[ConstProp] Handle insertelement constants

Previously ConstantFoldExtractElementInstruction() would only work with
insertelement instructions, not contants. This properly handles
insertelement constants as well.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D85865

1 files changed, 9 insertions, 0 deletions

diff --git a/llvm/lib/IR/ConstantFold.cpp b/llvm/lib/IR/ConstantFold.cpp
index f02246c..2d3afca 100644
--- a/llvm/lib/IR/ConstantFold.cpp
+++ b/llvm/lib/IR/ConstantFold.cpp
@@ -853,6 +853,15 @@ Constant *llvm::ConstantFoldExtractElementInstruction(Constant *Val,
       }
       return CE->getWithOperands(Ops, ValVTy->getElementType(), false,
                                  Ops[0]->getType()->getPointerElementType());
+    } else if (CE->getOpcode() == Instruction::InsertElement) {
+      if (const auto *IEIdx = dyn_cast<ConstantInt>(CE->getOperand(2))) {
+        if (APSInt::isSameValue(APSInt(IEIdx->getValue()),
+                                APSInt(CIdx->getValue()))) {
+          return CE->getOperand(1);
+        } else {
+          return ConstantExpr::getExtractElement(CE->getOperand(0), CIdx);
+        }
+      }
     }
   }