I'm just going to close this out, since I'm not sure we really need it -- D14963, D14962, and other work adjacent to T2450 found some general performance improvements in rendering. It's possible we might pursue this (and/or a highlighting cache) in the future, but I think we're fast enough now that the huge amount of upfront work the daemons would have to do to populate these caches probably doesn't make a ton of sense.

Had some free time today and implemented this. The improvement however is not as big as i expected.
Doing a blame on phabricators __phutil_library_map__.php will take ~20sec without and ~15sec with the cache on my dev vm.

• D5663 added a reasonable-looking way to detect these processes, but bin/phd never got the glue code to call it.
• This task is otherwise still relevant:
  • Add code in bin/phd stop and bin/phd restart to warn the user that there are processes which look like daemons that don't have corresponding PID files, and that they can use --force to terminate them.
  • With --force, terminate these processes.
  • The steps in https://secure.phabricator.com/T2374#8 should still let you reproduce this in modern phd.