InstructionSimplify: 'extractelement' with an undef index is undef

Summary:
An undef extract index can be arbitrarily chosen to be an
out-of-range index value, which would result in the instruction being undef.

This change closes a gap identified while working on lowering vector permute intrinsics
with variable index vectors to pure LLVM IR.

Reviewers: arsenm, spatel, majnemer

Reviewed By: arsenm, spatel

Subscribers: fhahn, nhaehnle, wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D40231

llvm-svn: 319910

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -3901,6 +3901,11 @@ static Value *SimplifyExtractElementInst(Value *Vec, Value *Idx, const SimplifyQ
     if (Value *Elt = findScalarElement(Vec, IdxC->getZExtValue()))
       return Elt;
 
+  // An undef extract index can be arbitrarily chosen to be an out-of-range
+  // index value, which would result in the instruction being undef.
+  if (isa<UndefValue>(Idx))
+    return UndefValue::get(Vec->getType()->getVectorElementType());
+
   return nullptr;
 }
 

llvm/test/Transforms/InstSimplify/2011-09-05-InsertExtractValue.ll

@@ -46,3 +46,10 @@ define i8 @test4(<8 x i8> %V) {
 ; CHECK-NEXT: %[[extract:.*]] = extractelement <8 x i8> %[[add]], i32 6
 ; CHECK-NEXT: ret i8 %[[extract]]
 }
+
+define i32 @test5(<4 x i32> %V) {
+  %extract = extractelement <4 x i32> %V, i32 undef
+  ret i32 %extract
+}
+; CHECK-LABEL: @test5(
+; CHECK: ret i32 undef