Teach the dag combiner to turn a truncate/sign_extend pair into a sextinreg

when the types match up.  This allows the X86 backend to compile:

sbyte %toggle_value(sbyte* %tmp.1) {
        %tmp.2 = load sbyte* %tmp.1
        ret sbyte %tmp.2
}

to this:

_toggle_value:
        mov %EAX, DWORD PTR [%ESP + 4]
        movsx %EAX, BYTE PTR [%EAX]
        ret

instead of this:

_toggle_value:
        mov %EAX, DWORD PTR [%ESP + 4]
        movsx %EAX, BYTE PTR [%EAX]
        movsx %EAX, %AL
        ret

noticed in Shootout/objinst.

-Chris

llvm-svn: 24630

1 files changed, 4 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 6cd66c7c19c..b7f6853f74c 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -1546,6 +1546,10 @@ SDOperand DAGCombiner::visitSIGN_EXTEND(SDNode *N) {
   // fold (sext (sextload x)) -> (sextload x)
   if (N0.getOpcode() == ISD::SEXTLOAD && VT == N0.getValueType())
     return N0;
+  // fold (sext (truncate x)) -> (sextinreg x) iff x size == sext size.
+  if (N0.getOpcode() == ISD::TRUNCATE && N0.getOperand(0).getValueType() == VT)
+    return DAG.getNode(ISD::SIGN_EXTEND_INREG, VT, N0.getOperand(0),
+                       DAG.getValueType(N0.getValueType()));
   // fold (sext (load x)) -> (sextload x)
   if (N0.getOpcode() == ISD::LOAD && N0.hasOneUse()) {
     SDOperand ExtLoad = DAG.getExtLoad(ISD::SEXTLOAD, VT, N0.getOperand(0),