From: John Garry <john.garry@huawei.com>
To: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
"Taylor Blau" <me@ttaylorr.com>
Cc: <git@vger.kernel.org>
Subject: Re: [Question] .git folder file updates for changing head commit
Date: Wed, 30 Mar 2022 11:48:56 +0100 [thread overview]
Message-ID: <7f23ed59-c237-dce1-53a5-43f35b92c972@huawei.com> (raw)
In-Reply-To: <220327.86tubje6bn.gmgdl@evledraar.gmail.com>
On 27/03/2022 16:48, Ævar Arnfjörð Bjarmason wrote:
> 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.
>
> 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.
> 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
next prev parent reply other threads:[~2022-03-30 10:49 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-03-23 15:19 [Question] .git folder file updates for changing head commit John Garry
2022-03-23 20:25 ` Taylor Blau
2022-03-24 12:56 ` John Garry
2022-03-27 15:48 ` Ævar Arnfjörð Bjarmason
2022-03-30 10:48 ` John Garry [this message]
2022-03-23 20:50 ` Glen Choo
2022-03-24 12:59 ` John Garry
2022-03-23 21:28 ` Andreas Schwab
2022-03-24 13:00 ` John Garry
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