[FFmpeg-devel] [PATCH v3] avutil/mips: Avoid instruction exception caused by gssqc1/gslqc1.

[Shiyou Yin]

>-----Original Message-----
>From: ffmpeg-devel-bounces at ffmpeg.org [mailto:ffmpeg-devel-bounces at ffmpeg.org] On Behalf Of
>Reimar D?ffinger
>Sent: Sunday, July 28, 2019 6:37 AM
>To: FFmpeg development discussions and patches
>Subject: Re: [FFmpeg-devel] [PATCH v3] avutil/mips: Avoid instruction exception caused by gssqc1/gslqc1.
>
>On 26.07.2019, at 07:18, Shiyou Yin <yinshiyou-hf at loongson.cn> wrote:
>
>> Ensure the address accesed by gssqc1/gslqc1 are 16-bits memory-aligned.
>
>Looks good to me if standard DECLARE_ALIGNED should work for stack on MIPS.
>(on x86 it used to be possible for the stack pointer to only be 8-byte aligned, in which case
>DECLARE_ALIGNED could not actually provide 16-byte aligned stack variables, so there were special
>macros - I have not checked how it works nowadays or if MIPS has that issue).
>Well, I guess regardless of that, this is better than before, so should be fine to apply either way.
>
DECLARE_ALIGNED is defined in ' libavutil/mem.h ' and related to compiler. No matter mips or x86,
it's definition is ' #define DECLARE_ALIGNED(n,t,v)      t __attribute__ ((aligned (n))) v' when build 
with gcc or clang (Specific implementation within the compiler is not considered here.).
In libavcodec/x86, DECLARE_ALIGNED is used to define 8/16/32-byte aligned variable too.

Here is a test on mips.
1) Defined variable with 'double temp[8];', and compiled with 'gcc -fstack-protector-all'.
In this case, address of temp is not 16-byte aligned and gssqc1/gslqc1 caused instruction exception. 
2) Replacing ' double temp[8];' with ' double __attribute__((aligned(16))) __back_temp[8];'.
Address of temp is 16-byte aligned and gssqc1/gslqc1 works properly.

There is a typo in the commit message and have revised in v4. Please help to apply.