[llvm] b65ab0b - [ValueTracking] Add comment clarifying missing `usub.sat` in `isKnownNonZero`; NFC

[Noah Goldstein via llvm-commits]

Author: Noah Goldstein
Date: 2024-04-08T23:33:06-05:00
New Revision: b65ab0b726ce421cc6cd7fdfbf51bf4aba17ce87

URL: https://github.com/llvm/llvm-project/commit/b65ab0b726ce421cc6cd7fdfbf51bf4aba17ce87
DIFF: https://github.com/llvm/llvm-project/commit/b65ab0b726ce421cc6cd7fdfbf51bf4aba17ce87.diff

LOG: [ValueTracking] Add comment clarifying missing `usub.sat` in `isKnownNonZero`; NFC

Closes #87700

Added: 
    

Modified: 
    llvm/lib/Analysis/ValueTracking.cpp

Removed: 
    


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diff  --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index d50ccd8af287d8..6602db424fc260 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -2816,6 +2816,8 @@ static bool isKnownNonZeroFromOperator(const Operator *I,
       case Intrinsic::bswap:
       case Intrinsic::ctpop:
         return isKnownNonZero(II->getArgOperand(0), DemandedElts, Depth, Q);
+        // NB: We don't do usub_sat here as in any case we can prove its
+        // non-zero, we will fold it to `sub nuw` in InstCombine.
       case Intrinsic::ssub_sat:
         return isNonZeroSub(DemandedElts, Depth, Q, BitWidth,
                             II->getArgOperand(0), II->getArgOperand(1));