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Add a simple optimization to simplify the input to

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truncate and truncstore instructions, based on the 
knowledge that they don't demand the top bits.

llvm-svn: 42952
This commit is contained in:
Chris Lattner 2007-10-13 06:35:54 +00:00
parent f3582ac0e0
commit 133baf6012
2 changed files with 43 additions and 1 deletions
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42
lib/CodeGen/SelectionDAG/DAGCombiner.cpp
View File

@ -295,6 +295,8 @@ namespace {
SDNode *MatchRotate(SDOperand LHS, SDOperand RHS);
SDOperand ReduceLoadWidth(SDNode *N);
SDOperand GetDemandedBits(SDOperand V, uint64_t Mask);
/// GatherAllAliases - Walk up chain skipping non-aliasing memory nodes,
/// looking for aliasing nodes and adding them to the Aliases vector.
void GatherAllAliases(SDNode *N, SDOperand OriginalChain,
@ -2793,6 +2795,24 @@ SDOperand DAGCombiner::visitANY_EXTEND(SDNode *N) {
return SDOperand();
}
/// GetDemandedBits - See if the specified operand can be simplified with the
/// knowledge that only the bits specified by Mask are used. If so, return the
/// simpler operand, otherwise return a null SDOperand.
SDOperand DAGCombiner::GetDemandedBits(SDOperand V, uint64_t Mask) {
switch (V.getOpcode()) {
default: break;
case ISD::OR:
case ISD::XOR:
// If the LHS or RHS don't contribute bits to the or, drop them.
if (DAG.MaskedValueIsZero(V.getOperand(0), Mask))
return V.getOperand(1);
if (DAG.MaskedValueIsZero(V.getOperand(1), Mask))
return V.getOperand(0);
break;
}
return SDOperand();
}
/// ReduceLoadWidth - If the result of a wider load is shifted to right of N
/// bits and then truncated to a narrower type and where N is a multiple
/// of number of bits of the narrower type, transform it to a narrower load
@ -2986,6 +3006,13 @@ SDOperand DAGCombiner::visitTRUNCATE(SDNode *N) {
return N0.getOperand(0);
}
// See if we can simplify the input to this truncate through knowledge that
// only the low bits are being used. For example "trunc (or (shl x, 8), y)"
// -> trunc y
SDOperand Shorter = GetDemandedBits(N0, MVT::getIntVTBitMask(VT));
if (Shorter.Val)
return DAG.getNode(ISD::TRUNCATE, VT, Shorter);
// fold (truncate (load x)) -> (smaller load x)
// fold (truncate (srl (load x), c)) -> (smaller load (x+c/evtbits))
return ReduceLoadWidth(N);
@ -4000,6 +4027,21 @@ SDOperand DAGCombiner::visitSTORE(SDNode *N) {
if (CombineToPreIndexedLoadStore(N) || CombineToPostIndexedLoadStore(N))
return SDOperand(N, 0);
// FIXME: is there such a think as a truncating indexed store?
if (ST->isTruncatingStore() && ST->getAddressingMode() == ISD::UNINDEXED &&
MVT::isInteger(Value.getValueType())) {
// See if we can simplify the input to this truncstore with knowledge that
// only the low bits are being used. For example:
// "truncstore (or (shl x, 8), y), i8" -> "truncstore y, i8"
SDOperand Shorter =
GetDemandedBits(Value, MVT::getIntVTBitMask(ST->getStoredVT()));
AddToWorkList(Value.Val);
if (Shorter.Val)
return DAG.getTruncStore(Chain, Shorter, Ptr, ST->getSrcValue(),
ST->getSrcValueOffset(), ST->getStoredVT(),
ST->isVolatile(), ST->getAlignment());
}
return SDOperand();
}

2
lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
View File

@ -2025,7 +2025,7 @@ SDOperand SelectionDAGLegalize::LegalizeOp(SDOperand Op) {
// probably means that we need to integrate dag combiner and legalizer
// together.
// We generally can't do this one for long doubles.
if (ConstantFPSDNode *CFP =dyn_cast<ConstantFPSDNode>(ST->getValue())) {
if (ConstantFPSDNode *CFP = dyn_cast<ConstantFPSDNode>(ST->getValue())) {
if (CFP->getValueType(0) == MVT::f32) {
Tmp3 = DAG.getConstant((uint32_t)CFP->getValueAPF().
convertToAPInt().getZExtValue(),