[InstSimplify] insertelement V, undef, ? --> V

This was part of InstCombine, but it's better placed in
InstSimplify. InstCombine also had an unreachable but weaker
fold for insertelement with undef index, so that is deleted.

llvm-svn: 361559

1 files changed, 5 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index b71841a1607..6e421dcaa73 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -4011,6 +4011,11 @@ Value *llvm::SimplifyInsertElementInst(Value *Vec, Value *Val, Value *Idx,
   if (isa<UndefValue>(Idx))
     return UndefValue::get(Vec->getType());
 
+  // Inserting an undef scalar? Assume it is the same value as the existing
+  // vector element.
+  if (isa<UndefValue>(Val))
+    return Vec;
+
   return nullptr;
 }