# 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 `Program`s, `ProgramBuilder`s are not part of the public Tint API. ## AST The Abstract Syntax Tree is a directed acyclic graph of `ast::Node`s 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::Node`s are not shared. The AST does not perform any verification of its content. For example, the `ast::StrideAttribute` 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::Node`s 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 a builtin 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::Node`s to `sem::Node`s. 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::Node`s, 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 `Reader`s. 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 `Program`s semantic information by analyzing the `Program`s 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 `Program`s 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).