[InstCombine] replace undef elements in vector constant when doing icmp folds (PR44383)

As shown in P44383:
https://bugs.llvm.org/show_bug.cgi?id=44383
...we can't safely propagate a vector constant through this icmp fold
if that vector constant contains undefined elements.

We know that each defined element of the constant is safe though, so
find the first of those and replicate it into the formerly undef lanes.

Differential Revision: https://reviews.llvm.org/D72101

1 files changed, 17 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 4abac988c2e..f38dc436722 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -3364,6 +3364,23 @@ static Value *foldICmpWithLowBitMaskedVal(ICmpInst &I,
     llvm_unreachable("All possible folds are handled.");
   }
 
+  // The mask value may be a vector constant that has undefined elements. But it
+  // may not be safe to propagate those undefs into the new compare, so replace
+  // those elements by copying an existing, defined, and safe scalar constant.
+  Type *OpTy = M->getType();
+  auto *VecC = dyn_cast<Constant>(M);
+  if (OpTy->isVectorTy() && VecC && VecC->containsUndefElement()) {
+    Constant *SafeReplacementConstant = nullptr;
+    for (unsigned i = 0, e = OpTy->getVectorNumElements(); i != e; ++i) {
+      if (!isa<UndefValue>(VecC->getAggregateElement(i))) {
+        SafeReplacementConstant = VecC->getAggregateElement(i);
+        break;
+      }
+    }
+    assert(SafeReplacementConstant && "Failed to find undef replacement");
+    M = Constant::replaceUndefsWith(VecC, SafeReplacementConstant);
+  }
+
   return Builder.CreateICmp(DstPred, X, M);
 }