[Pixman] [PATCH v2] build: improve control logic for enabling MMI.

[Matt Turner]

Thank you for the patch!

On Fri, Mar 6, 2020 at 3:28 AM Shiyou Yin <yinshiyou-hf at loongson.cn> wrote:
>
> From: Yin Shiyou <yinshiyou-hf at loongson>

Should be yinshiyou-hf at loongson*.cn*?

>
> 1. Replace LS_CFLAGS with MMI_CFLAGS to express its intention more accurately.
>    LS_CFLAGS is still available, but it is not recommended.

I'm not aware of any reasons why LS_CFLAGS needs to stay for
compatibility. Do we know of any distros that set it to override the
-march=... value?

> 2. Improve the control logic for enabling MMI.
>
> Three essential conditions for enabling MMI:
> 1) user have not specify --disable-loongson-mmi.
> 2) MMI options has been specified by MMI_CFLAGS,CC or compiler's default setting.
> 3) compiler supports these MMI options.
> ---
>  configure.ac | 69 ++++++++++++++++++++++++++++++++++++++++--------------------

We should also update meson.build. I expect/hope that the autotools
build system will go away sometime in the future.

I'm not sure I entirely understand the patch. I understand that the
objective is to make it possible to easily build pixman for Loongson3A
and use the pixman-mmx.c optimizations.

I think it's currently possible to build pixman on mips without
specifying -march=loongson* in CFLAGS and it will enable the
pixman-mmx.c paths and choose them at runtime. Is part of the goal to
keep that working? If so, could we just use the -mloongson-mmi flag to
compile pixman-mmx.c?

Or does that flag mean the Loongson3A variants of the instructions?
What happens if you compile with -march=loongson2f -mloongson-mmi?
Does GCC generate instructions compatible with 2F or 3A?