[InstSimplify] fold insertelement-of-extractelement

This was partly handled in InstCombine (only the constant
index case), so delete that and zap it more generally in
InstSimplify.

llvm-svn: 361576

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 6e421dcaa73..1f8245d30f6 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -4016,6 +4016,12 @@ Value *llvm::SimplifyInsertElementInst(Value *Vec, Value *Val, Value *Idx,
   if (isa<UndefValue>(Val))
     return Vec;
 
+  // If we are extracting a value from a vector, then inserting it into the same
+  // place, that's the input vector:
+  // insertelt Vec, (extractelt Vec, Idx), Idx --> Vec
+  if (match(Val, m_ExtractElement(m_Specific(Vec), m_Specific(Idx))))
+    return Vec;
+
   return nullptr;
 }