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

[YunQiang Su]

Matt Turner <mattst88 at gmail.com> 于2020年3月9日周一 上午8:31写道：
>
> On Sat, Feb 22, 2020 at 6:34 AM YunQiang Su <wzssyqa at gmail.com> wrote:
> >
> > 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.
>
> If you're referring to using -march=loongson2f in configure.ac, then I
> should point out that that was my choice, and I don't really know what
> other options I had -- or even have today. As far as I know,

MIPS has its baseline, aka
   MIPS I, MIPS II, MIPS III, MIPS IV, MIPS V
   MIPS64/32 r1/r2/r3/r5/r6 (no r4 exists)

Lots of vendor's has its extension based on these baseline.
For example Loongson 2F is MIPS III + some extensions, which we can
see on any other vendor's CPU.

If you are willing to make a Loongson 2F specific OS, -march=loongson2f is OK,
while if you are willing to make a generic OS, the above baseline is suggested.

> -march=loongson* was, until recently, the only way to enable the SIMD
> instructions, and worse, if I recall correctly Loongson 2E and 2F are
> not entirely binary compatible themselves!
>
> The only stable Loongson system I've ever had is a Yeeloong -- 2F, so
> it's what I chose. Like I said in another email, I even tried building
> pixman-mmx.c multiple times with different -march=... values, linking
> them all into libpixman, and choosing which to execute at runtime, but
> binutils does not allow linking object files that are compiled with
> different -march=... values on mips for reasons I do not know.

Yes. that's the problem of MIPS, it has toooo much chaos.
It is not your problem, because you cannot know that Loongson will change
their opcode in future...

The difference between -march=loongson3a and -march=mips64r2 are not
only about SIMD. They even have some different on their basic INSN.
So for generic-purpose OS, we cannot use some options like -march=loongson3a.

-- 
YunQiang Su