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

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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


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@24630 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2005-12-07 07:11:03 +00:00
parent 8b6cea1b95
commit b14ab8a10d
1 changed files with 4 additions and 0 deletions
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4
lib/CodeGen/SelectionDAG/DAGCombiner.cpp
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@ -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),