[Mesa-dev] [PATCH v2] glsls: Modify exec_list to avoid strict-aliasing violations

[Davin McCall]

Hi Eero,

On 25/06/15 12:27, Eero Tamminen wrote:
> Hi,
>
> On 06/25/2015 02:48 AM, Davin McCall wrote:
>> In terms of performance:
>>
>> (export LIBGL_ALWAYS_SOFTWARE=1; time glmark2)
>
> For Intel driver, INTEL_NO_HW=1 could be used.
>
> (Do other drivers have something similar?)

Unfortunately I do not have an Intel display set up. What exactly would 
INTEL_NO_HW=1 do?

>
>> Again, only a very small improvement when strict aliasing is enabled.
>> With the above in mind it is reasonable to question whether this patch
>> is worthwhile.
>
> IMHO to evaluate that, we would need data for more drivers, not just 
> for SW rendering.

Given that we are talking about using a compiler option when compiling 
Mesa itself, I do not imagine that we would see a more significant 
change in performance when using HW assisted rendering. The code that 
runs on the GPU is the same in either case.

Using the R600 driver (and Radeon 6770 hardware) I get:


With -fno-strict-aliasing:
glmark2 Score: 1614
real    5m31.729s
user    0m43.828s
sys    0m20.892s

With strict aliasing:
glmark2 Score: 1626
real    5m31.725s
user    0m43.496s
sys    0m20.924s


This is all on x86, 32-bit. It's possible that more of an improvement 
would be seen on less register-starved architectures (including Intel 
x86-64), since more register spills/loads can potentially be avoided. I 
don't have the capability to test that right now. It's entirely possible 
that compiling without -fno-strict-aliasing on x86-64 will not (at 
present, even with this patch applied) produce a working library.

Davin