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Fold (trunc (srl x, c)) -> (srl (trunc x), c) 

llvm-svn: 28138
This commit is contained in:
Chris Lattner 2006-05-06 00:11:52 +00:00
parent 907e392dba
commit 86a1467fc0
1 changed files with 32 additions and 0 deletions
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32
llvm/lib/Target/TargetLowering.cpp
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@ -608,9 +608,41 @@ bool TargetLowering::SimplifyDemandedBits(SDOperand Op, uint64_t DemandedMask,
break;
}
case ISD::TRUNCATE: {
// Simplify the input, using demanded bit information, and compute the known
// zero/one bits live out.
if (SimplifyDemandedBits(Op.getOperand(0), DemandedMask,
KnownZero, KnownOne, TLO, Depth+1))
return true;
// If the input is only used by this truncate, see if we can shrink it based
// on the known demanded bits.
if (Op.getOperand(0).Val->hasOneUse()) {
SDOperand In = Op.getOperand(0);
switch (In.getOpcode()) {
default: break;
case ISD::SRL:
// Shrink SRL by a constant if none of the high bits shifted in are
// demanded.
if (ConstantSDNode *ShAmt = dyn_cast<ConstantSDNode>(In.getOperand(1))){
uint64_t HighBits = MVT::getIntVTBitMask(In.getValueType());
HighBits &= ~MVT::getIntVTBitMask(Op.getValueType());
HighBits >>= ShAmt->getValue();
if (ShAmt->getValue() < MVT::getSizeInBits(Op.getValueType()) &&
(DemandedMask & HighBits) == 0) {
// None of the shifted in bits are needed. Add a truncate of the
// shift input, then shift it.
SDOperand NewTrunc = TLO.DAG.getNode(ISD::TRUNCATE,
Op.getValueType(),
In.getOperand(0));
return TLO.CombineTo(Op, TLO.DAG.getNode(ISD::SRL,Op.getValueType(),
NewTrunc, In.getOperand(1)));
}
}
break;
}
}
assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?");
uint64_t OutMask = MVT::getIntVTBitMask(Op.getValueType());
KnownZero &= OutMask;