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[InstCombine] (-NSW x) u< x --> x s< 0 (PR39480) 

Name: (-x) u< x  -->  x s< 0
%neg_x = sub nsw i8 0, %x ; %x must not be INT_MIN
%r = icmp ult i8 %neg_x, %x
  =>
%r = icmp slt i8 %x, 0

https://rise4fun.com/Alive/zSuf

https://bugs.llvm.org/show_bug.cgi?id=39480
This commit is contained in:
Roman Lebedev 2020-08-06 11:26:48 +03:00
parent 0e1241a3c9
commit 132be1f502
No known key found for this signature in database
GPG Key ID: 083C3EBB4A1689E0
2 changed files with 6 additions and 2 deletions
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5
llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
View File

@ -3758,6 +3758,11 @@ Instruction *foldICmpXNegX(ICmpInst &I) {
NewRHS = Constant::getNullValue(X->getType());
break;
case ICmpInst::ICMP_ULT:
NewPred = ICmpInst::ICMP_SLT;
NewRHS = Constant::getNullValue(X->getType());
break;
case ICmpInst::ICMP_EQ:
case ICmpInst::ICMP_NE:
NewPred = Pred;

3
llvm/test/Transforms/InstCombine/cmp-x-vs-neg-x.ll
View File

@ -91,8 +91,7 @@ define i1 @t5(i8 %x) {
define i1 @t6(i8 %x) {
; CHECK-LABEL: @t6(
; CHECK-NEXT: [[NEG_X:%.*]] = sub nsw i8 0, [[X:%.*]]
; CHECK-NEXT: [[CMP:%.*]] = icmp ult i8 [[NEG_X]], [[X]]
; CHECK-NEXT: [[CMP:%.*]] = icmp slt i8 [[X:%.*]], 0
; CHECK-NEXT: ret i1 [[CMP]]
;
%neg_x = sub nsw i8 0, %x