You can compile different files in parallel. You can probably even compile a single file in parallel. Consider this: If you start reading in the middle of a source file, can you make sense of what you see, or is it incomprehensible garbage unless you start at the top?
The answer is that it mostly makes sense except you may be tricked by #defines for example. Thus you could probably "optimistically parse" small chunks of the source code, all in parallel, with the understanding that you may have to throw out some intermediate results in light of new understandings.
Yeah, of course there are tasks that can be executed with coarse-grained parallelism, that can take advantage of multiple threads. It's not embarrassingly parallel though like with graphics, where you have millions of pixels that can be shaded independently of all the other pixels in an image, per rendering pass. In an optimizing compiler, compiled functions are not independent of each other, because data flow analysis will affect code generation, so you can't just compile each function on a different thread and combine the results. At least on a per-file basis, compilation has an inherently serial bottleneck, and that's not considering that 1. not all languages have separate compilation and linking like the C family, and 2. even with whole program optimization disabled, the linking phase can be fairly complicated in modern linker, so you have yet another serial bottleneck. And if you're implementing an non-optimizing compiler, then even with the naive, completely serial compiler, the whole process won't take long enough for it to be worth going to this trouble.
Glitch works this way. It will simply redo the tree when better data is available (via dependency tracing); so you can parse, type check various parts within and between files using what is essentially optimistic parallelism.
This is quite useful for non regular problems that are otherwise difficult to parallelize.
The answer is that it mostly makes sense except you may be tricked by #defines for example. Thus you could probably "optimistically parse" small chunks of the source code, all in parallel, with the understanding that you may have to throw out some intermediate results in light of new understandings.