[Pixman] [PATCH v2 2/3] build: use '-mloongson-mmi' for Loongson MMI.

[YunQiang Su]

Shiyou Yin <yinshiyou-hf at loongson.cn> 于2020年2月22日周六 下午9:26写道：
>
> >-----Original Message-----
> >From: Adam Jackson [mailto:ajax at redhat.com]
> >Sent: Friday, February 21, 2020 11:33 PM
> >To: Yin Shiyou; pixman at lists.freedesktop.org
> >Subject: Re: [Pixman] [PATCH v2 2/3] build: use '-mloongson-mmi' for Loongson MMI.
> >
> >On Thu, 2020-02-20 at 22:23 +0800, Yin Shiyou wrote:
> >> It's suggested to use '-mloongson-mmi' to enable MMI.
> >> To keep compatible with old processor, '-mloongson-mmi' will be
> >> setted for Loongson-3A only.
> >
> >The pattern we've used for other CPUs is to build support for as many
> >ISA extensions as possible, unless they are explicitly disabled.
> >Distributions tend to want to set their own minimum ISA levels, and if
> >they wanted to assert -mloongson-mmi they would already have added it
> >to CFLAGS globally.
> >
> >Do you have any performance data for this change?
> >
> >If setting -mloongson-mmi means the compiler can do useful
> >autovectorization, then that's probably true for other arches too (eg
> >amd64 vs avx2), and we should support this kind of thing more
> >generically. But as it stands I don't think this patch is a good idea.
> >
> First, that's introduce the history of '-march=loongson2f' and '-mloongson-mmi'.
> From loongson2f start, mmi is supported by loongson processor.

Yes. So that's why when we code, we should be very careful, especially
when we work on base part of a OS, just like pixman.
One, history mistake will make all of the people painful.

An exmaple is about time_t on 32bit system.

> Unfortunately, the compiler's support for MMI extention is not standardized.
> Gcc compiler use '-march=loongson2f' for loongson2f at first, but from Loongson-3A,
> opcode of mmi instruction has changed, and '-march=loongson3a' is in replaced.

That is the reason some of Loongson's extensions make upstream unhappy.
You need be always very careful when you design a CPU.
如履薄冰. No zuo no die.

> From last year, compile option for mmi instruction has been standardized.
> Just like -mmsa for mips MSA. (MMI,LSX,LASX is Loongson SIMD extention.)
> -mloongson-mmi   for MMI (-march=loongson3a still works, but -mloongson-mmi is recommended for new processors except Loongson2f. )
> -mloongson-sx     for LSX
> -mloongson-asx    for LASX

That is good news.

>
> Second, back to this patch itself.
> I meet a problem when compile pixman on my Loongson3a with gcc, MMI can't be enabled.
> configure check failure: " linking mips:loongson_2f module with previous mips:gs464 modules"
> It can be solved by assign LS_CFLAGS="-mloongson-mmi" while config.
> So I submit this patch in hope that no need to assign LS_CFLAGS explicitly.
> This won't have much impact on performance as I know.

Here is not about performance. You made a bad design, that is burden of history.

>
> Third, about the compatibility with Loongson2f.
> This patch does not change the result of pixman compilation and running on loongson2f.
> Before add this patch, LS_CFLAGS will be setted as '-march=loongson2f' by default,
> After add this patch, LS_CFLAGS is still setted as '-march=loongson2f' if we compile on the old environment.
> It only makes MMI can be enabled without assign LS_CFLAGS explicitly while compiling locally on loongson-3A.
>

That's why you need to guess from compiler.
You can read the code of config.guess:
   http://git.savannah.gnu.org/gitweb/?p=config.git;a=blob;f=config.guess

In fact I noticed that you have broken the build system of ffmpeg and
some other project.
Please STOP it.

>
>
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-- 
YunQiang Su