReduce()
+ : FunctionPass(ID), DL(nullptr), DT(nullptr), TTI(nullptr) {
initializeStraightLineStrengthReducePass(*PassRegistry::getPassRegistry());
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<DominatorTreeWrapperPass>();
+ AU.addRequired<ScalarEvolution>();
+ 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:
+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 allocateCandidateAndFindBasis(Instruction *I);
- // Given that I is in the form of "(B + Idx) * S", adds this form to
- // Candidates, and finds its immediate basis.
- void allocateCandidateAndFindBasis(Value *B, ConstantInt *Idx, Value *S,
- Instruction *I);
+ 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;
DominatorTree *DT;
+ ScalarEvolution *SE;
+ TargetTransformInfo *TTI;
ilist<Candidate> Candidates;
// Temporarily holds all instructions that are unlinked (but not deleted) by
// rewriteCandidateWithBasis. These instructions will be actually removed
// after all rewriting finishes.
- DenseSet<Instruction *> UnlinkedInstructions;
+ std::vector<Instruction *> UnlinkedInstructions;
};
} // anonymous namespace
INITIALIZE_PASS_BEGIN(StraightLineStrengthReduce, "slsr",
"Straight line strength reduction", false, false)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
+INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
INITIALIZE_PASS_END(StraightLineStrengthReduce, "slsr",
"Straight line strength reduction", false, false)
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 and stride.
- Basis.Base == C.Base &&
- Basis.Stride == C.Stride);
+ // They share the same base, stride, and candidate kind.
+ Basis.Base == C.Base && Basis.Stride == C.Stride &&
+ Basis.CandidateKind == C.CandidateKind);
+}
+
+// TODO: use TTI->getGEPCost.
+static bool isGEPFoldable(GetElementPtrInst *GEP,
+ const TargetTransformInfo *TTI,
+ const DataLayout *DL) {
+ GlobalVariable *BaseGV = nullptr;
+ int64_t BaseOffset = 0;
+ bool HasBaseReg = false;
+ int64_t Scale = 0;
+
+ if (GlobalVariable *GV = dyn_cast<GlobalVariable>(GEP->getPointerOperand()))
+ BaseGV = GV;
+ else
+ HasBaseReg = true;
+
+ gep_type_iterator GTI = gep_type_begin(GEP);
+ for (auto I = GEP->idx_begin(); I != GEP->idx_end(); ++I, ++GTI) {
+ if (isa<SequentialType>(*GTI)) {
+ int64_t ElementSize = DL->getTypeAllocSize(GTI.getIndexedType());
+ if (ConstantInt *ConstIdx = dyn_cast<ConstantInt>(*I)) {
+ BaseOffset += ConstIdx->getSExtValue() * ElementSize;
+ } else {
+ // Needs scale register.
+ if (Scale != 0) {
+ // No addressing mode takes two scale registers.
+ return false;
+ }
+ Scale = ElementSize;
+ }
+ } else {
+ StructType *STy = cast<StructType>(*GTI);
+ uint64_t Field = cast<ConstantInt>(*I)->getZExtValue();
+ BaseOffset += DL->getStructLayout(STy)->getElementOffset(Field);
+ }
+ }
+
+ unsigned AddrSpace = GEP->getPointerAddressSpace();
+ return TTI->isLegalAddressingMode(GEP->getType()->getElementType(), BaseGV,
+ BaseOffset, HasBaseReg, Scale, AddrSpace);
+}
+
+// Returns whether (Base + Index * Stride) can be folded to an addressing mode.
+static bool isAddFoldable(const SCEV *Base, ConstantInt *Index, Value *Stride,
+ TargetTransformInfo *TTI) {
+ return TTI->isLegalAddressingMode(Base->getType(), nullptr, 0, true,
+ Index->getSExtValue());
+}
+
+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, DL);
+ 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 to
-// the number of existing candidates. However, a better algorithm exists. We
-// could depth-first search the dominator tree, and maintain a hash table that
-// contains all candidates that dominate the node being traversed. 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::allocateCandidateAndFindBasis(Value *B,
- ConstantInt *Idx,
- Value *S,
- Instruction *I) {
- Candidate C(B, Idx, S, I);
- // Try to compute the immediate basis of C.
- unsigned NumIterations = 0;
- // Limit the scan radius to avoid running forever.
- 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;
+// 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.
+ // candidate list so that it can be the basis of other candidates.
Candidates.push_back(C);
}
-void StraightLineStrengthReduce::allocateCandidateAndFindBasis(Instruction *I) {
+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;
- // "(Base + Index) * Stride" must be a Mul instruction at the first hand.
- if (I->getOpcode() == Instruction::Mul) {
- if (IntegerType *ITy = dyn_cast<IntegerType>(I->getType())) {
- Value *LHS = I->getOperand(0), *RHS = I->getOperand(1);
- for (unsigned Swapped = 0; Swapped < 2; ++Swapped) {
- // Only handle the canonical operand ordering.
- if (match(LHS, m_Add(m_Value(B), m_ConstantInt(Idx)))) {
- // If LHS is in the form of "Base + Index", then I is in the form of
- // "(Base + Index) * RHS".
- allocateCandidateAndFindBasis(B, Idx, RHS, I);
- } else {
- // Otherwise, at least try the form (LHS + 0) * RHS.
- allocateCandidateAndFindBasis(LHS, ConstantInt::get(ITy, 0), RHS, I);
- }
- // Swap LHS and RHS so that we also cover the cases where LHS is the
- // stride.
- if (LHS == RHS)
- break;
- std::swap(LHS, RHS);
- }
- }
+ 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) {
+ if (!isa<SequentialType>(*GTI++))
+ continue;
+
+ const SCEV *OrigIndexExpr = IndexExprs[I - 1];
+ IndexExprs[I - 1] = SE->getConstant(OrigIndexExpr->getType(), 0);
+
+ // The base of this candidate is GEP's base plus the offsets of all
+ // indices except this current one.
+ const SCEV *BaseExpr = SE->getGEPExpr(GEP->getSourceElementType(),
+ SE->getSCEV(GEP->getPointerOperand()),
+ IndexExprs, GEP->isInBounds());
+ Value *ArrayIdx = GEP->getOperand(I);
+ uint64_t ElementSize = DL->getTypeAllocSize(*GTI);
+ 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))))
+ 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)->getType()->getElementType()));
+ APInt Q, R;
+ APInt::sdivrem(IndexOffset, ElementSize, Q, R);
+ if (R.getSExtValue() == 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.getSExtValue() == 1)
+ return C.Stride;
+ // Common case 2: if (i' - i) is -1, Bump = -S.
+ if (IndexOffset.getSExtValue() == -1)
+ 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;
- assert(C.Base == Basis.Base && C.Stride == Basis.Stride);
- // Basis = (B + i) * S
- // C = (B + i') * S
- // ==>
- // C = Basis + (i' - i) * S
+
IRBuilder<> Builder(C.Ins);
- ConstantInt *IndexOffset = ConstantInt::get(
- C.Ins->getContext(), C.Index->getValue() - Basis.Index->getValue());
- Value *Reduced;
- // TODO: preserve nsw/nuw in some cases.
- if (IndexOffset->isOne()) {
- // If (i' - i) is 1, fold C into Basis + S.
- Reduced = Builder.CreateAdd(Basis.Ins, C.Stride);
- } else if (IndexOffset->isMinusOne()) {
- // If (i' - i) is -1, fold C into Basis - S.
- Reduced = Builder.CreateSub(Basis.Ins, C.Stride);
- } else {
- Value *Bump = Builder.CreateMul(C.Stride, IndexOffset);
- Reduced = Builder.CreateAdd(Basis.Ins, Bump);
- }
+ 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);
- C.Ins->dropAllReferences();
// 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.insert(C.Ins);
+ UnlinkedInstructions.push_back(C.Ins);
}
bool StraightLineStrengthReduce::runOnFunction(Function &F) {
if (skipOptnoneFunction(F))
return false;
+ TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
+ SE = &getAnalysis<ScalarEvolution>();
// 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 (auto node = GraphTraits<DominatorTree *>::nodes_begin(DT);
node != GraphTraits<DominatorTree *>::nodes_end(DT); ++node) {
- BasicBlock *B = node->getBlock();
- for (auto I = B->begin(); I != B->end(); ++I) {
- allocateCandidateAndFindBasis(I);
- }
+ for (auto &I : *node->getBlock())
+ allocateCandidatesAndFindBasis(&I);
}
// Rewrite candidates in the reverse depth-first order. This order makes sure
}
// Delete all unlink instructions.
- for (auto I : UnlinkedInstructions) {
- delete I;
+ 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);
+ }
+ delete UnlinkedInst;
}
bool Ret = !UnlinkedInstructions.empty();
UnlinkedInstructions.clear();
projects / oota-llvm.git / blobdiff