Re: [FFmpeg-devel] [PATCH 1/4] configure: aarch64: Support assembling the dotprod and i8mm arch extensions

From: "Martin Storsjö" <martin@martin.st>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
Subject: Re: [FFmpeg-devel] [PATCH 1/4] configure: aarch64: Support assembling the dotprod and i8mm arch extensions
Date: Tue, 30 May 2023 15:25:25 +0300 (EEST)
Message-ID: <c872991-cafc-a014-1874-12d3b82e3d0@martin.st> (raw)
In-Reply-To: <9110736.CDJkKcVGEf@basile.remlab.net>

On Sun, 28 May 2023, Rémi Denis-Courmont wrote:

> Le sunnuntaina 28. toukokuuta 2023, 0.34.15 EEST Martin Storsjö a écrit :
>
>> I guess the alternative would be to just try to set .arch
>> <highest-supported-that-we-care-about>. I was worried that support for
>> e.g. armv8.6-a appeared later in toolchains than support for the
>> individual extension i8mm, but at least from a quick browse in binutils
>> history, they seem to have been added at the same time, so there's
>> probably no such drawback.
>> 
>> Or what's the situation with e.g. SVE2 - was ".arch_extension sve2"
>> supported significantly earlier than ".arch armv9-a"?
>
> I have not tested SVE on LLVM. AFAIK, SVE and SVE2 are optional from 8.2 and 
> 9.0 onward respectively, and not mandatory in any version, so if your 
> toolchain supports neither .arch with plus sign, nor .arch_extension, it is 
> game over.

I didn't meant specifically whether LLVM supports it here, just in general 
wrt binutils and how to enable the feature.

FWIW it seems like SVE2 is a mandatory part of 9.0 - assembling SVE2 
instructions can be done with ".arch armv9-a". But there are about 2 years 
worth of deployed binutils based toolchains that do recognize ".arch 
armv8.2-a; .arch_extension sve2" but don't recognize ".arch armv9-a".

So for the generic mechanism for enabling cpu features, I'd prefer to keep 
the mechanism using primarily .arch_extension (with .arch set as high as 
necessary) rather than relying solely on .arch <version> without any extra 
+<feature>.

>> If we'd do that, it does simplify the configure logic a fair bit and
>> reduces the number of configure variables we need by a lot. It does enable
>> a few more instruction set extensions than what we need though, but that's
>> probably not a real issue.
>
> Yes.

I made an attempt at simplifying the logic in configure and asm.S 
somewhat, while still primarily using .arch_extension, and while making 
sure we still can get the features assembled with current Clang with a 
high enough -march= setting. (Runtime enabled features are out of scope 
for Clang for now as we don't want to try to pass individual higher 
-march= options to the individual assembly files.)

// Martin
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