[PATCH] D40231: InstructionSimplify: 'extractelement' with an undef index is undef

[Zvi Rackover via Phabricator via llvm-commits]

zvi created this revision.
Herald added a subscriber: wdng.

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.


https://reviews.llvm.org/D40231

Files:
  lib/Analysis/InstructionSimplify.cpp
  test/Transforms/InstSimplify/2011-09-05-InsertExtractValue.ll


Index: test/Transforms/InstSimplify/2011-09-05-InsertExtractValue.ll
===================================================================
--- test/Transforms/InstSimplify/2011-09-05-InsertExtractValue.ll
+++ test/Transforms/InstSimplify/2011-09-05-InsertExtractValue.ll
@@ -46,3 +46,10 @@
 ; 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
Index: lib/Analysis/InstructionSimplify.cpp
===================================================================
--- lib/Analysis/InstructionSimplify.cpp
+++ lib/Analysis/InstructionSimplify.cpp
@@ -3863,6 +3863,11 @@
     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;
 }
 


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