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[ValueTracking] When calculating known bits for integer abs, make sure we're looking at a negate and not just any instruction with the nsw flag set.

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The matchSelectPattern code can match patterns like (x >= 0) ? x : -x
for absolute value. But it can also match ((x-y) >= 0) ? (x-y) : (y-x).
If the latter form was matched we can only use the nsw flag if its
set on both subtracts.

This match makes sure we're looking at the former case only.

Differential Revision: https://reviews.llvm.org/D65692

llvm-svn: 368195
This commit is contained in:
Craig Topper 2019-08-07 18:28:16 +00:00
parent f74d811f50
commit 3d068e1b37
2 changed files with 12 additions and 5 deletions
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10
lib/Analysis/ValueTracking.cpp
View File

@ -1095,7 +1095,8 @@ static void computeKnownBitsFromOperator(const Operator *I, KnownBits &Known,
// RHS from matchSelectPattern returns the negation part of abs pattern.
// If the negate has an NSW flag we can assume the sign bit of the result
// will be 0 because that makes abs(INT_MIN) undefined.
if (Q.IIQ.hasNoSignedWrap(cast<Instruction>(RHS)))
if (match(RHS, m_Neg(m_Specific(LHS))) &&
Q.IIQ.hasNoSignedWrap(cast<Instruction>(RHS)))
MaxHighZeros = 1;
}
@ -5611,7 +5612,7 @@ static void setLimitsForIntrinsic(const IntrinsicInst &II, APInt &Lower,
}
static void setLimitsForSelectPattern(const SelectInst &SI, APInt &Lower,
APInt &Upper) {
APInt &Upper, const InstrInfoQuery &IIQ) {
const Value *LHS, *RHS;
SelectPatternResult R = matchSelectPattern(&SI, LHS, RHS);
if (R.Flavor == SPF_UNKNOWN)
@ -5624,7 +5625,8 @@ static void setLimitsForSelectPattern(const SelectInst &SI, APInt &Lower,
// then the result of abs(X) is [0..SIGNED_MAX],
// otherwise it is [0..SIGNED_MIN], as -SIGNED_MIN == SIGNED_MIN.
Lower = APInt::getNullValue(BitWidth);
if (cast<Instruction>(RHS)->hasNoSignedWrap())
if (match(RHS, m_Neg(m_Specific(LHS))) &&
IIQ.hasNoSignedWrap(cast<Instruction>(RHS)))
Upper = APInt::getSignedMaxValue(BitWidth) + 1;
else
Upper = APInt::getSignedMinValue(BitWidth) + 1;
@ -5678,7 +5680,7 @@ ConstantRange llvm::computeConstantRange(const Value *V, bool UseInstrInfo) {
else if (auto *II = dyn_cast<IntrinsicInst>(V))
setLimitsForIntrinsic(*II, Lower, Upper);
else if (auto *SI = dyn_cast<SelectInst>(V))
setLimitsForSelectPattern(*SI, Lower, Upper);
setLimitsForSelectPattern(*SI, Lower, Upper, IIQ);
ConstantRange CR = ConstantRange::getNonEmpty(Lower, Upper);

7
test/Transforms/InstSimplify/icmp-abs-nabs.ll
View File

@ -404,7 +404,12 @@ define i1 @nabs_no_intersection(i32 %a) {
; We can't fold this to false unless both subs have nsw.
define i1 @abs_sub_sub_missing_nsw(i32 %x, i32 %y) {
; CHECK-LABEL: @abs_sub_sub_missing_nsw(
; CHECK-NEXT: ret i1 false
; CHECK-NEXT: [[A:%.*]] = sub i32 [[X:%.*]], [[Y:%.*]]
; CHECK-NEXT: [[B:%.*]] = sub nsw i32 [[Y]], [[X]]
; CHECK-NEXT: [[C:%.*]] = icmp sgt i32 [[A]], -1
; CHECK-NEXT: [[D:%.*]] = select i1 [[C]], i32 [[A]], i32 [[B]]
; CHECK-NEXT: [[E:%.*]] = icmp slt i32 [[D]], 0
; CHECK-NEXT: ret i1 [[E]]
;
%a = sub i32 %x, %y
%b = sub nsw i32 %y, %x