On Thu, 2011-01-13 at 20:38 +0100, Ralf Wildenhues wrote: > * Bob Friesenhahn wrote on Thu, Jan 13, 2011 at 08:32:01PM CET: > > On Thu, 13 Jan 2011, Ralf Wildenhues wrote: > > >You can implement hash-based dependencies inside GNU make if you like. > > >Or another make. Maybe someone has even done so already. It has little > > >to do with Automake. > > > > I have never heard of such a thing implemented in make. It would > > surely bloat the makefile by more megabytes or be excruciatingly > > slow. > > That's not what I meant. You can hack the source code of GNU make > with the goal to (optionally) replace timestamp-based dependencies > with file-hash-based depdendencies. No changes at all in any user > makefiles. It's not a large change even. IIRC somebody has done > this already, it just hasn't been merged upstream.
It's not QUITE so simple. The advantage to using timestamp-based dependencies is that the filesystem is your database of "out-of-dateness", and the kernel ensures that the behavior of that database is well-defined. If you want to use anything more sophisticated to determine "out-of-dateness", then make has to keep its own database to remember the previous state, so it can be compared. Granted this is not rocket science but there are some tricky bits to it: it needs to be pretty portable to the various architectures make supports; it needs to be well-behaved in the case of a missing database, and in the case of multiple instances of make running in the same directory at the same time; what happens if the database is corrupted or somehow contains the wrong information; etc. Making this robust enough to trust it with your builds requires some thought and care. > BTW, Nathanael's propositions in this thread go in the same direction: > http://gcc.gnu.org/ml/gcc-patches/2010-11/msg01504.html There was a set of work done to introduce "user-defined out-of-date functions" into GNU make, a few years ago. That didn't end up getting merged because it seemed too complex to me, both from a make developer and make user standpoint. I agree with Nathanael, although I take serious issue with his statement "the GNU make people were completely uninterested, since they didn't get why it was a good idea"--that is simply not true. What is true is that Nathanael posted a 10,000ft-level description of what he wanted, not completely thought-out, and when I replied to his message with some discussion of the idea and potential problems with it, he never followed up.
