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[ValueTracking] Use ConstantRange based overflow check for signed sub 

This is D59450, but for signed sub. This case is not NFC, because
the overflow logic in ConstantRange is more powerful than the existing
check. This resolves the TODO in the function.

I've added two tests to show that this indeed catches more cases than
the previous logic, but the main correctness test coverage here is in
the existing ConstantRange unit tests.

Differential Revision: https://reviews.llvm.org/D59617

llvm-svn: 356685
This commit is contained in:
Nikita Popov 2019-03-21 17:23:51 +00:00
parent 54dab70bb7
commit 3af5b28f47
2 changed files with 7 additions and 12 deletions
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15
llvm/lib/Analysis/ValueTracking.cpp
View File

@ -4190,17 +4190,12 @@ OverflowResult llvm::computeOverflowForSignedSub(const Value *LHS,
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;
ConstantRange LHSRange =
ConstantRange::fromKnownBits(LHSKnown, /*signed*/ true);
ConstantRange RHSRange =
ConstantRange::fromKnownBits(RHSKnown, /*signed*/ true);
return mapOverflowResult(LHSRange.signedSubMayOverflow(RHSRange));
}
bool llvm::isOverflowIntrinsicNoWrap(const IntrinsicInst *II,

4
llvm/test/Transforms/InstCombine/sub.ll
View File

@ -1271,7 +1271,7 @@ define i32 @nsw_inference1(i32 %x, i32 %y) {
; CHECK-LABEL: @nsw_inference1(
; CHECK-NEXT: [[X2:%.*]] = or i32 [[X:%.*]], 1024
; CHECK-NEXT: [[Y2:%.*]] = and i32 [[Y:%.*]], 1
; CHECK-NEXT: [[Z:%.*]] = sub nuw i32 [[X2]], [[Y2]]
; CHECK-NEXT: [[Z:%.*]] = sub nuw nsw i32 [[X2]], [[Y2]]
; CHECK-NEXT: ret i32 [[Z]]
;
%x2 = or i32 %x, 1024
@ -1284,7 +1284,7 @@ define i32 @nsw_inference2(i32 %x, i32 %y) {
; CHECK-LABEL: @nsw_inference2(
; CHECK-NEXT: [[X2:%.*]] = and i32 [[X:%.*]], -1025
; CHECK-NEXT: [[Y2:%.*]] = or i32 [[Y:%.*]], -2
; CHECK-NEXT: [[Z:%.*]] = sub i32 [[X2]], [[Y2]]
; CHECK-NEXT: [[Z:%.*]] = sub nsw i32 [[X2]], [[Y2]]
; CHECK-NEXT: ret i32 [[Z]]
;
%x2 = and i32 %x, -1025