Analysis: Reformulate WillNotOverflowUnsignedMul for reusability
WillNotOverflowUnsignedMul's smarts will live in ValueTracking as
computeOverflowForUnsignedMul. It now returns a tri-state result:
never overflows, always overflows and sometimes overflows.
llvm-svn: 225076
diff --git a/llvm/lib/Transforms/InstCombine/InstCombine.h b/llvm/lib/Transforms/InstCombine/InstCombine.h
index b4d1efc..96edc79 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombine.h
+++ b/llvm/lib/Transforms/InstCombine/InstCombine.h
@@ -286,7 +286,6 @@
bool WillNotOverflowSignedSub(Value *LHS, Value *RHS, Instruction *CxtI);
bool WillNotOverflowUnsignedSub(Value *LHS, Value *RHS, Instruction *CxtI);
bool WillNotOverflowSignedMul(Value *LHS, Value *RHS, Instruction *CxtI);
- bool WillNotOverflowUnsignedMul(Value *LHS, Value *RHS, Instruction *CxtI);
Value *EmitGEPOffset(User *GEP);
Instruction *scalarizePHI(ExtractElementInst &EI, PHINode *PN);
Value *EvaluateInDifferentElementOrder(Value *V, ArrayRef<int> Mask);
@@ -388,6 +387,10 @@
return llvm::ComputeSignBit(V, KnownZero, KnownOne, DL, Depth, AT, CxtI,
DT);
}
+ OverflowResult computeOverflowForUnsignedMul(Value *LHS, Value *RHS,
+ const Instruction *CxtI) {
+ return llvm::computeOverflowForUnsignedMul(LHS, RHS, DL, AT, CxtI, DT);
+ }
private:
/// SimplifyAssociativeOrCommutative - This performs a few simplifications for
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index ec6a613..34caf1a 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -440,24 +440,8 @@
}
case Intrinsic::umul_with_overflow: {
Value *LHS = II->getArgOperand(0), *RHS = II->getArgOperand(1);
- unsigned BitWidth = cast<IntegerType>(LHS->getType())->getBitWidth();
-
- APInt LHSKnownZero(BitWidth, 0);
- APInt LHSKnownOne(BitWidth, 0);
- computeKnownBits(LHS, LHSKnownZero, LHSKnownOne, 0, II);
- APInt RHSKnownZero(BitWidth, 0);
- APInt RHSKnownOne(BitWidth, 0);
- computeKnownBits(RHS, RHSKnownZero, RHSKnownOne, 0, II);
-
- // Get the largest possible values for each operand.
- APInt LHSMax = ~LHSKnownZero;
- APInt RHSMax = ~RHSKnownZero;
-
- // If multiplying the maximum values does not overflow then we can turn
- // this into a plain NUW mul.
- bool Overflow;
- LHSMax.umul_ov(RHSMax, Overflow);
- if (!Overflow) {
+ OverflowResult OR = computeOverflowForUnsignedMul(LHS, RHS, II);
+ if (OR == OverflowResult::NeverOverflows) {
return CreateOverflowTuple(II, Builder->CreateNUWMul(LHS, RHS), false);
}
} // FALL THROUGH
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index d444d33..255e587 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -165,39 +165,6 @@
return false;
}
-/// \brief Return true if we can prove that:
-/// (mul LHS, RHS) === (mul nuw LHS, RHS)
-bool InstCombiner::WillNotOverflowUnsignedMul(Value *LHS, Value *RHS,
- Instruction *CxtI) {
- // 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 zero 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();
- APInt LHSKnownZero(BitWidth, 0);
- APInt RHSKnownZero(BitWidth, 0);
- APInt TmpKnownOne(BitWidth, 0);
- computeKnownBits(LHS, LHSKnownZero, TmpKnownOne, 0, CxtI);
- computeKnownBits(RHS, RHSKnownZero, TmpKnownOne, 0, CxtI);
- // Note that underestimating the number of zero bits gives a more
- // conservative answer.
- unsigned ZeroBits = LHSKnownZero.countLeadingOnes() +
- RHSKnownZero.countLeadingOnes();
- // First handle the easy case: if we have enough zero bits there's
- // definitely no overflow.
- if (ZeroBits >= BitWidth)
- return true;
-
- // There is an ambiguous cases where there can be no overflow:
- // ZeroBits == BitWidth - 1
- // However, determining overflow requires calculating the sign bit of
- // LHS * RHS/2.
-
- return false;
-}
-
Instruction *InstCombiner::visitMul(BinaryOperator &I) {
bool Changed = SimplifyAssociativeOrCommutative(I);
Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
@@ -413,7 +380,9 @@
I.setHasNoSignedWrap(true);
}
- if (!I.hasNoUnsignedWrap() && WillNotOverflowUnsignedMul(Op0, Op1, &I)) {
+ if (!I.hasNoUnsignedWrap() &&
+ computeOverflowForUnsignedMul(Op0, Op1, &I) ==
+ OverflowResult::NeverOverflows) {
Changed = true;
I.setHasNoUnsignedWrap(true);
}