Concurrency in the compiler

So right now, we've identified five stages in compilation:

Lex
Parse
Collate
Compile
Codegen

So which, if any, of these can we run in parallel?

Lex- yes. The lexer simply operates on a series of files. These files could be each opened in parallel.
Parse- yes. The parser simply operates on the tokens produced by the lexer, so each file can be parsed in paralle.
Collate- yes. Thanks to the PPL, a concurrent_unordered_map ought to do just fine here to allow us to collate concurrently.

Compile - Unknown. As this stage currently has no implementation, there's no way to know if I can parallelise that implementation. However, right now, I think the answer may yet be "yes". For example, given a simple binary expression of the form "expr + expr", then logically, each expression could be constructed concurrently. In addition, any statement that does not change the symbol table could be evaluated concurrently.

Codegen - Unlikely. LLVM does not appear to offer much concurrent compilation functionality, so this could get rather messy if concurrency is aimed for.