Subject: Re: [PATCH v2] Makefile: make the "sparse" target non-.PHONY

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Fri, 24 Sep 2021 03:30:10 +0200

On Thu, Sep 23 2021, Junio C Hamano wrote:

> Jeff King <[email-masked]> writes:
>
>> On Thu, Sep 23, 2021 at 02:07:16AM +0200, Ævar Arnfjörð Bjarmason wrote:
>>
>>> We ensure that the recursive dependencies are correct by depending on
>>> the *.o file, which in turn will have correct dependencies by either
>>> depending on all header files, or under
>>> "COMPUTE_HEADER_DEPENDENCIES=yes" the headers it needs.
>>> 
>>> This means that a plain "make sparse" is much slower, as we'll now
>>> need to make the *.o files just to create the *.sp files, but
>>> incrementally creating the *.sp files is *much* faster and less
>>> verbose, it thus becomes viable to run "sparse" along with "all" as
>>> e.g. "git rebase --exec 'make all sparse'".
>>
>> OK. I think this solves the dependency issues sufficiently. It is a
>> tradeoff that you must do the normal build in order to do the sparse
>> check now. That is certainly fine for my workflow (I am building Git all
>> the time, and only occasionally run "make sparse"). I don't know if
>> others would like it less (e.g., if Ramsay is frequently running sparse
>> checks without having just built).
>>
>> (I'd say "I do not care that much either way", but then I do not care
>> all that much either way about incremental sparse checks either, so I'm
>> not sure my opinion really matters).
>
> My build procedure runs "make sparse" before the primary build,
> simply because the former tends to be much faster to fail when there
> is an issue in the code.  I can understand that depending on .o is a
> cheap way to piggyback on the dependencies it has, but my latency
> will get much slower if this goes in _and_ I keep trying to pick up
> potentially problematic patches from the list.

Would you be OK with just running something like this instead?:

    make -j $(nproc) objects CC=cgcc CFLAGS="-no-compile -Wsparse-error -Wno-pointer-arith -Wno-memcpy-max-count"

It gives you almost exactly the same thing as the old "sparse" target. I
think the way it worked is really not something we needed a special
target for in the first place, or perhaps just a .PHONY alias.

The "almost" is because those -Wno-* are now per-file via
SP_EXTRA_FLAGS.

I.e. if we have a $(CC) that's willing to accept CC-like options but
just won't create output files we can use "objects" (or other targets I
added in 029bac01a87 (Makefile: add {program,xdiff,test,git,fuzz}-objs &
objects targets, 2021-02-23)). Even if the tool itself created broken
*.o files a change of $(CC) to the "real" compiler would ensure that
they'd get regenerated.