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[InstCombine] Moving overflow computation logic from InstCombine to ValueTracking; NFC 

Differential Revision: https://reviews.llvm.org/D46704

Change-Id: Ifabcbe431a2169743b3cc310f2a34fd706f13f02
llvm-svn: 332026
This commit is contained in:
Omer Paparo Bivas 2018-05-10 19:46:19 +00:00
parent b5322e385e
commit fbb83deef7
5 changed files with 129 additions and 86 deletions
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15
llvm/include/llvm/Analysis/ValueTracking.h
View File

@ -384,6 +384,11 @@ class Value;
AssumptionCache *AC,
const Instruction *CxtI,
const DominatorTree *DT);
OverflowResult computeOverflowForSignedMul(const Value *LHS, const Value *RHS,
const DataLayout &DL,
AssumptionCache *AC,
const Instruction *CxtI,
const DominatorTree *DT);
OverflowResult computeOverflowForUnsignedAdd(const Value *LHS,
const Value *RHS,
const DataLayout &DL,
@ -401,6 +406,16 @@ class Value;
AssumptionCache *AC = nullptr,
const Instruction *CxtI = nullptr,
const DominatorTree *DT = nullptr);
OverflowResult computeOverflowForUnsignedSub(const Value *LHS, const Value *RHS,
const DataLayout &DL,
AssumptionCache *AC,
const Instruction *CxtI,
const DominatorTree *DT);
OverflowResult computeOverflowForSignedSub(const Value *LHS, const Value *RHS,
const DataLayout &DL,
AssumptionCache *AC,
const Instruction *CxtI,
const DominatorTree *DT);
/// Returns true if the arithmetic part of the \p II 's result is
/// used only along the paths control dependent on the computation

83
llvm/lib/Analysis/ValueTracking.cpp
View File

@ -3704,6 +3704,48 @@ OverflowResult llvm::computeOverflowForUnsignedMul(const Value *LHS,
return OverflowResult::MayOverflow;
}
OverflowResult llvm::computeOverflowForSignedMul(const Value *LHS,
const Value *RHS,
const DataLayout &DL,
AssumptionCache *AC,
const Instruction *CxtI,
const DominatorTree *DT) {
// Multiplying n * m significant bits yields a result of n + m significant
// bits. If the total number of significant bits does not exceed the
// result bit width (minus 1), there is no overflow.
// This means if we have enough leading sign bits in the operands
// we can guarantee that the result does not overflow.
// Ref: "Hacker's Delight" by Henry Warren
unsigned BitWidth = LHS->getType()->getScalarSizeInBits();
// Note that underestimating the number of sign bits gives a more
// conservative answer.
unsigned SignBits = ComputeNumSignBits(LHS, DL, 0, AC, CxtI, DT) +
ComputeNumSignBits(RHS, DL, 0, AC, CxtI, DT);
// First handle the easy case: if we have enough sign bits there's
// definitely no overflow.
if (SignBits > BitWidth + 1)
return OverflowResult::NeverOverflows;
// There are two ambiguous cases where there can be no overflow:
// SignBits == BitWidth + 1 and
// SignBits == BitWidth
// The second case is difficult to check, therefore we only handle the
// first case.
if (SignBits == BitWidth + 1) {
// It overflows only when both arguments are negative and the true
// product is exactly the minimum negative number.
// E.g. mul i16 with 17 sign bits: 0xff00 * 0xff80 = 0x8000
// For simplicity we just check if at least one side is not negative.
KnownBits LHSKnown = computeKnownBits(LHS, DL, /*Depth=*/0, AC, CxtI, DT);
KnownBits RHSKnown = computeKnownBits(RHS, DL, /*Depth=*/0, AC, CxtI, DT);
if (LHSKnown.isNonNegative() || RHSKnown.isNonNegative())
return OverflowResult::NeverOverflows;
}
return OverflowResult::MayOverflow;
}
OverflowResult llvm::computeOverflowForUnsignedAdd(const Value *LHS,
const Value *RHS,
const DataLayout &DL,
@ -3833,6 +3875,47 @@ static OverflowResult computeOverflowForSignedAdd(const Value *LHS,
return OverflowResult::MayOverflow;
}
OverflowResult llvm::computeOverflowForUnsignedSub(const Value *LHS,
const Value *RHS,
const DataLayout &DL,
AssumptionCache *AC,
const Instruction *CxtI,
const DominatorTree *DT) {
// If the LHS is negative and the RHS is non-negative, no unsigned wrap.
KnownBits LHSKnown = computeKnownBits(LHS, DL, /*Depth=*/0, AC, CxtI, DT);
KnownBits RHSKnown = computeKnownBits(RHS, DL, /*Depth=*/0, AC, CxtI, DT);
if (LHSKnown.isNegative() && RHSKnown.isNonNegative())
return OverflowResult::NeverOverflows;
return OverflowResult::MayOverflow;
}
OverflowResult llvm::computeOverflowForSignedSub(const Value *LHS,
const Value *RHS,
const DataLayout &DL,
AssumptionCache *AC,
const Instruction *CxtI,
const DominatorTree *DT) {
// If LHS and RHS each have at least two sign bits, the subtraction
// cannot overflow.
if (ComputeNumSignBits(LHS, DL, 0, AC, CxtI, DT) > 1 &&
ComputeNumSignBits(RHS, DL, 0, AC, CxtI, DT) > 1)
return OverflowResult::NeverOverflows;
KnownBits LHSKnown = computeKnownBits(LHS, DL, 0, AC, CxtI, DT);
KnownBits RHSKnown = computeKnownBits(RHS, DL, 0, AC, CxtI, DT);
// Subtraction of two 2's complement numbers having identical signs will
// never overflow.
if ((LHSKnown.isNegative() && RHSKnown.isNegative()) ||
(LHSKnown.isNonNegative() && RHSKnown.isNonNegative()))
return OverflowResult::NeverOverflows;
// TODO: implement logic similar to checkRippleForAdd
return OverflowResult::MayOverflow;
}
bool llvm::isOverflowIntrinsicNoWrap(const IntrinsicInst *II,
const DominatorTree &DT) {
#ifndef NDEBUG

42
llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
View File

@ -856,48 +856,6 @@ Value *FAddCombine::createAddendVal(const FAddend &Opnd, bool &NeedNeg) {
return createFMul(OpndVal, Coeff.getValue(Instr->getType()));
}
/// Return true if we can prove that:
/// (sub LHS, RHS) === (sub nsw LHS, RHS)
/// This basically requires proving that the add in the original type would not
/// overflow to change the sign bit or have a carry out.
/// TODO: Handle this for Vectors.
bool InstCombiner::willNotOverflowSignedSub(const Value *LHS,
const Value *RHS,
const Instruction &CxtI) const {
// If LHS and RHS each have at least two sign bits, the subtraction
// cannot overflow.
if (ComputeNumSignBits(LHS, 0, &CxtI) > 1 &&
ComputeNumSignBits(RHS, 0, &CxtI) > 1)
return true;
KnownBits LHSKnown = computeKnownBits(LHS, 0, &CxtI);
KnownBits RHSKnown = computeKnownBits(RHS, 0, &CxtI);
// Subtraction of two 2's complement numbers having identical signs will
// never overflow.
if ((LHSKnown.isNegative() && RHSKnown.isNegative()) ||
(LHSKnown.isNonNegative() && RHSKnown.isNonNegative()))
return true;
// TODO: implement logic similar to checkRippleForAdd
return false;
}
/// Return true if we can prove that:
/// (sub LHS, RHS) === (sub nuw LHS, RHS)
bool InstCombiner::willNotOverflowUnsignedSub(const Value *LHS,
const Value *RHS,
const Instruction &CxtI) const {
// If the LHS is negative and the RHS is non-negative, no unsigned wrap.
KnownBits LHSKnown = computeKnownBits(LHS, /*Depth=*/0, &CxtI);
KnownBits RHSKnown = computeKnownBits(RHS, /*Depth=*/0, &CxtI);
if (LHSKnown.isNegative() && RHSKnown.isNonNegative())
return true;
return false;
}
// Checks if any operand is negative and we can convert add to sub.
// This function checks for following negative patterns
// ADD(XOR(OR(Z, NOT(C)), C)), 1) == NEG(AND(Z, C))

34
llvm/lib/Transforms/InstCombine/InstCombineInternal.h
View File

@ -442,11 +442,22 @@ private:
}
bool willNotOverflowSignedSub(const Value *LHS, const Value *RHS,
const Instruction &CxtI) const;
const Instruction &CxtI) const {
return computeOverflowForSignedSub(LHS, RHS, &CxtI) ==
OverflowResult::NeverOverflows;
}
bool willNotOverflowUnsignedSub(const Value *LHS, const Value *RHS,
const Instruction &CxtI) const;
const Instruction &CxtI) const {
return computeOverflowForUnsignedSub(LHS, RHS, &CxtI) ==
OverflowResult::NeverOverflows;
}
bool willNotOverflowSignedMul(const Value *LHS, const Value *RHS,
const Instruction &CxtI) const;
const Instruction &CxtI) const {
return computeOverflowForSignedMul(LHS, RHS, &CxtI) ==
OverflowResult::NeverOverflows;
}
bool willNotOverflowUnsignedMul(const Value *LHS, const Value *RHS,
const Instruction &CxtI) const {
@ -597,6 +608,12 @@ public:
return llvm::computeOverflowForUnsignedMul(LHS, RHS, DL, &AC, CxtI, &DT);
}
OverflowResult computeOverflowForSignedMul(const Value *LHS,
const Value *RHS,
const Instruction *CxtI) const {
return llvm::computeOverflowForSignedMul(LHS, RHS, DL, &AC, CxtI, &DT);
}
OverflowResult computeOverflowForUnsignedAdd(const Value *LHS,
const Value *RHS,
const Instruction *CxtI) const {
@ -609,6 +626,17 @@ public:
return llvm::computeOverflowForSignedAdd(LHS, RHS, DL, &AC, CxtI, &DT);
}
OverflowResult computeOverflowForUnsignedSub(const Value *LHS,
const Value *RHS,
const Instruction *CxtI) const {
return llvm::computeOverflowForUnsignedSub(LHS, RHS, DL, &AC, CxtI, &DT);
}
OverflowResult computeOverflowForSignedSub(const Value *LHS, const Value *RHS,
const Instruction *CxtI) const {
return llvm::computeOverflowForSignedSub(LHS, RHS, DL, &AC, CxtI, &DT);
}
/// Maximum size of array considered when transforming.
uint64_t MaxArraySizeForCombine;

41
llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
View File

@ -125,47 +125,6 @@ static Constant *getLogBase2(Type *Ty, Constant *C) {
return ConstantVector::get(Elts);
}
/// Return true if we can prove that:
/// (mul LHS, RHS) === (mul nsw LHS, RHS)
bool InstCombiner::willNotOverflowSignedMul(const Value *LHS,
const Value *RHS,
const Instruction &CxtI) const {
// Multiplying n * m significant bits yields a result of n + m significant
// bits. If the total number of significant bits does not exceed the
// result bit width (minus 1), there is no overflow.
// This means if we have enough leading sign bits in the operands
// we can guarantee that the result does not overflow.
// Ref: "Hacker's Delight" by Henry Warren
unsigned BitWidth = LHS->getType()->getScalarSizeInBits();
// Note that underestimating the number of sign bits gives a more
// conservative answer.
unsigned SignBits =
ComputeNumSignBits(LHS, 0, &CxtI) + ComputeNumSignBits(RHS, 0, &CxtI);
// First handle the easy case: if we have enough sign bits there's
// definitely no overflow.
if (SignBits > BitWidth + 1)
return true;
// There are two ambiguous cases where there can be no overflow:
// SignBits == BitWidth + 1 and
// SignBits == BitWidth
// The second case is difficult to check, therefore we only handle the
// first case.
if (SignBits == BitWidth + 1) {
// It overflows only when both arguments are negative and the true
// product is exactly the minimum negative number.
// E.g. mul i16 with 17 sign bits: 0xff00 * 0xff80 = 0x8000
// For simplicity we just check if at least one side is not negative.
KnownBits LHSKnown = computeKnownBits(LHS, /*Depth=*/0, &CxtI);
KnownBits RHSKnown = computeKnownBits(RHS, /*Depth=*/0, &CxtI);
if (LHSKnown.isNonNegative() || RHSKnown.isNonNegative())
return true;
}
return false;
}
Instruction *InstCombiner::visitMul(BinaryOperator &I) {
bool Changed = SimplifyAssociativeOrCommutative(I);
Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);