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[X86] Avoid introducing extra shuffles when lowering packed vector shifts.

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When lowering a vector shift node, the backend checks if the shift count is a
shuffle with a splat mask. If so, then it introduces an extra dag node to
extract the splat value from the shuffle. The splat value is then used
to generate a shift count of a target specific shift.

However, if we know that the shift count is a splat shuffle, we can use the
splat index 'I' to extract the I-th element from the first shuffle operand.
The advantage is that the splat shuffle may become dead since we no longer
use it.

Example:

;;
define <4 x i32> @example(<4 x i32> %a, <4 x i32> %b) {
  %c = shufflevector <4 x i32> %b, <4 x i32> undef, <4 x i32> zeroinitializer
  %shl = shl <4 x i32> %a, %c
  ret <4 x i32> %shl
}
;;

Before this patch, llc generated the following code (-mattr=+avx):
  vpshufd $0, %xmm1, %xmm1   # xmm1 = xmm1[0,0,0,0]
  vpxor  %xmm2, %xmm2
  vpblendw $3, %xmm1, %xmm2, %xmm1 # xmm1 = xmm1[0,1],xmm2[2,3,4,5,6,7]
  vpslld %xmm1, %xmm0, %xmm0
  retq

With this patch, the redundant splat operation is removed from the code.
  vpxor  %xmm2, %xmm2
  vpblendw $3, %xmm1, %xmm2, %xmm1 # xmm1 = xmm1[0,1],xmm2[2,3,4,5,6,7]
  vpslld %xmm1, %xmm0, %xmm0
  retq


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223461 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Andrea Di Biagio
2014-12-05 12:13:30 +00:00
parent 1610d6e878
commit 54529ed1c4
3 changed files with 173 additions and 16 deletions
Download Patch File Download Diff File Expand all files Collapse all files

27
lib/Target/X86/X86ISelLowering.cpp
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@@ -18444,30 +18444,27 @@ static SDValue LowerScalarVariableShift(SDValue Op, SelectionDAG &DAG,
} else {
if (Amt.getOpcode() == ISD::EXTRACT_SUBVECTOR)
Amt = Amt.getOperand(0);
if (Amt.getOpcode() == ISD::VECTOR_SHUFFLE &&
cast<ShuffleVectorSDNode>(Amt)->isSplat()) {
ShuffleVectorSDNode *SVN = dyn_cast<ShuffleVectorSDNode>(Amt);
if (SVN && SVN->isSplat()) {
unsigned SplatIdx = (unsigned)SVN->getSplatIndex();
SDValue InVec = Amt.getOperand(0);
if (InVec.getOpcode() == ISD::BUILD_VECTOR) {
unsigned NumElts = InVec.getValueType().getVectorNumElements();
unsigned i = 0;
for (; i != NumElts; ++i) {
SDValue Arg = InVec.getOperand(i);
if (Arg.getOpcode() == ISD::UNDEF) continue;
BaseShAmt = Arg;
break;
}
assert((SplatIdx < InVec.getValueType().getVectorNumElements()) &&
"Unexpected shuffle index found!");
BaseShAmt = InVec.getOperand(SplatIdx);
} else if (InVec.getOpcode() == ISD::INSERT_VECTOR_ELT) {
if (ConstantSDNode *C =
dyn_cast<ConstantSDNode>(InVec.getOperand(2))) {
unsigned SplatIdx =
cast<ShuffleVectorSDNode>(Amt)->getSplatIndex();
if (C->getZExtValue() == SplatIdx)
BaseShAmt = InVec.getOperand(1);
}
}
if (!BaseShAmt.getNode())
BaseShAmt = DAG.getNode(ISD::EXTRACT_VECTOR_ELT, dl, EltVT, Amt,
DAG.getIntPtrConstant(0));
if (!BaseShAmt)
// Avoid introducing an extract element from a shuffle.
BaseShAmt = DAG.getNode(ISD::EXTRACT_VECTOR_ELT, dl, EltVT, InVec,
DAG.getIntPtrConstant(SplatIdx));
}
}