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[X86][SSE] Improve comments describing combineTruncatedArithmetic. NFCI.

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llvm-svn: 292502
This commit is contained in:
Simon Pilgrim 2017-01-19 18:18:32 +00:00
parent 7de57d9c60
commit 915800c7e4
1 changed files with 7 additions and 0 deletions
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7
lib/Target/X86/X86ISelLowering.cpp
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@ -32350,9 +32350,14 @@ static SDValue combineTruncatedArithmetic(SDNode *N, SelectionDAG &DAG,
auto IsRepeatedOpOrFreeTruncation = [VT](SDValue Op0, SDValue Op1) {
unsigned TruncSizeInBits = VT.getScalarSizeInBits();
// Repeated operand, so we are only trading one output truncation for
// one input truncation.
if (Op0 == Op1)
return true;
// See if either operand has been extended from a smaller/equal size to
// the truncation size, allowing a truncation to combine with the extend.
unsigned Opcode0 = Op0.getOpcode();
if ((Opcode0 == ISD::ANY_EXTEND || Opcode0 == ISD::SIGN_EXTEND ||
Opcode0 == ISD::ZERO_EXTEND) &&
@ -32365,6 +32370,8 @@ static SDValue combineTruncatedArithmetic(SDNode *N, SelectionDAG &DAG,
Op1.getOperand(0).getScalarValueSizeInBits() <= TruncSizeInBits)
return true;
// See if either operand is a single use constant which can be constant
// folded.
SDValue BC0 = peekThroughOneUseBitcasts(Op0);
SDValue BC1 = peekThroughOneUseBitcasts(Op1);
return ISD::isBuildVectorOfConstantSDNodes(BC0.getNode()) ||