[Mesa-dev] [PATCH 2/2] nir: add lowering for gl_HelperInvocation

[Roland Scheidegger]

Am 12.06.2018 um 00:32 schrieb Jason Ekstrand:
> On Wed, Jun 6, 2018 at 7:43 AM, Rob Clark <robdclark at gmail.com
> <mailto:robdclark at gmail.com>> wrote:
> 
>     Signed-off-by: Rob Clark <robdclark at gmail.com
>     <mailto:robdclark at gmail.com>>
>     ---
>     I can't say for sure that this will work on all drivers, but it is
>     what the blob driver does, and it seems to make deqp happy.  I could
>     move this to it's own pass inside ir3, but that seemed like overkill
> 
>      src/compiler/nir/nir.h                      | 10 ++++++++++
>      src/compiler/nir/nir_lower_system_values.c  | 17 +++++++++++++++++
>      src/gallium/drivers/freedreno/ir3/ir3_nir.c |  1 +
>      3 files changed, 28 insertions(+)
> 
>     diff --git a/src/compiler/nir/nir.h b/src/compiler/nir/nir.h
>     index 073ab4e82ea..de3d55d83af 100644
>     --- a/src/compiler/nir/nir.h
>     +++ b/src/compiler/nir/nir.h
>     @@ -1963,6 +1963,16 @@ typedef struct nir_shader_compiler_options {
>          */
>         bool lower_base_vertex;
> 
>     +   /**
>     +    * If enabled, gl_HelperInvocation will be lowered as:
>     +    *
>     +    *   !((1 << gl_SampleID) & gl_SampleMaskIN[0]))
> 
> 
> This only works for multi-sampling.  What about the single-sampled case?
It doesn't make sense to me for msaa neither.
gl_SampleID forces per-sample execution, which clearly isn't what you
want. Plus, gl_SampleMaskIN is specified to only contain bits for the
current shader invocation, so for msaa with forced per-sample execution
that would only contain the single bit corresponding to gl_SampleID
anyway (that is my interpretation at least - I know hw sample mask
inputs will probably always contains all the bits from rasterization,
regardless of gl_SampleID), so the "1 << gl_SampleID &" part will do
nothing at all. So I think that part is more about lowering the hw
rasterization sample mask to gl_SampleMaskIN rather than lowering to
gl_HelperInvocation.
But yes, !gl_SampleMaskIN should give gl_HelperInvocation - I think all
hw can give you raster sample mask even without msaa but I'm not
entirely sure if it's guaranteed to work in GL with single sampling
(similar for sample id, which should just be stuck at 0). But using the
gl names here looks very, very fishy to me here.

Roland




> 
> --Jason
> 
>  
> 
>     +    *
>     +    * TODO any hw w/ more than 32 samples?  For them (if they
>     +    * used this option), a bit more math would be involved.
>     +    */
>     +   bool lower_helper_invocation;
>     +
>         bool lower_cs_local_index_from_id;
> 
>         bool lower_device_index_to_zero;
>     diff --git a/src/compiler/nir/nir_lower_system_values.c
>     b/src/compiler/nir/nir_lower_system_values.c
>     index 487da042620..6668cbb5dcd 100644
>     --- a/src/compiler/nir/nir_lower_system_values.c
>     +++ b/src/compiler/nir/nir_lower_system_values.c
>     @@ -136,6 +136,23 @@ convert_block(nir_block *block, nir_builder *b)
>                                    nir_load_first_vertex(b));
>               break;
> 
>     +      case SYSTEM_VALUE_HELPER_INVOCATION:
>     +         if (b->shader->options->lower_helper_invocation) {
>     +            nir_ssa_def *tmp;
>     +
>     +            tmp = nir_ushr(b,
>     +                           nir_imm_int(b, 1),
>     +                           nir_load_sample_id(b));
>     +
>     +            tmp = nir_iand(b,
>     +                           nir_load_sample_mask_in(b),
>     +                           tmp);
>     +
>     +            sysval = nir_inot(b, nir_i2b(b, tmp));
>     +         }
>     +
>     +         break;
>     +
>            case SYSTEM_VALUE_INSTANCE_INDEX:
>               sysval = nir_iadd(b,
>                                 nir_load_instance_id(b),
>     diff --git a/src/gallium/drivers/freedreno/ir3/ir3_nir.c
>     b/src/gallium/drivers/freedreno/ir3/ir3_nir.c
>     index cd1f9c526f2..341d990b269 100644
>     --- a/src/gallium/drivers/freedreno/ir3/ir3_nir.c
>     +++ b/src/gallium/drivers/freedreno/ir3/ir3_nir.c
>     @@ -51,6 +51,7 @@ static const nir_shader_compiler_options options = {
>                     .lower_extract_byte = true,
>                     .lower_extract_word = true,
>                     .lower_all_io_to_temps = true,
>     +               .lower_helper_invocation = true,
>      };
> 
>      struct nir_shader *
>     -- 
>     2.17.0
> 
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