Use the new TargetLowering::ComputeNumSignBits method to eliminate

sign_extend_inreg operations.  Though ComputeNumSignBits is still rudimentary,
this is enough to compile this:

short test(short X, short x) {
  int Y = X+x;
  return (Y >> 1);
}
short test2(short X, short x) {
  int Y = (short)(X+x);
  return Y >> 1;
}

into:

_test:
        add r2, r3, r4
        srawi r3, r2, 1
        blr
_test2:
        add r2, r3, r4
        extsh r2, r2
        srawi r3, r2, 1
        blr

instead of:

_test:
        add r2, r3, r4
        srawi r2, r2, 1
        extsh r3, r2
        blr
_test2:
        add r2, r3, r4
        extsh r2, r2
        srawi r2, r2, 1
        extsh r3, r2
        blr

llvm-svn: 28146

1 files changed, 5 insertions, 5 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index b967855eeeb..16ce87a5361 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -1937,6 +1937,11 @@ SDOperand DAGCombiner::visitSIGN_EXTEND_INREG(SDNode *N) {
     SDOperand Truncate = DAG.getConstant(N0C->getValue(), EVT);
     return DAG.getNode(ISD::SIGN_EXTEND, VT, Truncate);
   }
+  
+  // If the input is already sign extended, just drop the extend.
+  if (TLI.ComputeNumSignBits(N0) >= MVT::getSizeInBits(VT)-EVTBits+1)
+    return N0;
+  
   // fold (sext_in_reg (sext_in_reg x, VT2), VT1) -> (sext_in_reg x, minVT) pt1
   if (N0.getOpcode() == ISD::SIGN_EXTEND_INREG && 
       cast<VTSDNode>(N0.getOperand(1))->getVT() <= EVT) {
@@ -1947,11 +1952,6 @@ SDOperand DAGCombiner::visitSIGN_EXTEND_INREG(SDNode *N) {
       EVT < cast<VTSDNode>(N0.getOperand(1))->getVT()) {
     return DAG.getNode(ISD::SIGN_EXTEND_INREG, VT, N0.getOperand(0), N1);
   }
-  // fold (sext_in_reg (assert_sext x)) -> (assert_sext x)
-  if (N0.getOpcode() == ISD::AssertSext && 
-      cast<VTSDNode>(N0.getOperand(1))->getVT() <= EVT) {
-    return N0;
-  }
   // fold (sext_in_reg (sextload x)) -> (sextload x)
   if (N0.getOpcode() == ISD::SEXTLOAD && 
       cast<VTSDNode>(N0.getOperand(3))->getVT() <= EVT) {