From: John Garry <john.garry@huawei.com>
Date: Wed, 30 Mar 2022 10:48:56 GMT
Subject: Re: [Question] .git folder file updates for changing head commit
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:
> 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