[LLVMdev] Commutability of X86 FMA3 instructions.

[Lang Hames]

Hi all,

The 213 variant of the FMA3 instructions is currently marked
commutable (see X86InstrFMA.td). Is that safe? According to the ISA
the FMA3 instructions aren't commutable for non-numeric results, so
I'd have thought commuting this would only be valid in fast-math mode?

For the curious, the reason that I'm asking is that we currently
always select the 213 variant, but this introduces an extra copies in
accumulator-style loops. Something like:

while (...)
  accumulator = x * y + accumulator;

yields:

loop:
  vfmadd.213 y, x, acc
  vmovaps acc, x
  decl count
  jne loop

instead of

loop:
  vfmadd.231 acc, x, y
  decl count
  jne loop

I have started writing a patch to generate the 231 variant by default,
and I want to know whether I need to go to the trouble of adding
custom commute logic. If these things aren't commutable then I don't
need to worry at all. If they are commutable, but only in fast-math
mode, then I can support that too.

Thanks for the help!

- Lang.