[Mesa-dev] [PATCH] mesa: use u_bit_scan() in update_program_texture_state()

[Timothy Arceri]

On 10/05/17 20:06, Samuel Pitoiset wrote:
> On 05/10/2017 03:01 AM, Timothy Arceri wrote:
>> I made this same change a while back but found I couldn't really 
>> measure any performance increase in practice. I suspect it's because 
>> we already
>> limit the max loops with (1 << s) <= prog[i]->SamplersUsed and there 
>> is generally no big gaps in used samples.
>>
>> Did you manage to measure any improvement here?
> 
> No, I didn't but I don't think it will improve anything. Though it's a 
> common pratice to use a bit scan for bitfields, and it makes the code a 
> bit cleaner.

It does have potential to provide a small improvement as this is a 
hotpath, hence the opt you are replacing. Anyway looks good so feel free 
to add:

Reviewed-by: Timothy Arceri <tarceri at itsqueeze.com>

> 
>>
>> On 10/05/17 07:33, Samuel Pitoiset wrote:
>>> The check in update_single_program_texture() can also be
>>> removed.
>>>
>>> Signed-off-by: Samuel Pitoiset <samuel.pitoiset at gmail.com>
>>> ---
>>>   src/mesa/main/texstate.c | 13 +++++--------
>>>   1 file changed, 5 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/src/mesa/main/texstate.c b/src/mesa/main/texstate.c
>>> index 830b230b5d..c11a2e95fe 100644
>>> --- a/src/mesa/main/texstate.c
>>> +++ b/src/mesa/main/texstate.c
>>> @@ -620,9 +620,6 @@ update_single_program_texture(struct gl_context 
>>> *ctx, struct gl_program *prog,
>>>      struct gl_sampler_object *sampler;
>>>      int unit;
>>> -   if (!(prog->SamplersUsed & (1 << s)))
>>> -      return NULL;
>>> -
>>>      unit = prog->SamplerUnits[s];
>>>      texUnit = &ctx->Texture.Unit[unit];
>>> @@ -676,16 +673,16 @@ update_program_texture_state(struct gl_context 
>>> *ctx, struct gl_program **prog,
>>>      int i;
>>>      for (i = 0; i < MESA_SHADER_STAGES; i++) {
>>> +      GLbitfield mask;
>>>         int s;
>>>         if (!prog[i])
>>>            continue;
>>> -      /* We can't only do the shifting trick as the loop condition 
>>> because if
>>> -       * sampler 31 is active, the next iteration tries to shift by 
>>> 32, which is
>>> -       * undefined.
>>> -       */
>>> -      for (s = 0; s < MAX_SAMPLERS && (1 << s) <= 
>>> prog[i]->SamplersUsed; s++) {
>>> +      mask = prog[i]->SamplersUsed;
>>> +
>>> +      while (mask) {
>>> +         const int s = u_bit_scan(&mask);
>>>            struct gl_texture_object *texObj;
>>>            texObj = update_single_program_texture(ctx, prog[i], s);
>>>