On Fri, 2025-02-28 at 19:26 +0100, Ben Boeckel via Gcc wrote: > > In POSIX make, including GNU Make, if a command doesn't modify the > > modification time of the target then that target is not considered > > updated, and other targets which list it as a prerequisite are not > > invoked: > > Hmm. My understanding was that if a recipe is run, its outputs are > assumed to have been updated. I forget where I saw that, but it > sticks in my mind for some reason. Experimenting with GNU Make indeed > does exhibit `restat = 1` behavior out-of-the-box. That's good to > know :). I'll update my priors.
There is one issue related to this: I don't know if Ninja addresses this or not: maybe this is what people were thinking of? Because the modification time of the target is not updated, the recipe for the the target will always be run (but targets that depend on it will not be run). E.g., in my previous example I had: ~$ touch three; make MKTWO=echo echo two two Now if I run it again after touching "three", the command for "two" is again run as you'd expect, and "one" is not since "two"'s timestamp is not changed: ~$ touch three; make MKTWO=echo echo two two BUT! Now if I do NOT update "three", "two" is STILL updated, and will continue to be as long as the "two" target's timestamp is not changed: ~$ make MKTWO=echo echo two two ~$ make MKTWO=echo echo two two If the recipe to create "two" is complex/long-running then this is not great. Only when we allow "two" to actually be modified (in my example, by not overriding the MKTWO variable), will we get back to "normal" behavior: ~$ make touch two touch one ~$ make make: 'one' is up to date. This is the best that make can do, because it relies solely on the filesystem as its database of "build state" and doesn't preserve any build state of its own. Since "two"'s timestamp was not modified make has no way to know that its recipe was actually run previously. If the recipe to build "two" is expensive, it's probably better to just bite the bullet and allow "two" to be updated whenever its prerequisites change. The only way for make to do better would be for it to keep its own database of up-to-date notations, alongside or in addition to the filesystem's modification time. That would be a major structural change, of course.
