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Fix a minor bug in DAGCombiner's folding of SELECT. Folding "select C, 0, 1"

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to "C ^ 1" is only valid when C is known to be either 0 or 1.  Most of the
similar foldings in this function only handle "i1" types, but this one appears
intentionally written to handle larger integer types.  If C has an integer
type larger than "i1", this needs to check if the high bits of a boolean
are known to be zero.  I also changed the comment to describe this folding as
"C ^ 1" instead of "~C", since that is what the code does and since the latter
would only be valid for "i1" types.  The good news is that most LLVM targets
use TargetLowering::ZeroOrOneBooleanContent so this change will not disable
the optimization; the bad news is that I've been unable to come up with a
testcase to demonstrate the problem.

I have also removed a "FIXME" comment for folding "select C, X, 0" to "C & X",
since the code looks correct to me.  It could be made more aggressive by not
limiting the type to "i1", but that would then require checking for
TargetLowering::ZeroOrNegativeOneBooleanContent.  Similar changes could be
done for the other SELECT foldings, but it was decided to be not worth the
trouble and complexity (see e.g., r44663).

llvm-svn: 62790
This commit is contained in:
Bob Wilson 2009-01-22 22:05:48 +00:00
parent a6e5948fce
commit d79e684d1e
1 changed files with 5 additions and 3 deletions
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8
lib/CodeGen/SelectionDAG/DAGCombiner.cpp
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@ -2746,8 +2746,11 @@ SDValue DAGCombiner::visitSELECT(SDNode *N) {
// fold select C, 1, X -> C | X
if (VT == MVT::i1 && N1C && N1C->getAPIntValue() == 1)
return DAG.getNode(ISD::OR, VT, N0, N2);
// fold select C, 0, 1 -> ~C
if (VT.isInteger() && VT0.isInteger() &&
// fold select C, 0, 1 -> C ^ 1
if (VT.isInteger() &&
(VT0 == MVT::i1 ||
(VT0.isInteger() &&
TLI.getBooleanContents() == TargetLowering::ZeroOrOneBooleanContent)) &&
N1C && N2C && N1C->isNullValue() && N2C->getAPIntValue() == 1) {
SDValue XORNode = DAG.getNode(ISD::XOR, VT0, N0, DAG.getConstant(1, VT0));
if (VT == VT0)
@ -2770,7 +2773,6 @@ SDValue DAGCombiner::visitSELECT(SDNode *N) {
return DAG.getNode(ISD::OR, VT, NOTNode, N1);
}
// fold select C, X, 0 -> C & X
// FIXME: this should check for C type == X type, not i1?
if (VT == MVT::i1 && N2C && N2C->isNullValue())
return DAG.getNode(ISD::AND, VT, N0, N1);
// fold X ? X : Y --> X ? 1 : Y --> X | Y