[RISCV] Commute True in foldVMergeToMask #156499
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In order to fold a vmerge into a pseudo, the pseudo's passthru needs to be the same as vmerge's false operand.
If they don't match we can try and commute the instruction if possible, e.g. here we can commute v9 and v8 to fold the vmerge:
vsetvli zero, a0, e32, m1, ta, ma
vfmadd.vv v9, v10, v8
vsetvli zero, zero, e32, m1, tu, ma
vmerge.vvm v8, v8, v9, v0
vsetvli zero, a0, e32, m1, tu, mu
vfmacc.vv v8, v9, v10, v0.t
Previously this wasn't possible because we did the peephole in SelectionDAG, but now that it's been migrated to MachineInstr in llvm#144076 we can reuse the commuting infrastructure in TargetInstrInfo.
This fixes the extra vmv.v.v in the "mul" example here: llvm#123069 (comment)
It should also allow us to remove the isel patterns described in llvm#141885 later.
topperc
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Sep 2, 2025
lukel97
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Sep 3, 2025
Now that RISCVVectorPeephole can commute operands to fold vmerge into a pseudo to make it masked in llvm#156499, we can remove the remaining VPatMultiplyAccVL_VV_VX/VPatFPMulAccVL_VV_VF_RM patterns. It also looks like we can remove the vmerge_vl patterns for _TIED psuedos too. Tested on SPEC CPU 2017 and llvm-test-suite to confirm there's no codegen change. Fixes llvm#141885
lukel97
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Sep 3, 2025
Now that RISCVVectorPeephole can commute operands to fold vmerge into a pseudo to make it masked in #156499, we can remove the remaining VPatMultiplyAccVL_VV_VX/VPatFPMulAccVL_VV_VF_RM patterns. It also looks like we can remove the vmerge_vl patterns for _TIED psuedos too. I suspect they're handled by convertAllOnesVMergeToVMv and foldVMV_V_V Tested on SPEC CPU 2017 and llvm-test-suite to confirm there's no codegen change. Fixes #141885
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In order to fold a vmerge into a pseudo, the pseudo's passthru needs to be the same as vmerge's false operand.
If they don't match we can try and commute the instruction if possible, e.g. here we can commute v9 and v8 to fold the vmerge:
Previously this wasn't possible because we did the peephole in SelectionDAG, but now that it's been migrated to MachineInstr in #144076 we can reuse the commuting infrastructure in TargetInstrInfo.
This fixes the extra vmv.v.v in the "mul" example here: #123069 (comment)
It should also allow us to remove the isel patterns described in #141885 later.