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NIR Texture Instructions
========================
Even though texture instructions *could* be supported as intrinsics, the vast
number of combinations mean that doing so is practically impossible. Instead,
NIR has a dedicated texture instruction. There are several texture operations:
.. c:autoenum:: nir_texop
:file: src/compiler/nir/nir.h
:members:
As with other instruction types, there is still an array of sources, except
that each source also has a *type* associated with it. There are various
source types, each corresponding to a piece of information that the different
texture operations require.
.. c:autoenum:: nir_tex_src_type
:members:
Of particular interest are the texture/sampler deref/index/handle source types.
First, note that textures and samplers are specified separately in NIR. While
not required for OpenGL, this is required for Vulkan and OpenCL. Some
OpenGL [ES] drivers have to deal with hardware that does not have separate
samplers and textures. While not recommended, an OpenGL-only driver may assume
that the texture and sampler derefs will always point to the same resource, if
needed. Note that this pretty well paints your compiler into a corner and
makes any future port to Vulkan or OpenCL harder, so such assumptions should
really only be made if targeting OpenGL ES 2.0 era hardware.
Also, like a lot of other resources, there are multiple ways to represent a
texture in NIR. It can be referenced by a variable dereference, an index, or a
bindless handle. When using an index or a bindless handle, the texture type
information is generally not available. To handle this, various information
from the type is redundantly stored in the :c:struct:`nir_tex_instr` itself.
.. c:autostruct:: nir_tex_instr
:members:
.. c:autostruct:: nir_tex_src
:members:
Texture instruction helpers
---------------------------
There are a number of helper functions for working with NIR texture
instructions. They are documented here in no particular order.
.. c:autofunction:: nir_tex_instr_create
.. c:autofunction:: nir_tex_instr_need_sampler
.. c:autofunction:: nir_tex_instr_result_size
.. c:autofunction:: nir_tex_instr_dest_size
.. c:autofunction:: nir_tex_instr_is_query
.. c:autofunction:: nir_tex_instr_has_implicit_derivative
.. c:autofunction:: nir_tex_instr_src_type
.. c:autofunction:: nir_tex_instr_src_size
.. c:autofunction:: nir_tex_instr_src_index
.. c:autofunction:: nir_tex_instr_add_src
.. c:autofunction:: nir_tex_instr_remove_src
Texture instruction lowering
----------------------------
Because most hardware only supports some subset of all possible GLSL/SPIR-V
texture operations, NIR provides a quite powerful lowering pass which is able
to implement more complex texture operations in terms of simpler ones.
.. c:autofunction:: nir_lower_tex
.. c:autostruct:: nir_lower_tex_options
:members:
.. c:autoenum:: nir_lower_tex_packing
:members:
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