NIR Texture Instructions — The Mesa 3D Graphics Library latest documentation

# 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:

enum nir_texop

  Texture instruction opcode

  enumerator nir_texop_tex

    Regular texture look-up

  enumerator nir_texop_txb

    Texture look-up with LOD bias

  enumerator nir_texop_txl

    Texture look-up with explicit LOD

  enumerator nir_texop_txd

    Texture look-up with partial derivatives

  enumerator nir_texop_txf

    Texel fetch with explicit LOD

  enumerator nir_texop_txf_ms

    Multisample texture fetch

  enumerator nir_texop_txf_ms_fb

    Multisample texture fetch from framebuffer

  enumerator nir_texop_txf_ms_mcs_intel

    Multisample compression value fetch

  enumerator nir_texop_txs

    Texture size

  enumerator nir_texop_lod

    Texture lod query

  enumerator nir_texop_tg4

    Texture gather

  enumerator nir_texop_query_levels

    Texture levels query

  enumerator nir_texop_texture_samples

    Texture samples query

  enumerator nir_texop_samples_identical

    Query whether all samples are definitely identical.

  enumerator nir_texop_tex_prefetch

    Regular texture look-up, eligible for pre-dispatch

  enumerator nir_texop_lod_bias

    Returns the sampler’s LOD bias (if sampler LOD bias is lowered)

  enumerator nir_texop_fragment_fetch_amd

    Multisample fragment color texture fetch

  enumerator nir_texop_fragment_mask_fetch_amd

    Multisample fragment mask texture fetch

  enumerator nir_texop_descriptor_amd

    Returns a buffer or image descriptor.

  enumerator nir_texop_sampler_descriptor_amd

    Returns a sampler descriptor.

  enumerator nir_texop_image_min_lod_agx

    Returns the image view’s min LOD

  enumerator nir_texop_has_custom_border_color_agx

    Returns a bool indicating that the sampler uses a custom border colour

  enumerator nir_texop_custom_border_color_agx

    Returns the sampler’s custom border colour (if has_custom_border_agx)

  enumerator nir_texop_hdr_dim_nv

    Maps to TXQ.DIMENSION

  enumerator nir_texop_tex_type_nv

    Maps to TXQ.TEXTURE_TYPE

  enumerator nir_texop_sample_pos_nv

    Maps to TXQ.SAMPLER_POS

  enumerator nir_texop_sample_weighted_qcom

    Returns the weighted average of a region of texels in the texture, using the filter kernel sampled from ref_texture. (VK_QCOM_image_processing)

  enumerator nir_texop_box_filter_qcom

    Returns the result of a weighted average of the texels in a box of size box_size centered at coord. (VK_QCOM_image_processing)

  enumerator nir_texop_block_match_sad_qcom

    Returns the result of SAD/SSD block matching on 2D textures. (VK_QCOM_image_processing)

    The textures are always 2D dim, and the coord and ref_coord texture sources are ivec2s. The size of the block is specified in the block_size texture source (uvec2 from SPIRV, packed u32 in the backend)

  enumerator nir_texop_resinfo_intel

    txs in .xyz and query_levels in .w

  enumerator nir_texop_sparse_residency_intel

    Returns red data and a sparse residency code for sampling ops

  enumerator nir_texop_sparse_residency_txf_intel

    Returns red data and a sparse residency code for texel fetches

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.

enum nir_tex_src_type

  Texture instruction source type

  enumerator nir_tex_src_coord

    Texture coordinate

    Must have `nir_tex_instr.coord_components` components.

  enumerator nir_tex_src_projector

    Projector

    The texture coordinate (except for the array component, if any) is divided by this value before LOD computation and sampling.

    Must be a float scalar.

  enumerator nir_tex_src_comparator

    Shadow comparator

    For shadow sampling, the fetched texel values are compared against the shadow comparator using the compare op specified by the sampler object and converted to 1.0 if the comparison succeeds and 0.0 if it fails. Interpolation happens after this conversion so the actual result may be anywhere in the range [0.0, 1.0].

    Only valid if `nir_tex_instr.is_shadow` and must be a float scalar.

  enumerator nir_tex_src_offset

    Coordinate offset

    An integer value that is added to the texel address before sampling. This is only allowed with operations that take an explicit LOD as it is applied in integer texel space after LOD selection and not normalized coordinate space.

  enumerator nir_tex_src_bias

    LOD bias

    This value is added to the computed LOD before mip-mapping.

  enumerator nir_tex_src_lod

    Explicit LOD

  enumerator nir_tex_src_min_lod

    Min LOD

    The computed LOD is clamped to be at least as large as min_lod before mip-mapping.

  enumerator nir_tex_src_max_lod_kk

    Max LOD

    The computed LOD is clamped to be at most as large as max_lod before mip-mapping.

  enumerator nir_tex_src_lod_bias_min_agx

    LOD bias + min LOD packed together into 32-bits. This is the common case for texturing on Honeykrisp with DX12, where both LOD bias and min LOD are emulated and passed in a single hardware source together. So it’s important to optimize so e.g. nir_opt_preamble can make good decisions that avoid extra moves.

  enumerator nir_tex_src_ms_index

    MSAA sample index

  enumerator nir_tex_src_ms_mcs_intel

    Intel-specific MSAA compression data

  enumerator nir_tex_src_ddx

    Explicit horizontal (X-major) coordinate derivative

  enumerator nir_tex_src_ddy

    Explicit vertical (Y-major) coordinate derivative

  enumerator nir_tex_src_texture_deref

    Texture variable dereference

  enumerator nir_tex_src_sampler_deref

    Sampler variable dereference

  enumerator nir_tex_src_texture_offset

    Texture index offset

    This is added to `nir_tex_instr.texture_index`. Unless `nir_tex_instr.texture_non_uniform` is set, this is guaranteed to be dynamically uniform.

  enumerator nir_tex_src_sampler_offset

    Dynamically uniform sampler index offset

    This is added to `nir_tex_instr.sampler_index`. Unless `nir_tex_instr.sampler_non_uniform` is set, this is guaranteed to be dynamically uniform. This should not be present until GLSL ES 3.20, GLSL 4.00, or ARB_gpu_shader5, because in ES 3.10 and GL 3.30 samplers said “When aggregated into arrays within a shader, samplers can only be indexed with a constant integral expression.”

  enumerator nir_tex_src_texture_handle

    Bindless texture handle

    This is, unfortunately, a bit overloaded at the moment. There are generally two types of bindless handles:

    > 1. For GL_ARB_bindless bindless handles. These are part of the GL/Gallium-level API and are always a 64-bit integer.

    > 2. HW-specific handles. GL_ARB_bindless handles may be lowered to these. Also, these are used by many Vulkan drivers to implement descriptor sets, especially for UPDATE_AFTER_BIND descriptors. The details of hardware handles (bit size, format, etc.) is HW-specific.

    Because of this overloading and the resulting ambiguity, we currently don’t validate anything for these.

  enumerator nir_tex_src_sampler_handle

    Bindless sampler handle

    See nir_tex_src_texture_handle,

  enumerator nir_tex_src_texture_heap_offset

    Texture descriptor heap offset (in bytes)

  enumerator nir_tex_src_sampler_heap_offset

    Sampler descriptor heap offset (in bytes)

  enumerator nir_tex_src_sampler_deref_intrinsic

    Tex src intrinsic

    This is an intrinsic used before function inlining i.e. before we know if a bindless value has been given as function param for use as a tex src.

  enumerator nir_tex_src_plane

    Plane index for multi-plane YCbCr textures

  enumerator nir_tex_src_backend1

    Backend-specific vec4 tex src argument.

    Can be used to have NIR optimization (copy propagation, lower_vec_to_regs) apply to the packing of the tex srcs. This lowering must only happen after nir_lower_tex().

    The nir_tex_instr_src_type() of this argument is float, so no lowering will happen if nir_lower_int_to_float is used.

  enumerator nir_tex_src_backend2

    Second backend-specific vec4 tex src argument, see nir_tex_src_backend1.

  enumerator nir_tex_src_ref_coord

    VK_QCOM_image_processing sources

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 `nir_tex_instr` itself.

struct nir_tex_instr

  Represents a texture instruction

  nir_instr instr

    Base instruction

  nir_def def

    Destination

  enum glsl_sampler_dim sampler_dim

    Dimensionality of the texture operation

    This will typically match the dimensionality of the texture deref type if a nir_tex_src_texture_deref is present. However, it may not if texture lowering has occurred.

  nir_alu_type dest_type

    ALU type of the destination

    This is the canonical sampled type for this texture operation and may not exactly match the sampled type of the deref type when a nir_tex_src_texture_deref is present. For OpenCL, the sampled type of the texture deref will be GLSL_TYPE_VOID and this is allowed to be anything. With SPIR-V, the signedness of integer types is allowed to differ. For all APIs, the bit size may differ if the driver has done any sort of mediump or similar lowering since texture types always have 32-bit sampled types.

  nir_texop op

    Texture opcode

  nir_tex_src *src

    Array of sources

    This array has `nir_tex_instr.num_srcs` elements

  unsigned int num_srcs

    Number of sources

  unsigned int coord_components

    Number of components in the coordinate, if any.

    This applies to the nir_tex_src_coord and nir_tex_src_ref_coord src types.

  bool is_array

    True if the texture instruction acts on an array texture

  bool is_shadow

    True if the texture instruction performs a shadow comparison

    If this is true, the texture instruction must have a nir_tex_src_comparator.

  bool is_new_style_shadow

    If is_shadow is true, whether this is the old-style shadow that outputs 4 components or the new-style shadow that outputs 1 component.

  bool is_sparse

    True if this texture instruction should return a sparse residency code. The code is in the last component of the result.

  unsigned int component

    nir_texop_tg4 component selector

    This determines which RGBA component is gathered.

  unsigned int array_is_lowered_cube

    Validation needs to know this for gradient component count

  unsigned int is_gather_implicit_lod

    True if this tg4 instruction has an implicit LOD or LOD bias, instead of using level 0

  unsigned int skip_helpers

    True if helper invocation loads can be skipped

    This is set by nir_opt_load_skip_helpers().

  int8_t tg4_offsets[4][2]

    Gather offsets

  bool texture_non_uniform

    True if the texture index or handle is not dynamically uniform

  bool sampler_non_uniform

    True if the sampler index or handle is not dynamically uniform.

    This may be set when VK_EXT_descriptor_indexing is supported and the appropriate capability is enabled.

    This should always be false in GLSL (GLSL ES 3.20 says “When aggregated into arrays within a shader, opaque types can only be indexed with a dynamically uniform integral expression”, and GLSL 4.60 says “When aggregated into arrays within a shader, [texture, sampler, and samplerShadow] types can only be indexed with a dynamically uniform expression, or texture lookup will result in undefined values.”).

  bool embedded_sampler

    True if this texture instruction uses an embedded sampler.

    In this case, sampler_index is the index in embedded sampler table.

  bool offset_non_uniform

    True if the offset is not dynamically uniform

  bool can_speculate

    True whether this returns the same result anywhere in the shader and doesn’t cause page faults.

  unsigned int texture_index

    The texture index

    If this texture instruction has a nir_tex_src_texture_offset source, then the texture index is given by texture_index + texture_offset.

  unsigned int sampler_index

    The sampler index

    The following operations do not require a sampler and, as such, this field should be ignored:

    > - nir_texop_txf

    > - nir_texop_txf_ms

    > - nir_texop_txs

    > - nir_texop_query_levels

    > - nir_texop_texture_samples

    > - nir_texop_samples_identical

    If this texture instruction has a nir_tex_src_sampler_offset source, then the sampler index is given by sampler_index + sampler_offset.

struct nir_tex_src

  A texture instruction source

  nir_src src

    Base source

  nir_tex_src_type src_type

    Type of this source

## Texture instruction helpers

There are a number of helper functions for working with NIR texture instructions. They are documented here in no particular order.

nir_tex_instr *nir_tex_instr_create(nir_shader *shader, unsigned int num_srcs)

  Creates a NIR texture instruction

bool nir_tex_instr_need_sampler(const nir_tex_instr *instr)

  Returns true if the texture operation requires a sampler as a general rule

  Note that the specific hw/driver backend could require to a sampler object/configuration packet in any case, for some other reason.

  See also `nir_tex_instr.sampler_index`.

unsigned int nir_tex_instr_result_size(const nir_tex_instr *instr)

  Returns the number of components returned by this nir_tex_instr

  Useful for code building texture instructions when you don’t want to think about how many components a particular texture op returns. This does not include the sparse residency code.

static inline unsigned int nir_tex_instr_dest_size(const nir_tex_instr *instr)

  Returns the destination size of this nir_tex_instr including the sparse residency code, if any.

bool nir_tex_instr_is_query(const nir_tex_instr *instr)

  Returns true if this texture operation queries something about the texture rather than actually sampling it.

bool nir_tex_instr_has_implicit_derivative(const nir_tex_instr *instr)

  Returns true if this texture instruction does implicit derivatives

  This is important as there are extra control-flow rules around derivatives and texture instructions which perform them implicitly.

nir_alu_type nir_tex_instr_src_type(const nir_tex_instr *instr, unsigned int src)

  Returns the ALU type of the given texture instruction source

unsigned int nir_tex_instr_src_size(const nir_tex_instr *instr, unsigned int src)

  Returns the number of components required by the given texture instruction source

static inline int nir_tex_instr_src_index(const nir_tex_instr *instr, nir_tex_src_type type)

  Returns the index of the texture instruction source with the given nir_tex_src_type or -1 if no such source exists.

void nir_tex_instr_add_src(nir_tex_instr *tex, nir_tex_src_type src_type, nir_def *src)

  Adds a source to a texture instruction

void nir_tex_instr_remove_src(nir_tex_instr *tex, unsigned int src_idx)

  Removes a source from a texture instruction

## 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.

bool nir_lower_tex(nir_shader *shader, const nir_lower_tex_options *options)

  Lowers complex texture instructions to simpler ones

struct nir_lower_tex_options
  unsigned int lower_txp

    bitmask of (1 << GLSL_SAMPLER_DIM_x) to control for which sampler types a texture projector is lowered.

  bool lower_txp_array

    If true, lower texture projector for any array sampler dims

  bool lower_txf_offset

    If true, lower away nir_tex_src_offset for all texelfetch instructions.

  bool lower_rect_offset

    If true, lower away nir_tex_src_offset for all rect textures.

  nir_instr_filter_cb lower_offset_filter

    If not NULL, this filter will return true for tex instructions that should lower away nir_tex_src_offset.

  bool lower_rect

    If true, lower rect textures to 2D, using txs to fetch the texture dimensions and dividing the texture coords by the texture dims to normalize.

  bool lower_1d

    If true, lower 1D textures to 2D. This requires the GL/VK driver to map 1D textures to 2D textures with height=1.

    lower_1d_shadow does this lowering for shadow textures only.

  unsigned int lower_y_uv_external

    If true, convert yuv to rgb.

  unsigned int saturate_s

    To emulate certain texture wrap modes, this can be used to saturate the specified tex coord to [0.0, 1.0]. The bits are according to sampler #, ie. if, for example:

    > (conf->saturate_s & (1 << n))

    is true, then the s coord for sampler n is saturated.

    Note that clamping must happen after projector lowering so any projected texture sample instruction with a clamped coordinate gets automatically lowered, regardless of the ‘lower_txp’ setting.

  unsigned int lower_srgb

    Bitmap of textures that need srgb to linear conversion. If (lower_srgb & (1 << texture_index)) then the rgb (xyz) components of the texture are lowered to linear.

  bool lower_txd_cube_map

    If true, lower nir_texop_txd on cube maps with nir_texop_txl.

  bool lower_txd_3d

    If true, lower nir_texop_txd on 3D surfaces with nir_texop_txl.

  bool lower_txd_array

    If true, lower nir_texop_txd any array surfaces with nir_texop_txl.

  bool lower_txd_shadow

    If true, lower nir_texop_txd on shadow samplers (except cube maps) with nir_texop_txl. Notice that cube map shadow samplers are lowered with lower_txd_cube_map.

  bool lower_txd

    If true, lower nir_texop_txd on all samplers to a nir_texop_txl. Implies lower_txd_cube_map and lower_txd_shadow.

  bool lower_txd_clamp

    If true, lower nir_texop_txd when it uses min_lod.

  bool lower_txb_shadow_clamp

    If true, lower nir_texop_txb that try to use shadow compare and min_lod at the same time to a nir_texop_lod, some math, and nir_texop_tex.

  bool (*lower_txd_cb)(const nir_tex_instr *tex, const void *data)

    A generic callback to nir_texop_txd.

  bool lower_txs_lod

    If true, lower nir_texop_txs with a non-0-lod into nir_texop_txs with 0-lod followed by a nir_ishr.

  bool lower_txs_cube_array

    If true, lower nir_texop_txs for cube arrays to a nir_texop_txs with a 2D array type followed by a nir_idiv by 6.

  bool lower_tg4_broadcom_swizzle

    If true, apply a .bagr swizzle on tg4 results to handle Broadcom’s mixed-up tg4 locations.

  bool lower_tg4_offsets

    If true, lowers tg4 with 4 constant offsets to 4 tg4 calls

  bool lower_to_fragment_fetch_amd

    Lower txf_ms to fragment_mask_fetch and fragment_fetch and samples_identical to fragment_mask_fetch.

  enum nir_lower_tex_packing (*lower_tex_packing_cb)(const nir_tex_instr *tex, const void *data)

    To lower packed sampler return formats. This will be called for all tex instructions.

  bool lower_lod_zero_width

    If true, lower nir_texop_lod to return -FLT_MAX if the sum of the absolute values of derivatives is 0 for all coordinates.

  bool lower_sampler_lod_bias

    If true, emulates sampler descriptor LOD bias by adding the sampler LOD bias to every texture instruction’s LOD.

  void *callback_data

    Payload data to be sent to callback / filter functions.

enum nir_lower_tex_packing
  enumerator nir_lower_tex_packing_none = 0

    No packing

  enumerator nir_lower_tex_packing_16

    The sampler returns up to 2 32-bit words of half floats or 16-bit signed or unsigned ints based on the sampler type

  enumerator nir_lower_tex_packing_8

    The sampler returns 1 32-bit word of 4x8 unorm