[InstCombine] Compute bits of first operand for multi-use sub

Otherwise the result will always be unknown anyway. This fixes one
of the last inconsistencies in results between computeKnownBits()
and SimplifyDemandedBits().

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
index 5a7ff48..ac824b2 100644 (file)
--- a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -1090,6 +1090,7 @@ Value *InstCombinerImpl::SimplifyMultipleUseDemandedBits(
       return I->getOperand(0);
 
     bool NSW = cast<OverflowingBinaryOperator>(I)->hasNoSignedWrap();
+    computeKnownBits(I->getOperand(0), LHSKnown, Depth + 1, CxtI);
     Known = KnownBits::computeForAddSub(/*Add*/ false, NSW, LHSKnown, RHSKnown);
     break;
   }