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Emit power-of-two constant multiply as shift #13128

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Aug 2, 2017
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Emit power-of-two constant multiply as shift #13128

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13 changes: 13 additions & 0 deletions src/jit/codegenxarch.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -982,6 +982,19 @@ void CodeGen::genCodeForMul(GenTreeOp* treeNode)
unsigned int scale = (unsigned int)(imm - 1);
getEmitter()->emitIns_R_ARX(INS_lea, size, targetReg, rmOp->gtRegNum, rmOp->gtRegNum, scale, 0);
}
else if (!requiresOverflowCheck && rmOp->isUsedFromReg() && (imm == genFindLowestBit(imm)) && (imm != 0))
{
// Use shift for constant multiply when legal
uint64_t zextImm = static_cast<uint64_t>(static_cast<size_t>(imm));
unsigned int shiftAmount = genLog2(zextImm);

if (targetReg != rmOp->gtRegNum)
{
// Copy reg src to dest register
inst_RV_RV(ins_Copy(targetType), targetReg, rmOp->gtRegNum, targetType);
}
inst_RV_SH(INS_shl, size, targetReg, shiftAmount);
}
else
{
// use the 3-op form with immediate
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