dawn-cmake/docs/arch.md

Tint Architecture

                   ┏━━━━━━━━┓                   ┏━━━━━━┓
                   ┃ SPIR━V ┃                   ┃ WGSL ┃
                   ┗━━━━┃━━━┛                   ┗━━━┃━━┛
                        ▼                           ▼
              ┏━━━━━━━━━┃━━━━━━━━━━━━━━━━━━━━━━━━━━━┃━━━━━━━━┓
              ┃         ┃          Reader           ┃        ┃
              ┃         ┃                           ┃        ┃
              ┃ ┏━━━━━━━┻━━━━━━┓             ┏━━━━━━┻━━━━━━┓ ┃
              ┃ ┃ SPIRV-Reader ┃             ┃ WGSL-Reader ┃ ┃
              ┃ ┗━━━━━━━━━━━━━━┛             ┗━━━━━━━━━━━━━┛ ┃
              ┗━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━┛
                                      ▼
                    ┏━━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━━┓
                    ┃           ProgramBuilder          ┃
                    ┃             (mutable)             ┃
      ┏━━━━━━━━━━━━►┫   ┏━━━━━┓ ┏━━━━━━━┓ ┏━━━━━━━━━┓   ┃
      ┃             ┃   ┃ AST ┃ ┃ Types ┃ ┃ Symbols ┃   ┃
      ┃             ┃   ┗━━━━━┛ ┗━━━━━━━┛ ┗━━━━━━━━━┛   ┃
      ┃             ┗━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━┛
      ┃                               ▼
      ┃             ┌┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┃┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┐
      ▲             ┆ Build           ▼                ┆
  ┏━━━┻━━━┓         ┆        ┏━━━━━━━━┻━━━━━━━━┓       ┆
  ┃ Clone ┃         ┆        ┃    Resolver     ┃       ┆
  ┗━━━┳━━━┛         ┆        ┗━━━━━━━━━━━━━━━━━┛       ┆
      ▲             └┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┃┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┘
      ┃                               ▼
      ┃       ┏━━━━━━━━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━━━━━━━┓
      ┃       ┃                    Program                   ┃
      ┃       ┃                  (immutable)                 ┃
      ┣━━━━━━◄┫  ┏━━━━━┓ ┏━━━━━━━┓ ┏━━━━━━━━━━┓ ┏━━━━━━━━━┓  ┃
      ┃       ┃  ┃ AST ┃ ┃ Types ┃ ┃ Semantic ┃ ┃ Symbols ┃  ┃
      ┃       ┃  ┗━━━━━┛ ┗━━━━━━━┛ ┗━━━━━━━━━━┛ ┗━━━━━━━━━┛  ┃
      ┃       ┗━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━┛
      ▲                               ▼
┏━━━━━┻━━━━━┓                         ┃             ┏━━━━━━━━━━━┓
┃ Transform ┃◄━━━━━━━━━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━►┃ Inspector ┃
┗━━━━━━━━━━━┛                         ┃             ┗━━━━━━━━━━━┛
                                      ▼
┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┓
┃                                  Writer                                  ┃
┃                                                                          ┃
┃ ┏━━━━━━━━━━━━━━┓    ┏━━━━━━━━━━━━━┓    ┏━━━━━━━━━━━━━┓    ┏━━━━━━━━━━━━┓ ┃
┃ ┃ SPIRV-Writer ┃    ┃ WGSL-Writer ┃    ┃ HLSL-Writer ┃    ┃ MSL-Writer ┃ ┃
┃ ┗━━━━━━━┳━━━━━━┛    ┗━━━━━━┳━━━━━━┛    ┗━━━━━━┳━━━━━━┛    ┗━━━━━━┳━━━━━┛ ┃
┗━━━━━━━━━┃━━━━━━━━━━━━━━━━━━┃━━━━━━━━━━━━━━━━━━┃━━━━━━━━━━━━━━━━━━┃━━━━━━━┛
          ▼                  ▼                  ▼                  ▼
     ┏━━━━┻━━━┓          ┏━━━┻━━┓           ┏━━━┻━━┓            ┏━━┻━━┓
     ┃ SPIR-V ┃          ┃ WGSL ┃           ┃ HLSL ┃            ┃ MSL ┃
     ┗━━━━━━━━┛          ┗━━━━━━┛           ┗━━━━━━┛            ┗━━━━━┛

Reader

Readers are responsible for parsing a shader program and populating a ProgramBuilder with the parsed AST, type and symbol information.

The WGSL reader is a recursive descent parser. It closely follows the WGSL grammar in the naming of the parse methods.

ProgramBuilder

A ProgramBuilder is the primary interface to construct an immutable Program. There are a number of methods exposed which make creating of the Program simpler. A ProgramBuilder can only be used once, and must be discarded after the Program is constructed.

A Program is built from the ProgramBuilder by std::move()ing the ProgramBuilder to a new Program object. When built, resolution is performed so the produced Program will contain all the needed semantic information.

At any time before building the Program, ProgramBuilder::IsValid() may be called to ensure the AST is structurally correct. This checks that things like if statements have a condition and body attached.

If further changes to the Program are needed (say via a Transform) then a new ProgramBuilder can be produced by cloning the Program into a new ProgramBuilder.

Unlike Programs, ProgramBuilders are not part of the public Tint API.

AST

The Abstract Syntax Tree is a directed acyclic graph of ast::Nodes which encode the syntactic structure of the WGSL program.

The root of the AST is the ast::Module class which holds each of the declared functions, variables and user defined types (type aliases and structures).

Each ast::Node represents a single part of the program's source, and so ast::Nodes are not shared.

The AST does not perform any verification of its content. For example, the ast::StrideDecoration node has numeric stride parameter, which is a count of the number of bytes from the start of one array element to the start of the next. The AST node itself does not constrain the set of stride values that you can set, aside from storing it as an unsigned integer.

Types

Types are constructed during the Reader and resolution phases, and are held by the Program or ProgramBuilder. AST and semantic nodes can both reference types.

Each type::Type node uniquely represents a particular spelling of a WGSL type within the program, so you can compare type::Type* pointers to check for equivalence of type expressions. For example, there is only one type::Type node for the i32 type, no matter how many times it is mentioned in the source program. However, if MyI32 is a type alias for i32, then they will have two different type nodes.

Semantic information

Semantic information is held by sem::Nodes which describe the program at a higher / more abstract level than the AST. This includes information such as the resolved type of each expression, the resolved overload of an intrinsic function call, and the module scoped variables used by each function.

Semantic information is generated by the Resolver when the Program is built from a ProgramBuilder.

The sem::Info class holds a map of ast::Nodes to sem::Nodes. This map is many-to-one - i.e. while a AST node might have a single corresponding semantic node, the reverse may not be true. For example: many ast::IdentifierExpression nodes may map to a single sem::Variable, and so the sem::Variable does not have a single corresponding ast::Node.

Unlike ast::Nodes, semantic nodes may not necessarily form a directed acyclic graph, and the semantic graph may contain diamonds.

Symbols

Symbols represent a unique string identifier in the source program. These string identifiers are transformed into symbols within the Readers.

During the Writer phase, symbols may be emitted as strings using a Namer. A Namer may output the symbol in any form that preserves the uniqueness of that symbol.

Resolver

The Resolver will automatically run when a Program is built. A Resolver creates the Programs semantic information by analyzing the Programs AST and type information.

The Resolver will validate to make sure the generated Program is semantically valid.

Program

A Program holds an immutable version of the information from the ProgramBuilder along with semantic information generated by the Resolver.

Like ProgramBuilder, Program::IsValid() may be called to ensure the AST is structurally correct and semantically valid, and that the Resolver did not report any errors.

Unlike the ProgramBuilder, a Program is fully immutable, and is part of the public Tint API. The immutable nature of Programs make these entirely safe to share between multiple threads without the use of synchronization primitives.

Inspector

The inspectors job is to go through the Program and pull out various pieces of information. The information may be used to pass information into the downstream compilers (things like specialization constants) or may be used to pass into transforms to update the AST before generating the resulting code.

The input Program to the inspector must be valid (pass validation).

Transforms

There maybe various transforms we want to run over the Program. This is for things like Vertex Pulling or Robust Buffer Access.

A transform operates by cloning the input Program into a new ProgramBuilder, applying the required changes, and then finally building and returning a new output Program. As the resolver is always run when a Program is built, Transforms will always emit a Program with semantic information.

The input Program to a transform must be valid (pass validation). If the input Program of a transform is valid then the transform must guarantee that the output program is also valid.

Writers

A writer is responsible for writing the Program in the target shader language.

The input Program to a writer must be valid (pass validation).