[Mesa-dev] [PATCH] nir: add lowering for gl_HelperInvocation

[Rob Clark]

On Mon, Jun 25, 2018 at 2:29 PM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
> On Mon, Jun 25, 2018 at 2:23 PM, Ian Romanick <idr at freedesktop.org> wrote:
>> On 06/19/2018 08:24 AM, Rob Clark wrote:
>>> v2: reword comment about lower_helper_invocations to be more clear
>>>     that it might not work on all hardware
>>> v3: add special variant of load_sample_id which does not imply per-
>>>     sample shading
>>>
>>> Signed-off-by: Rob Clark <robdclark at gmail.com>
>>> ---
>>> Yeah, new intrinsic name is a bit funny sounding, but I couldn't
>>> think of anything better.
>>>
>>>  src/compiler/nir/nir.h                          | 12 ++++++++++++
>>>  src/compiler/nir/nir_intrinsics.py              |  3 +++
>>>  src/compiler/nir/nir_lower_system_values.c      | 17 +++++++++++++++++
>>>  .../drivers/freedreno/ir3/ir3_compiler_nir.c    |  1 +
>>>  src/gallium/drivers/freedreno/ir3/ir3_nir.c     |  1 +
>>>  5 files changed, 34 insertions(+)
>>>
>>> diff --git a/src/compiler/nir/nir.h b/src/compiler/nir/nir.h
>>> index 2d620454796..cb9a4af64cf 100644
>>> --- a/src/compiler/nir/nir.h
>>> +++ b/src/compiler/nir/nir.h
>>> @@ -1981,6 +1981,18 @@ typedef struct nir_shader_compiler_options {
>>>      */
>>>     bool lower_base_vertex;
>>>
>>> +   /**
>>> +    * If enabled, gl_HelperInvocation will be lowered as:
>>> +    *
>>> +    *   !((1 << sample_id) & sample_mask_in))
>>
>> I recently encountered something related to this.  Issue #22 of the
>> GL_ARB_shader_image_load_store spec suggests using (gl_SampleMaskIn[0]
>> == 0) to detect a helper invocation.  There's a note that
>> implementations that support > 32 samples will also need to check
>> gl_SampleMaskIn[1].

note that for adreno I just have up to 4x samples.. in fact, do any
mesa drivers support > 32 samples?

>  Did you try that?  It should generate less code.
>>
>> See also piglit commit c89439fcaba7259b25d090df43a6bf2aac6483d7.
>
> Based on what I've seen of adreno, that sysval is likely actually not
> gl_SampleMaskIn, but rather a full pixel coverage mask. This only
> matters with sample-rate shading which I don't think Rob has
> investigated yet.
>
> I think gl_SampleMaskIn[0] == 0 should work in the general case, but I
> suspect that freedreno will have to implement gl_SampleMaskIn as
> covmask & (1 << sampleid) for sample-rate shading.
>

I didn't try that but I think gl_SampleMaskIn[0] will work as long as
there isn't sample-rate shading.  Blob always does 'covmask & (1 <<
sampleid)'.  But I guess as long as there isn't any GL state that can
also trigger sample-rate shading (?), I could switch lowering based on
whether shader reads sample_id, and use the simpler
gl_SampleMaskIn[0]==0 in the non-sample-rate case...

BR,
-R

> Cheers,
>
>   -ilia