Re: optimize lookups in snapshot [sub]xip arrays

On Sat, Jul 30, 2022 at 12:02:02PM +0700, John Naylor wrote:
> On Fri, Jul 29, 2022 at 4:34 AM Nathan Bossart <nathandbossart(at)gmail(dot)com>
> wrote:
>> * I borrowed USE_SSE2 from one of John Naylor's patches [0]. I'm not
> sure
>> whether this is committable,
>
> I'll be the first to say it's not committable and needs some thought. Since
> there are several recently proposed patches that take advantage of SSE2, it
> seems time for me to open a new thread and get that prerequisite settled.
> I'll do that next week.

Awesome. I will help test and review.

>> so I would welcome thoughts on the proper
>> form. Given the comment says that SSE2 is supported by all x86-64
>> hardware, I'm not seeing why we need the SSE 4.2 checks. Is it not
>> enough to check for __x86_64__ and _M_AMD64?
>
> That's enough for emitting instructions that the target CPU can run, but
> says nothing (I think) about the host compiler's ability to understand the
> intrinsics and associated headers. The architecture is old enough that
> maybe zero compilers in the buildfarm that target AMD64 fail to understand
> SSE2 intrinsics, but I hadn't looked into it. The SSE 4.2 intrinsics check
> is not necessary, but it was sufficient and already present, so I borrowed
> it for the PoC.

Got it, makes sense.

--
Nathan Bossart
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