[ccache] Using git file hashes for ccache

[Joel Rosdahl]

On 2010-12-31 19:06, Justin Lebar wrote:
> I expect that approach B would speed up ccache direct mode hits
> significantly, as most of the time, you'd only hash the source file,
> and you'd use the cached hashes of the files it includes.  If it runs
> faster, presumably there would be less to gain by invoking ccache less
> often.
> 
> I'm very skeptical that we want to add to ccache the kind of
> complexity (and tight coupling!) that option C requires.  Furthermore,
> it seems to me that some of this logic (e.g. "don't build me because,
> although my mtime has changed, my contents haven't") belongs in the
> build system.
> 
> I'd also guess that C wouldn't be much faster than B, since in the
> steady state, B hashes only the source file and has cached hashes of
> most or all of the source file's includes.

I too think that option B sounds interesting, at least as an experiment
to get a hint of the potential gain.

-- Joel