Sunny Cove wide

By: Travis Downs (travis.downs.delete@this.gmail.com), December 16, 2018 11:32 am
Room: Moderated Discussions
anon (spam.delete.delete.delete@this.this.this.spam.com) on December 15, 2018 9:55 am wrote:

> > FMA4 is a different story because it's not about non-destructive, it's about
> > have 4 total inputs and outputs. Sure, that might mess with your uop format.
> >
>
> You are aware that FMA3 uops do not write to the same physical register they
> read from so after the renamer the uop format looks exactly like FMA4 would?
>


Yes, I am (note that this also applies to my mov-fusion reasoning: after the renamer 2-arg destructive ops look pretty much the same as 3-arg non-destructive).

Many of the important restrictions are in the uop format before the renamer (i.e., that which appears in the IDQ and uop cache) and in the renamer itself, and that's a reason why it would be hard to support 4-argument instructions.

3-argument (counting flags if used as input) seems like a sweet spot for "before renamer" uop format, since it covers a large variety of popular instructions: cmov and friends, all the 3-arg non-flag consuming instructions like the BMI ones, adc, sbb, etc. Only FMA4 would be an outlier needing 4 args.

There was definitely a time where it was clear Intel didn't support 4-argument "after renamer" ops, which is why adc, cmov, etc all needed 2 uops (and they even special cased adc reg, 0 to 1 uop since that was important enough apparently), but that restriction was removed in Broadwell and in Skylake it was taken full advantage of).
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