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Implement bit propagation through sub nodes, this (re)implements

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PowerPC/div-2.ll


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@26392 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2006-02-27 01:00:42 +00:00
parent 71f5dfa8e9
commit a6bc5a4d21
1 changed files with 29 additions and 3 deletions
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32
lib/CodeGen/SelectionDAG/TargetLowering.cpp
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@ -598,6 +598,12 @@ bool TargetLowering::SimplifyDemandedBits(SDOperand Op, uint64_t DemandedMask,
}
}
break;
case ISD::SUB:
// Just use ComputeMaskedBits to compute output bits, there are no
// simplifications that can be done here, and sub always demands all input
// bits.
ComputeMaskedBits(Op, DemandedMask, KnownZero, KnownOne, Depth);
break;
}
// If we know the value of all of the demanded bits, return this as a
@ -861,12 +867,32 @@ void TargetLowering::ComputeMaskedBits(SDOperand Op, uint64_t Mask,
KnownOne = 0;
return;
}
case ISD::SUB:
case ISD::SUB: {
ConstantSDNode *CLHS = dyn_cast<ConstantSDNode>(Op.getOperand(0));
if (!CLHS) return;
// 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. Remember to update
// SimplifyDemandedBits if/when this is implemented.
// positive if we can prove that X is >= 0 and < 16.
MVT::ValueType VT = CLHS->getValueType(0);
if ((CLHS->getValue() & MVT::getIntVTSignBit(VT)) == 0) { // sign bit clear
unsigned NLZ = CountLeadingZeros_64(CLHS->getValue()+1);
uint64_t MaskV = (1ULL << (63-NLZ))-1; // NLZ can't be 64 with no sign bit
MaskV = ~MaskV & MVT::getIntVTBitMask(VT);
ComputeMaskedBits(Op.getOperand(1), MaskV, KnownZero, KnownOne, Depth+1);
// If all of the MaskV bits are known to be zero, then we know the output
// top bits are zero, because we now know that the output is from [0-C].
if ((KnownZero & MaskV) == MaskV) {
unsigned NLZ2 = CountLeadingZeros_64(CLHS->getValue());
KnownZero = ~((1ULL << (64-NLZ2))-1) & Mask; // Top bits known zero.
KnownOne = 0; // No one bits known.
} else {
KnownOne = KnownOne = 0; // Otherwise, nothing known.
}
}
return;
}
default:
// Allow the target to implement this method for its nodes.
if (Op.getOpcode() >= ISD::BUILTIN_OP_END)