[LLVMdev] Question about FMA formation

[Lang Hames]

Hi Michael, Shuxin,


> Shuxin was showing some more complicated patterns that required
> re-association to match (fast-math flags permitting). For those, we're
> considering if having a re-associate-for-FMA functionality in
> codegen-prepare would solve that problem. Thus, we can re-associate in
> codegen-prepare and emit FMA in fast-isel.
>

Yep. I misread the association on Shuxin's example, but even ((a*b) +
(c*d)) + e would match to a 3-instructions:
(fadd (fma a b (fmul c d)) e).

If there are hairier examples that really require reassociation my vote
would be for this last scheme: An FMA-friendly reassociation pass run
before isel that exposes simple patterns for isel to match.

- Lang.
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