[PATCH] D124463: [RISCV] Add isCommutable to scalar FMA instructions.

[Craig Topper via Phabricator via llvm-commits]

craig.topper added a comment.

In D124463#3475151 <https://reviews.llvm.org/D124463#3475151>, @reames wrote:

> Declaring a three operand opcode as commutative when only two of the arguments can be safely commuted feels rather suspect to me.

The commutable operands are determined by TargetInstrInfo::findCommutedOpIndices. The default implementation picks the first two source operands. X86 relies on this for commuting AMD's FMA4 instructions. For the FMA3 instructions, X86 overrides TargetInstrInfo::findCommutedOpIndices and calls X86InstrInfo::findThreeSrcCommutedOpIndices. An additional override of TargetInstrInfo::commuteInstructionImpl is used to change the opcode for FMA3.

Should I add more tests or more comments or both?


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https://reviews.llvm.org/D124463