Re: [FFmpeg-devel] [PATCH] Replace br return with ret

[Casey Smalley <casey.smalley@arm.com>]

Hi,

Just wondering what current thoughts on the patch was. It looks as
though the change is fine, but if there is still an issue I can submit a
new patch using BTI landing pads instead.

Best regards,

Casey.

On 7/27/23 18:22, Reimar Döffinger wrote:
> 
>
>
>> On 27 Jul 2023, at 15:55, Rémi Denis-Courmont <remi@remlab.net> wrote:
>>
>> Hi,
>>
>> The use of RET vs BR also has microarchitectural side effects. AFAIU, RET should always be paired with an earlier BL/BLR to avoid interfering with branch prediction.
>>
>> So depending on the circumstances, either one of these should be addressed:
>> * Clarify that this is actually a function return , and RET should be used anyway, regardless of BTI.
>> * Keep BR and add BTI J landing pads where appropriate, if this wasn't really a function return.
> Yes BL and RET is best to match up.
>
> For this function:
> % git grep func_tr_32x4
> libavcodec/aarch64/hevcdsp_idct_neon.S:function func_tr_32x4_\name
> libavcodec/aarch64/hevcdsp_idct_neon.S:        bl              func_tr_32x4_firstpass
> libavcodec/aarch64/hevcdsp_idct_neon.S:        bl              func_tr_32x4_secondpass_\bitdepth
> libavcodec/arm/hevcdsp_idct_neon.S:function func_tr_32x4_\name
> libavcodec/arm/hevcdsp_idct_neon.S:        bl              func_tr_32x4_firstpass
> libavcodec/arm/hevcdsp_idct_neon.S:        bl              func_tr_32x4_secondpass_\bitdepth
>
> It is always used with "bl", thus ret is also more correct from
> that aspect.
> Was your comment only on checking that, or did you mean that this should
> be mentioned in the commit message?
> (if you are wondering why the code did not use ret before, I guess it's
> because it was ported from the 32-bit arm assembler and it slipped by code review)
>
> Best regards,
> Reimar
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