Fixing miscompilation due to undef picking the wrong branch

llvm · Aug 8, 2024 · 05379be · 05379be
1 parent df7456e
commit 05379be
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Showing 2 changed files with 5 additions and 6 deletions.
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -1735,17 +1735,15 @@ Value *InstCombinerImpl::SimplifyDemandedVectorElts(Value *V,
     APInt DemandedLHS(DemandedElts), DemandedRHS(DemandedElts);
     if (auto *CV = dyn_cast<ConstantVector>(Sel->getCondition())) {
       for (unsigned i = 0; i < VWidth; i++) {
-        // isNullValue() always returns false when called on a ConstantExpr.
-        // Skip constant expressions to avoid propagating incorrect information.
         Constant *CElt = CV->getAggregateElement(i);
-        if (isa<ConstantExpr>(CElt))
-          continue;
+
         // TODO: If a select condition element is undef, we can demand from
         // either side. If one side is known undef, choosing that side would
         // propagate undef.
+        // isNullValue() always returns false when called on a ConstantExpr.
         if (CElt->isNullValue())
           DemandedLHS.clearBit(i);
-        else
+        else if (CElt->isOneValue())
           DemandedRHS.clearBit(i);
       }
     }

diff --git a/llvm/test/Transforms/InstCombine/pr98435.ll b/llvm/test/Transforms/InstCombine/pr98435.ll
@@ -4,7 +4,8 @@
 define <2 x i1> @pr98435(<2 x i1> %val) {
 ; CHECK-LABEL: define <2 x i1> @pr98435(
 ; CHECK-SAME: <2 x i1> [[VAL:%.*]]) {
-; CHECK-NEXT:    ret <2 x i1> <i1 poison, i1 true>
+; CHECK-NEXT:    [[VAL1:%.*]] = select <2 x i1> <i1 undef, i1 true>, <2 x i1> <i1 true, i1 true>, <2 x i1> [[VAL]]
+; CHECK-NEXT:    ret <2 x i1> [[VAL1]]
 ;
   %val1 = select <2 x i1> <i1 undef, i1 true>, <2 x i1> <i1 true, i1 true>, <2 x i1> %val
   ret <2 x i1> %val1