Fix a ValueTracking rule: RHS means operand 1, not 0. Add a simple

ashr instcombine to help expose this code. And apply the fix to
SelectionDAG's copy of this code too.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@65364 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 6 insertions, 1 deletions

diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp
index 7f2cab96ec..62554a5e91 100644
--- a/lib/Transforms/Scalar/InstructionCombining.cpp
+++ b/lib/Transforms/Scalar/InstructionCombining.cpp
@@ -7031,7 +7031,12 @@ Instruction *InstCombiner::visitAShr(BinaryOperator &I) {
       MaskedValueIsZero(Op0,
                       APInt::getSignBit(I.getType()->getPrimitiveSizeInBits())))
     return BinaryOperator::CreateLShr(Op0, I.getOperand(1));
-  
+
+  // Arithmetic shifting an all-sign-bit value is a no-op.
+  unsigned NumSignBits = ComputeNumSignBits(Op0);
+  if (NumSignBits == Op0->getType()->getPrimitiveSizeInBits())
+    return ReplaceInstUsesWith(I, Op0);
+
   return 0;
 }