The documentation may come in handy:

1. https://ccache.dev/manual/4.11.3.html#_how_ccache_works

2. https://ccache.dev/manual/4.11.3.html#_cache_statistics

and so forth.

[1] https://ccache.dev (ccache - a fast C/C++ compiler cache)

Another thing I'd consider as interesting is parse caching from token to AST. Most headers don't change, so even when a TU needs to be recompiled, most parts of the AST could be reused. (Some kind of more clever and transparent precompiled headers.) This is likely to need some changes in the AST data structures for fast serialization and loading/inserting. And that makes me think that maybe the text book approach of generating an AST is a bad idea if we care about fast compilation.

Tangentially, I'm astonished that they claim correctness while a large amount of IR is inadequately (if at all) captured in the hash (comdat, symbol visibility, aliases, constant exprs, block address, calling convention/attributes for indirect calls, phi nodes, fast math flags, GEP type, ....). I'm also a bit annoyed, because this is the type of research that is very sloppily implemented, only evaluates projects where compile time is not a big problem and then only achieves small absolute savings, and papers over inherent difficulties (here: capturing the IR, parse time) that makes this unlikely to be used in practice.