Module msl

Backend for MSL (Metal Shading Language).

This backend does not support the SHADER_INT64_ATOMIC_ALL_OPS capability.

Binding model

Metal’s bindings are flat per resource. Since there isn’t an obvious mapping from SPIR-V’s descriptor sets, we require a separate mapping provided in the options. This mapping may have one or more resource end points for each descriptor set + index pair.

Entry points

Even though MSL and our IR appear to be similar in that the entry points in both can accept arguments and return values, the restrictions are different. MSL allows the varyings to be either in separate arguments, or inside a single [[stage_in]] struct. We gather input varyings and form this artificial structure. We also add all the (non-Private) globals into the arguments.

At the beginning of the entry point, we assign the local constants and re-compose the arguments as they are declared on IR side, so that the rest of the logic can pretend that MSL doesn’t have all the restrictions it has.

For the result type, if it’s a structure, we re-compose it with a temporary value holding the result.

Pointer-typed bounds-checked expressions and OOB locals

MSL (unlike HLSL and GLSL) has native support for pointer-typed function arguments. When the BoundsCheckPolicy is ReadZeroSkipWrite and an out-of-bounds index expression is used for such an argument, our strategy is to pass a pointer to a dummy variable. These dummy variables are called “OOB locals”. We emit at most one OOB local per function for each type, since all expressions producing a result of that type can share the same OOB local. (Note that the OOB local mechanism is not actually implementing “skip write”, nor even “read zero” in some cases of read-after-write, but doing so would require additional effort and the difference is unlikely to matter.)

External textures

Support for crate::ImageClass::External textures is implemented by lowering each external texture global variable to 3 texture2d<float, sample>s, and a constant buffer of type NagaExternalTextureParams. This provides up to 3 planes of texture data (for example single planar RGBA, or separate Y, Cb, and Cr planes), and the parameters buffer containing information describing how to handle these correctly. The bind target to use for each of these globals is specified via the BindTarget::external_texture field of the relevant entries in EntryPointResources::resources.

External textures are supported by WGSL’s textureDimensions(), textureLoad(), and textureSampleBaseClampToEdge() built-in functions. These are implemented using helper functions. See the following functions for how these are generated:

• Writer::write_wrapped_image_query
• Writer::write_wrapped_image_load
• Writer::write_wrapped_image_sample

The lowered global variables for each external texture global are passed to the entry point as separate arguments (see “Entry points” above). However, they are then wrapped in a struct to allow them to be conveniently passed to user defined and helper functions. See writer::EXTERNAL_TEXTURE_WRAPPER_STRUCT.

Modules

keywords 🔒

sampler

writer 🔒

Structs

AttributeMapping

A mapping of vertex buffers and their attributes to shader locations.

BindExternalTextureTarget

Binding information for a Naga External image global variable.

BindTarget

BindingMapSerialization 🔒

EntryPointResources

Options

PipelineOptions

A subset of options that are meant to be changed per pipeline.

TranslationInfo

Information about a translated module that is required for the use of the result.

VertexBufferMapping

A description of a vertex buffer with all the information we need to address the attributes within it.

Writer

Enums

BindSamplerTarget

EntryPointError

Error

LocationMode 🔒

Points in the MSL code where we might emit a pipeline input or output.

ResolvedBinding 🔒

ResolvedInterpolation 🔒

VertexFormat

Corresponds to WebGPU GPUVertexFormat.

Functions

deserialize_binding_map 🔒

write_string

Type Aliases

BindingMap

EntryPointResourceMap

InlineSamplerIndex

Slot