Someone can make the lowering optional later if they want something
different for their hardware.
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
The GL_ARB_shader_ballot spec says that gl_SubGroupSizeARB is declared
as a uniform. This means that it cannot change across an invocation
such as a draw call or a compute dispatch. For compute shaders, we're
ok because we only ever use one dispatch size. For fragment, however,
the hardware dynamically chooses between SIMD8 and SIMD16 which violates
the spec. Instead, let's just pick a subgroup size based on the shader
stage. The fixed size we choose for compute shaders is a bit higher
than strictly needed but there's no real harm in that. The advantage is
that, if they do anything interesting with the value, NIR will see it as
an immediate and can optimize better.
Acked-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Ballot intrinsics return a bitfield of subgroups. In GLSL and some
SPIR-V extensions, they return a uint64_t. In SPV_KHR_shader_ballot,
they return a uvec4. Also, some back-ends would rather pass around
32-bit values because it's easier than messing with 64-bit all the time.
To solve this mess, we make nir_lower_subgroups take a new parameter
called ballot_bit_size and it lowers whichever thing it gets in from the
source language (uint64_t or uvec4) to a scalar with the specified
number of bits. This replaces a chunk of the old lowering code.
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
This commit pulls nir_lower_read_invocations_to_scalar along with most
of the guts of nir_opt_intrinsics (which mostly does subgroup lowering)
into a new nir_lower_subgroups pass. There are various other bits of
subgroup lowering that we're going to want to do so it makes a bit more
sense to keep it all together in one pass. We also move it in i965 to
happen after nir_lower_system_values to ensure that because we want to
handle the subgroup mask system value intrinsics here.
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
