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[ValueTracking] Compute range for abs without nsw

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This is a small followup to D59511. The code that was moved into
computeConstantRange() there is a bit overly conversative: If the
abs is not nsw, it does not compute any range. However, abs without
nsw still has a well-defined contiguous unsigned range from 0 to
SIGNED_MIN. This is a lot less useful than the usual 0 to SIGNED_MAX
range, but if we're already here we might as well specify it...

Differential Revision: https://reviews.llvm.org/D59563

llvm-svn: 356586
This commit is contained in:
Nikita Popov 2019-03-20 18:16:02 +00:00
parent b4e86424c1
commit 8f1e59217b
2 changed files with 9 additions and 12 deletions
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15
lib/Analysis/ValueTracking.cpp
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@ -5673,14 +5673,15 @@ static void setLimitsForSelectPattern(const SelectInst &SI, APInt &Lower,
unsigned BitWidth = SI.getType()->getScalarSizeInBits();
// matchSelectPattern() returns the negation part of an abs pattern in RHS.
// If the negate has an NSW flag, abs(INT_MIN) is undefined. Without that
// constraint, we can't make a contiguous range for the result of abs.
if (R.Flavor == SelectPatternFlavor::SPF_ABS &&
cast<Instruction>(RHS)->hasNoSignedWrap()) {
// The result of abs(X) is >= 0 (with nsw).
if (R.Flavor == SelectPatternFlavor::SPF_ABS) {
// If the negation part of the abs (in RHS) has the NSW flag,
// 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);
Upper = APInt::getSignedMaxValue(BitWidth) + 1;
if (cast<Instruction>(RHS)->hasNoSignedWrap())
Upper = APInt::getSignedMaxValue(BitWidth) + 1;
else
Upper = APInt::getSignedMinValue(BitWidth) + 1;
return;
}

6
test/Transforms/InstSimplify/icmp-abs-nabs.ll
View File

@ -150,11 +150,7 @@ define i1 @abs_nsw_is_not_negative_wrong_range(i32 %x) {
; Even if we don't have nsw, the range is still limited in the unsigned domain.
define i1 @abs_positive_or_signed_min(i32 %x) {
; CHECK-LABEL: @abs_positive_or_signed_min(
; CHECK-NEXT: [[CMP:%.*]] = icmp slt i32 [[X:%.*]], 0
; CHECK-NEXT: [[NEGX:%.*]] = sub i32 0, [[X]]
; CHECK-NEXT: [[ABS:%.*]] = select i1 [[CMP]], i32 [[NEGX]], i32 [[X]]
; CHECK-NEXT: [[R:%.*]] = icmp ult i32 [[ABS]], -2147483647
; CHECK-NEXT: ret i1 [[R]]
; CHECK-NEXT: ret i1 true
;
%cmp = icmp slt i32 %x, 0
%negx = sub i32 0, %x