On Wed, Mar 23 2022, Taylor Blau wrote: > On Wed, Mar 23, 2022 at 03:19:06PM +0000, John Garry wrote: >> For building the linux perf tool we use the git head commit id as part of >> the tool version sting. To save time in re-building, the Makefile rule has a >> dependency on .git/HEAD for rebuilding. An alternative approach would be to >> compare git log output to check current versus previous build head commit, >> but that is seen as inefficient time-wise. > > Having a Makefile recipe that depends on $GIT_DIR/HEAD seems strange to > me. > > Presumably your Makefile rules would map out which parts of your program > depend on each other, and would get invalidated when the source itself > changes, no? > > Perhaps you also care about the commit you're building from in order to > embed something into your program. But it seems like you could inject > the output of "git rev-parse HEAD" when you construct the version > identifier whenever you do need to rebuild. Our very own build process for git.git relies on this, see how version.o needs a GIT-VERSION-FILE, which we generate by shelling out and including. It is unfortunate that we don't have an advertised way to do this, with the ref backend I think trickery using $(file) to read the HEAD will work to do it in pure-make, i.e. you'd need to parse it to see if it's a symref, then depend on the target it points to. Or you could recursively depend on a glob of the whole refspace for generating the version file ... It would be nice if we had a way to guarantee that we'd write some file on HEAD updates, AFAIK not even the new reference-transaction hook will do that (due to "git reset --hard" and friends). And yes, this does actually matter. There's a huge performance difference between a Makefile that needs to shell out for every little thing. I've been optimizing our own Makefile incrementally from running in ~500ms down to 100-200ms for noop runs over over the last few months. It's possible to get it down to 10-20ms at least by getting rid of the remaining shell-outs. That makes a big difference when e.g. using "make" with "git rebase -i -x".
