dawn-cmake/docs/codegen.md

Dawn's code generators.

Dawn relies on a lot of code generation to produce boilerplate code, especially webgpu.h-related code. They start by reading some JSON files (and sometimes XML too), process the data into an in-memory representation that's then used by some Jinja2 templates to generate the code. This is similar to the model/view separation in Web development.

Generators are based on generator_lib.py which provides facilities for integrating in build systems and using Jinja2. Templates can be found in generator/templates and the generated files are in out/<Debug/Release/foo>/gen/src when building Dawn in standalone. Generated files can also be found in Chromium's code search.

Dawn "JSON API" generators

Most of the code generation is done from dawn.json which is a JSON description of the WebGPU API with extra annotation used by some of the generators. The code for all the "Dawn JSON" generators is in dawn_json_generator.py (with templates in the regular template dir).

At this time it is used to generate:

• the Dawn, Emscripten, and upstream webgpu-native webgpu.h C header
• the Dawn and Emscripten webgpu_cpp.cpp/h C++ wrapper over the C header
• libraries that implements webgpu.h by calling in a static or thread_local proc table
• other parts of the Emscripten WebGPU implementation
• a GMock version of the API with its proc table for testing
• validation helper functions for dawn_native
• the definition of dawn_native's proc table
• dawn_native's internal version of the webgpu.h types
• utilities for working with dawn_native's chained structs
• a lot of dawn_wire parts, see below

Internally dawn.json is a dictionary from the "canonical name" of things to their definition. The "canonical name" is a space-separated (mostly) lower-case version of the name that's parsed into a Name Python object. Then that name can be turned into various casings with .CamelCase() .SNAKE_CASE(), etc. When dawn.json things reference each other, it is always via these "canonical names".

The "_metadata" key in the JSON file is used by flexible templates for generating various Web Standard API that contains following metadata:

• "api" a string, the name of the Web API
• "namespace" a string, the namespace of C++ wrapper
• "c_prefix" (optional) a string, the prefix of C function and data type, it will default to upper-case of "namespace" if it's not provided.
• "copyright_year" (optional) a string, templates will use the the year of copyright.

The basic schema is that every entry is a thing with a "category" key what determines the sub-schema to apply to that thing. Categories and their sub-shema are defined below. Several parts of the schema use the concept of "record" which is a list of "record members" which are a combination of a type, a name and other metadata. For example the list of arguments of a function is a record. The list of structure members is a record. This combined concept is useful for the dawn_wire generator to generate code for structure and function calls in a very similar way.

Most items and sub-items can include a list of "tags", which, if specified, conditionally includes the item if any of its tags appears in the enabled_tags configuration passed to parse_json. This is used to include and exclude various items for Dawn, Emscripten, or upstream header variants. Tags are applied in the "parse_json" step (rather than later): this has the benefit of automatically catching when, for a particular tag configuration, an included item references an excluded item.

A record is a list of record members, each of which is a dictionary with the following schema:

• "name" a string
• "type" a string, the name of the base type for this member
• "annotation" a string, default to "value". Define the C annotation to apply to the base type. Allowed annotations are "value" (the default), "*", "const*", "const*const*"
• "length" (default to 1 if not set), a string. Defines length of the array pointed to for pointer arguments. If not set the length is implicitly 1 (so not an array), but otherwise it can be set to the name of another member in the same record that will contain the length of the array (this is heavily used in the fooCount foos pattern in the API). As a special case "strlen" can be used for const char* record members to denote that the length should be determined with strlen.
• "optional" (default to false) a boolean that says whether this member is optional. Member records can be optional if they are pointers (otherwise dawn_wire will always try to dereference them), objects (otherwise dawn_wire will always try to encode their ID and crash), or if they have a "default" key. Optional pointers and objects will always default to nullptr.
• "default" (optional) a number or string. If set the record member will use that value as default value. Depending on the member's category it can be a number, a string containing a number, or the name of an enum/bitmask value.

"native", doesn't have any other key. This is used to define native types that can be referenced by name in other things.

"typedef" (usually only used for gradual deprecations):

• "name": the name of the things this is a typedef for.

"enum" an uint32_t-based enum value.

• "values" an array of enum values. Each value is a dictionary containing:
  • "name" a string
  • "value" a number that can be decimal or hexadecimal
  • "jsrepr" (optional) a string to allow overriding how this value map to Javascript for the Emscripten bits
  • "valid" (defaults to true) a boolean that controls whether the dawn_native validation utilities will consider this enum value valid.
• "emscripten_no_enum_table" (optional) if true, skips generating an enum table in library_webgpu_enum_tables.js

"bitmask" an uint32_t-based bitmask. It is similar to "enum" but can be output differently.

"callback" defines a function pointer type that can be used by other things (usually callbacks passed to method calls)

• "args" a record, so an array of record members

"structure"

• "members" a record, so an array of record members
• "extensible" (defaults to false) a boolean defining if this is an "extensible" WebGPU structure (i.e. has nextInChain). "descriptor" structures should usually have this set to true.
• "chained" (defaults to false) a boolean defining if this is a structure that can be "chained" in a WebGPU structure (i.e. has nextInChain and sType)

"object"

• **methods** an array of methods for this object. Note that "release" and "reference" don't need to be specified. Each method is a dictionary containing:
  • "name" a string
  • "return_type" (default to no return type) a string that's the name of the return type.
  • "arguments" a record, so an array of record members

Dawn "wire" generators

The generator for the pieces of dawn_wire need additional data which is found in dawn_wire_json. Examples of pieces that are generated are:

• WireCmd.cpp/.h the most important piece: the meat of the serialization / deserialization code for WebGPU structures and commands
• ServerHandlers/Doers.cpp that does the complete handling of all regular WebGPU methods in the server
• ApiProcs.cpp that implements the complete handling of all regular WebGPU methods in the client

Most of the WebGPU methods can be handled automatically by the wire client/server but some of them need custom handling (for example because they handle callbacks or need client-side state tracking). dawn_wire.json defines which methods need special handling, and extra wire commands that can be used by that special handling (and will get WireCmd support).

The schema of dawn_wire.json is a dictionary with the following keys:

• "commands" an array of records defining extra client->server commands that can be used in special-cased code path.
  • Each record member can have an extra "skip_serialize" key that's a boolean that default to false and makes WireCmd skip it on its on-wire format.
• "return commands" like "commands" but in revers, an array of records defining extra server->client commands
• "special items" a dictionary containing various lists of methods or object that require special handling in places in the dawn_wire autogenerated files
  • "client_side_structures": a list of structure that we shouldn't generate serialization/deserialization code for because they are client-side only
  • "client_handwritten_commands": a list of methods that are written manually and won't be automatically generated in the client
  • "client_side_commands": a list of methods that won't be automatically generated in the server. Gets added to "client_handwritten_commands"
  • "client_special_objects": a list of objects that need special manual state-tracking in the client and won't be autogenerated
  • "server_custom_pre_handler_commands": a list of methods that will run custom "pre-handlers" before calling the autogenerated handlers in the server
  • "server_handwrittten_commands": a list of methods that are written manually and won't be automatically generated in the server.
  • server_reverse_object_lookup_objects: a list of objects for which the server will maintain an object -> ID mapping.

OpenGL loader generator

The code to load OpenGL entrypoints from a GetProcAddress function is generated from gl.xml and the list of extensions it supports.