# Compilation

The compilation process goes through 3 steps :

1. Parsing of shader mixins
2. Generation of SPIR-V from mixins, resulting in SPIR-V mixins
3. Mixing of those SPIR-V bytecode into a final shader

The previous implementation was working through the AST to do the mixin system, going for the SPIR-V mixin system helps us mix with binary data instead of textual. This also enables us to use SPIR-V Cross and Naga for cross shader compilation.

## Parsing

### Grammars

Parsing is done with Eto.Parse, [the SDSL grammar](https://github.com/ykafia/SDSLParser/tree/master/src/SDSL/Parsers/Grammars/SDSLGrammar) is what defines the language.

There is also a grammar for [comments](https://github.com/ykafia/SDSLParser/blob/master/src/SDSL/Parsers/Grammars/CommentGrammar.cs) and [directives](https://github.com/ykafia/SDSLParser/tree/master/src/SDSL/Parsers/Grammars/DirectiveGrammar), the latter is not used fully functional so directives are handled by [CPP.NET](https://github.com/MonoGame/CppNet).

For the langage there is first a comment prepass where comments are removed, then the directive compilation and finally the resulting code is transformed into an AST

### AST

The [Abstract syntax tree](https://github.com/ykafia/SDSLParser/tree/master/src/SDSL/Parsers/AST/Shader) is built right after the syntax parse tree is generated in [the GetToken method](https://github.com/ykafia/SDSLParser/blob/master/src/SDSL/Parsers/AST/Shader/ShaderToken.cs#L29). 

As opposed to the previous shader system, those AST are not supposed to be operated on as they will only serve to generate partial spir-v binaries.

### Semantic analysis

Some semantic analysis passes include : 
* Type checking
* Scope resolution
* Declaration rules
* Flow control checks
* Unique ID checks
* Control flow analysis

The control flow analysis generates a control flow graph (CFG) that will help heavily in generating the SPIR-V assembly.

## SPIR-V Assembler

The backend will use [SoftTouch.Spirv](https://github.com/SoftTouch.Spirv) to generate spir-v assembly. In this library the spir-v specification has been extended to include instructions for SDSL, helping the compiler build final spir-v shaders from mixins.

In this library a `Mixer` can be created to generate a new mixin. It can do operations like `Inherit`, `Compose` and also generate functions through a small abstraction over spir-v.

It's designed to have very low allocations, meaning all unecessary allocations are avoided, to make sure shader compilation doesn't hinder on performance.