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The type-legalizer often scalarizes code. One of the common patterns is extract-and-truncate.

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In this patch we optimize this pattern and convert the sequence into extract op of a narrow type.
This allows the BUILD_VECTOR dag optimizations to construct efficient shuffle operations in many cases.




git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@149692 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Nadav Rotem
2012-02-03 13:18:25 +00:00
parent a02556679e
commit 7e413e9c94
3 changed files with 43 additions and 1 deletions
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34
lib/CodeGen/SelectionDAG/DAGCombiner.cpp
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@@ -4957,6 +4957,7 @@ SDValue DAGCombiner::visitSIGN_EXTEND_INREG(SDNode *N) {
SDValue DAGCombiner::visitTRUNCATE(SDNode *N) {
SDValue N0 = N->getOperand(0);
EVT VT = N->getValueType(0);
bool isLE = TLI.isLittleEndian();
// noop truncate
if (N0.getValueType() == N->getValueType(0))
@@ -4984,6 +4985,39 @@ SDValue DAGCombiner::visitTRUNCATE(SDNode *N) {
return N0.getOperand(0);
}
// Fold Extract-and-trunc into a narrow extract:
// trunc(extract(x)) -> extract(bitcast(x))
// We only run this optimization after type legalization (which often
// creates this pattern) and before operation legalization after which
// we need to be more careful about the vector instructions that we generate.
if (N0.getOpcode() == ISD::EXTRACT_VECTOR_ELT &&
LegalTypes && !LegalOperations && N0->hasOneUse()) {
EVT VecTy = N0.getOperand(0).getValueType();
EVT ExTy = N0.getValueType();
EVT TrTy = N->getValueType(0);
unsigned NumElem = VecTy.getVectorNumElements();
unsigned SizeRatio = ExTy.getSizeInBits()/TrTy.getSizeInBits();
EVT NVT = EVT::getVectorVT(*DAG.getContext(), TrTy, SizeRatio * NumElem);
assert(NVT.getSizeInBits() == VecTy.getSizeInBits() && "Invalid Size");
SDValue EltNo = N0->getOperand(1);
if (isa<ConstantSDNode>(EltNo) && isTypeLegal(NVT)) {
int Elt = cast<ConstantSDNode>(EltNo)->getZExtValue();
int Index = isLE ? (Elt*SizeRatio) : (Elt*SizeRatio + (SizeRatio-1));
SDValue V = DAG.getNode(ISD::BITCAST, N->getDebugLoc(),
NVT, N0.getOperand(0));
return DAG.getNode(ISD::EXTRACT_VECTOR_ELT,
N->getDebugLoc(), TrTy, V,
DAG.getConstant(Index, MVT::i32));
}
}
// 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