[Mesa-dev] [PATCH 3/3] radeonsi: use #pragma pack to pack si_shader_key

[Marek Olšák]

On Tue, Jun 27, 2017 at 10:02 AM, Nicolai Hähnle <nhaehnle at gmail.com> wrote:
> On 27.06.2017 00:15, Marek Olšák wrote:
>>
>> On Thu, Jun 22, 2017 at 9:19 AM, Nicolai Hähnle <nhaehnle at gmail.com>
>> wrote:
>>>
>>> On 20.06.2017 20:00, Marek Olšák wrote:
>>>>
>>>>
>>>> From: Marek Olšák <marek.olsak at amd.com>
>>>>
>>>> sizeof(struct si_shader_key):
>>>>     Before reverting the 2 commits: 120 bytes
>>>>     After reverting the 2 commits: 128 bytes
>>>>     With #pragma pack: 107 bytes
>>>>
>>>> I'm not sure if memcmp with a byte-aligned size is a good idea.
>>>
>>>
>>>
>>> Does this have a measurable impact? The code is nicer to read with the
>>> structure after the patches, but I'm not sure it's worth the risk of
>>> getting
>>> misaligned data somewhere.
>>
>>
>> It decreases the time spent in si_update_shaders by 6% when going from
>> 128 bytes to 106 for the shader key.
>>
>> What do you mean by the risk?
>
>
> If we end up accidentally misaligning a member, that could cause a
> performance hit. But those 6% improvement does seem worth it. I do think
> Emil's suggestion of adding the attribute directly to the structures is a
> good idea. With that, the patch is

I can't add the attributes inside the structures, because every
structure member would have to have the attribute, and that would be
too much noise in the code. There are MSVC and gcc attributes. gcc
supports both kinds. MSVC attributes are the global #pragmas. gcc
attributes are per-member or on structure types. If the attribute is
on a structure type, it only specifies alignment for the whole
structure, not individual members.

Marek