Mark Roulo (nothanks.delete@this.xxx.com) on May 24, 2022 7:41 am wrote:
> The argument against heterogeneous cores isn't that SIMD isn't useful.
I understand, but the outcome might be surprisingly similar: if the first step is, "we won't allow AVX-512 because it's not on the E cores", then developers might react with "AVX2 is not as good/easy to program so if that's all we get, perhaps we won't bother with SIMD (or this CPU) at all".

> You seem to be saying something like: "It is useful enough to provide, but not so useful to provide
> to all the cores. And having the developers deal with it is an acceptable solution."
AVX-512 lets us produce 1 GB/s of sorted output on a single core. That seems useful. But is it feasible to put on all types of cores?
I'd certainly prefer it be on all cores rather than worrying about heterogeneity. However, that's not under our control, is it?

Perhaps the concept of heterogeneous cores (with same ISA) actually makes sense and branchy/background tasks can run on E cores. It's not clear to me that that's worth the trouble of hinting to the OS what should run where, and trading down to an 11 year old SIMD ISA.

So if there's an either-or, how about
1) only homogeneous cores OR
2) proper heterogeneous without crippling the big cores: either a) all cores support AVX-512 or b) let software still use AVX-512, perhaps with an "I know how" opt-in.

Given that we have been surprised once, it seems prudent to prepare for more heterogeneity rather than just hope for 1) or 2a).

FYI I may be slow to reply for the rest of the week.