Re: [Question] .git folder file updates for changing head commit
- From
- John Garry <john.garry@huawei.com>
- Date
- Mar 30, 2022, 10:48 UTC
- Message-ID
- <7f23ed59-c237-dce1-53a5-43f35b92c972@huawei.com>
- In-Reply-To
- <220327.86tubje6bn.gmgdl@evledraar.gmail.com>
On 27/03/2022 16:48, Ævar Arnfjörð Bjarmason wrote:
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> 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.
I see.
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> > 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'm not sure if I mentioned it before, but the main motivation for adding the dependency was that "git describe" may be slow. We have since moved away from that (using "describe", which I hope turns out to be ok), but your GIT-VERSION-GEN still uses it. I just found that "git describe" is unreliable for us as it depends on tags being pushed to a clone git.
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> 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". > .
ok, thanks, John