[ccache] BSDiff for cache objects

[Bogdan Harjoc]

I just did a quick search, and couldn't find discussions on the idea of
caching compiled objects as binary diffs from other existing objects.

Basically, before writing a new object file, ccache could find a similar
object in the cache (based on object-code or source-code hashes for
example) and store the delta (using bsdiff, xdelta, ...) instead of the
complete file. For most minor source code changes, the savings should be
worth the extra effort.

Alternatively, a "compact" operation could be run periodically, that
compresses the cache using the same approach.

My question is whether ccache's real-world use would benefit from a feature
like this. I can put together a test that looks through people's .ccache
and reports how many "good bsdiff candidates" there are, and what the
savings would be.

Any opinions ?