Follow-up Comment #4, bug #20394 (project make):
Something like that could be tricky to accomplish, since make is highly
recursive: when make starts checking for timestamps on files it doesn't know
whether that file was found using implicit or explicit rules, etc.
Plus, I think it would be even more confusing to have some kinds of targets
(implicit) behave unexpectedly while others (explicit) behaved as expected,
but more slowly.
To fix this problem we need to add a way to determine if the cache is stale.
My idea on how to do this is for make to keep a counter that is incremented
every time make invokes a subprocess. Or maybe, from a correctness point of
view, it should be updated every time a subprocess _finishes_. Then when a
directory was cached it would store the current value of that counter. The
next time the cached data was consulted, if the current counter was greater
than the stored counter the cache would be considered stale and new data
would be read.
This would significantly reduce the efficiency of the cache, but it would
increase correctness and the cache would still be useful while make is
looking for things to build--for situations where there's very little or
nothing to do the cache would still be a big win.
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