[Mesa-dev] [PATCH 2/2] nir: add lowering for gl_HelperInvocation
Rob Clark
robdclark at gmail.com
Wed Jun 13 01:04:56 UTC 2018
On Tue, Jun 12, 2018 at 3:36 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
> On Tue, Jun 12, 2018 at 10:49 AM, Rob Clark <robdclark at gmail.com> wrote:
>>
>> On Mon, Jun 11, 2018 at 10:59 PM, Roland Scheidegger <sroland at vmware.com>
>> wrote:
>> > Am 12.06.2018 um 01:17 schrieb Rob Clark:
>> >> On Mon, Jun 11, 2018 at 6:59 PM, Roland Scheidegger
>> >> <sroland at vmware.com> wrote:
>> >>> 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.
>> >>
>> >> open to suggestions about how to try and force blob to do something
>> >> different.. I tried a few things and the blob popped out shader code
>> >> that worked the same way in all cases, but ofc I could have missed
>> >> something.
>> >>
>> >>> 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.
>> >>
>> >> I *guess* this is relying on behaviour that gl does not guarantee, but
>> >> also doesn't prohibit, but the hw happens work that way and it works
>> >> out..
>> >>
>> >> Maybe the answer is to simply better document the assumptions that
>> >> this lowering depends on?
>> > Well I think if you have a nir shader which uses sample id but is
>> > supposed to not be run per-sample that is quite confusing (I'd just
>> > assume that adrenos will use sample id 0 just like in single sampled
>> > case). Hopefully noone will deduce shader frequency from the nir
>> > shader...
>> > I'm definitely no NIR expert but there's quite a lot of hacky
>> > assumptions in there:
>> > - sample id in nir doesn't cause per-sample execution (and needs to be 0
>> > in case of msaa but no per-sample execution)
>> > - sampleMaskIn is available in single-sampled
>> > - sampleMaskIn doesn't actually have gl semantics but hw ones (so simply
>> > the mask from rasterization, not taking into account the sample id
>> > already if there's per-sample execution (for legitimate reasons),
>> > although as mentioned I think there's still some debate about this and
>> > not all drivers may actually agree what the semantics of it are in case
>> > of per-sample execution)
>> >
>> > As such I'm not sure it really makes sense to do that as a generic
>> > lowering pass? Albeit you're quite right that it might work for a lot of
>> > hw... But some comments would at the very least imho definitely be
>> > required.
>> >
>>
>> so I reworded the comment about the enable flag a bit:
>>
>> /**
>> * If enabled, gl_HelperInvocation will be lowered as:
>> *
>> * !((1 << sample_id) & sample_mask_in))
>> *
>> * This depends on some possibly hw implementation details, which may
>> * not be true for all hw. In particular that the FS is only executed
>> * for covered samples or for helper invocations. So, do not blindly
>> * enable this option.
>> */
>> bool lower_helper_invocation;
>>
>> But I guess I probably do need to introduce a new intrinsic to access
>> sample_id without necessarily triggering per-sample mode.. hmm..
>
>
> Interesting side-note: Issue 22 of the ARB_shader_image_load_store spec
> explicitly calls out gl_SampleMaskIn[0] as a mechanism for detecting helper
> pixels. So, apart from the whole implicitly causing per-sample shading,
> this does appear to be valid if you're willing to take an issue in an
> extension as spec text. :-)
>
I guess that might be worth referencing as a hint that this might work
on more than one hw, but I guess maybe it is not safe to count on as a
"this lowering option is guaranteed to work on all hw" thing..
Anyways, I'll respin this patch w/ the above comment (perhaps
embellished a bit) when I can think up a decent name for an intrinsic
to access sample_id without implying that you need to execute the FS
per-sample.. picking names is hard ;-)
BR,
-R
> --Jason
>
>
>>
>> BR,
>> -R
>>
>>
>> > Roland
>> >
>> >
>> >
>> >>
>> >> BR,
>> >> -R
>> >>
>> >>>
>> >>> 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
>> >>>>
>> >>>> _______________________________________________
>> >>>> mesa-dev mailing list
>> >>>> mesa-dev at lists.freedesktop.org
>> >>>> <mailto:mesa-dev at lists.freedesktop.org>
>> >>>>
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>> >>>>
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>> >>>>
>> >>>>
>> >>>>
>> >>>>
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>> >>>>
>> >>>
>> >
>
>
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