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implement CodeGen/PowerPC/div-2.ll:test2-4 by propagating zero bits through

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C-X's


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@23662 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner
2005-10-07 15:30:32 +00:00
parent a77e86e73e
commit c4ced268d8
2 changed files with 41 additions and 1 deletions
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18
lib/CodeGen/SelectionDAG/DAGCombiner.cpp
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@@ -205,6 +205,24 @@ static bool MaskedValueIsZero(const SDOperand &Op, uint64_t Mask,
return MaskedValueIsZero(Op.getOperand(0), NewVal, TLI);
}
return false;
case ISD::SUB:
if (ConstantSDNode *CLHS = dyn_cast<ConstantSDNode>(Op.getOperand(0))) {
// We know that the top bits of C-X are clear if X contains less bits
// than C (i.e. no wrap-around can happen). For example, 20-X is
// positive if we can prove that X is >= 0 and < 16.
unsigned Bits = MVT::getSizeInBits(CLHS->getValueType(0));
if ((CLHS->getValue() & (1 << (Bits-1))) == 0) { // sign bit clear
unsigned NLZ = CountLeadingZeros_64(CLHS->getValue()+1);
uint64_t MaskV = (1ULL << (63-NLZ))-1;
if (MaskedValueIsZero(Op.getOperand(1), ~MaskV, TLI)) {
// High bits are clear this value is known to be >= C.
unsigned NLZ2 = CountLeadingZeros_64(CLHS->getValue());
if ((Mask & ((1ULL << (64-NLZ2))-1)) == 0)
return true;
}
}
}
break;
case ISD::CTTZ:
case ISD::CTLZ:
case ISD::CTPOP: