[llvm] 5379f76 - [InstCombine] try harder to preserve 'nsz' in fneg-of-select transform

[Sanjay Patel via llvm-commits]

Author: Sanjay Patel
Date: 2022-02-24T10:43:53-05:00
New Revision: 5379f76e6328ecfa2e15b3c7ab32c4a053f4a663

URL: https://github.com/llvm/llvm-project/commit/5379f76e6328ecfa2e15b3c7ab32c4a053f4a663
DIFF: https://github.com/llvm/llvm-project/commit/5379f76e6328ecfa2e15b3c7ab32c4a053f4a663.diff

LOG: [InstCombine] try harder to preserve 'nsz' in fneg-of-select transform

The corner case where 'nsz' needs to be removed is very narrow
as discussed here:
https://reviews.llvm.org/rG3cdd05e519dd

If the select condition is not undef, there's no problem with
propagating 'nsz':
https://alive2.llvm.org/ce/z/4GWJdq

Added: 
    

Modified: 
    llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
    llvm/test/Transforms/InstCombine/fneg.ll

Removed: 
    


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diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 8a881924ab09f..37c05529c6501 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -2301,7 +2301,8 @@ Instruction *InstCombinerImpl::visitFNeg(UnaryOperator &I) {
     auto propagateSelectFMF = [&](SelectInst *S) {
       S->copyFastMathFlags(&I);
       if (auto *OldSel = dyn_cast<SelectInst>(Op))
-        if (!OldSel->hasNoSignedZeros())
+        if (!OldSel->hasNoSignedZeros() &&
+            !isGuaranteedNotToBeUndefOrPoison(OldSel->getCondition()))
           S->setHasNoSignedZeros(false);
     };
     // -(Cond ? -P : Y) --> Cond ? P : -Y

diff  --git a/llvm/test/Transforms/InstCombine/fneg.ll b/llvm/test/Transforms/InstCombine/fneg.ll
index 4c60a20d09ab3..b8df66ee74aa7 100644
--- a/llvm/test/Transforms/InstCombine/fneg.ll
+++ b/llvm/test/Transforms/InstCombine/fneg.ll
@@ -648,12 +648,12 @@ define float @select_fneg_false_no_nsz(float %x, float %y, i1 %b) {
   ret float %r
 }
 
-; TODO: The removal of nsz in this pattern is not needed if the select condition can't be poison.
+; The removal of nsz in this pattern is not needed if the select condition can't be poison.
 
 define float @select_fneg_false_nsz_ok(float %x, float %y, i1 noundef %b) {
 ; CHECK-LABEL: @select_fneg_false_nsz_ok(
 ; CHECK-NEXT:    [[X_NEG:%.*]] = fneg nnan ninf nsz float [[X:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = select nnan ninf i1 [[B:%.*]], float [[X_NEG]], float [[Y:%.*]]
+; CHECK-NEXT:    [[R:%.*]] = select nnan ninf nsz i1 [[B:%.*]], float [[X_NEG]], float [[Y:%.*]]
 ; CHECK-NEXT:    ret float [[R]]
 ;
   %ny = fneg float %y