Overview of the Front End

The BitC front end is conceptually divided into three major phases: surface syntax processing, validation, and code generation.

• The surface phase covers everything that is needed to generate an AST. This is basically the parser and the lexer. The main reason that this stage is distinguished is that we are planning a re-work of the language surface syntax.

• The validation phase covers everything required to know that you have a valid unit of compilation, or in interactive mode, a valid top-level form. This includes a few AST simplification passes, the symbol resolver, and various semantic checking passes. It also includes a hoisting pass that converts lambda instance forms into top level functions. This hoisting pass should probably be moved into to the next phase. If compilation makes it to the end of the UOC phase, the rest of the compilation should (in theory) proceed without error unless compiler memory is exhausted.

• The code generation phase implements closure conversion, tail recursion marking, and transformation of the code into an SSA-like form. In essence, this covers all of the ``flattening'' activities that are necessary to emit an IR representation of BitC code for use by the back end.

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