[LLVMdev] Question about FMA formation

[Eric Christopher]

On Wed, Dec 12, 2012 at 8:34 PM, Lang Hames <lhames at gmail.com> wrote:

> 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.
>
>

Agreed. I don't think fast isel should be attempting to form any new
patterns.

-eric
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