[DAGCombine] Add (sext_in_reg (zext x)) -> (sext x) combine

Handle the case where a sign extension has ended up being split into separate stages (typically to get around vector legal ops) and a zext + sext_in_reg gets inserted.

Differential Revision: https://reviews.llvm.org/D27461

llvm-svn: 288842

1 files changed, 9 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 590b10b5ce2..0fa0b919355 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -7137,6 +7137,15 @@ SDValue DAGCombiner::visitSIGN_EXTEND_INREG(SDNode *N) {
       return DAG.getNode(ISD::SIGN_EXTEND, SDLoc(N), VT, N00, N1);
   }
 
+  // fold (sext_in_reg (zext x)) -> (sext x)
+  // iff we are extending the source sign bit.
+  if (N0.getOpcode() == ISD::ZERO_EXTEND) {
+    SDValue N00 = N0.getOperand(0);
+    if (N00.getScalarValueSizeInBits() == EVTBits &&
+        (!LegalOperations || TLI.isOperationLegal(ISD::SIGN_EXTEND, VT)))
+      return DAG.getNode(ISD::SIGN_EXTEND, SDLoc(N), VT, N00, N1);
+  }
+
   // fold (sext_in_reg x) -> (zext_in_reg x) if the sign bit is known zero.
   if (DAG.MaskedValueIsZero(N0, APInt::getBitsSet(VTBits, EVTBits-1, EVTBits)))
     return DAG.getZeroExtendInReg(N0, SDLoc(N), EVT.getScalarType());