[gentoo-portage-dev] Add caching to a few commonly used functions

[Chun-Yu Shei]

Hi,

I was recently interested in whether portage could be speed up, since
dependency resolution can sometimes take a while on slower machines.
After generating some flame graphs with cProfile and vmprof, I found 3
functions which seem to be called extremely frequently with the same
arguments: catpkgsplit, use_reduce, and match_from_list.  In the first
two cases, it was simple to cache the results in dicts, while
match_from_list was a bit trickier, since it seems to be a requirement
that it return actual entries from the input "candidate_list".  I also
ran into some test failures if I did the caching after the
mydep.unevaluated_atom.use and mydep.repo checks towards the end of the
function, so the caching is only done up to just before that point.

The catpkgsplit change seems to definitely be safe, and I'm pretty sure
the use_reduce one is too, since anything that could possibly change the
result is hashed.  I'm a bit less certain about the match_from_list one,
although all tests are passing.

With all 3 patches together, "emerge -uDvpU --with-bdeps=y @world"
speeds up from 43.53 seconds to 30.96 sec -- a 40.6% speedup.  "emerge
-ep @world" is just a tiny bit faster, going from 18.69 to 18.22 sec
(2.5% improvement).  Since the upgrade case is far more common, this
would really help in daily use, and it shaves about 30 seconds off
the time you have to wait to get to the [Yes/No] prompt (from ~90s to
60s) on my old Sandy Bridge laptop when performing normal upgrades.

Hopefully, at least some of these patches can be incorporated, and please
let me know if any changes are necessary.

Thanks,
Chun-Yu