Re: [PATCH v2 1/1] git-compat-util: add a test balloon for C99 support

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

On Wed, Nov 17 2021, Carlo Arenas wrote:

> On Wed, Nov 17, 2021 at 3:18 PM brian m. carlson
> <sandals@crustytoothpaste.net> wrote:
>>
>> On 2021-11-17 at 03:01:57, Jeff King wrote:
>> > My thinking was that breaking older compilers was preferable to breaking
>> > non-gnu ones, because at least old ones go away eventually. But your
>> > other email makes me wonder if those non-GNU ones may already be
>> > overriding CFLAGS.
>>
>> Our only problem platform, as far as I can tell, is RHEL/CentOS 7.  That
>> uses GCC 4.8, and even Ubuntu 18.04 ships with GCC 7.
>
> There are several odd BSD platforms that are still stuck in pre-GPLv3
> gcc (AKA gcc 4.2.1) like OpenBSD Alpha, hppa, landisk (and maybe also
> SPARC64 which is tier1) and that will need the same, there is indeed
> also luna88k that uses an even older gcc but hopefully will be able to
> work if it understands enough C99 and can be told to use it by this
> flag.
>
>> > Still, if we can come up with a solution that breaks neither (with some
>> > light auto-detection or heuristics in the Makefile), that could be the
>> > best of both worlds.
>>
>> I can move COMPILER_FEATURES out of config.mak.dev and into Makefile so
>> that we can make use of it.  We'll need to depend on GCC 6 for this
>> because we lack a way to distinguish 5.1 (which should work) from 5.0
>> (which will not).
>
> 5.0 works AFAIK, is anything older than 5 than does not as reported[1]
> before, but it won't be still a good fit, since it only works for gcc
> and clang AS-IS.
>
> Carlo
>
> [1] https://lore.kernel.org/git/CAPUEsphnCvK+RZ+h30ZarA1zo9yZ=ndEBrcAbKGf4W92j647vA@mail.gmail.com/

Rather than moving around COMPILER_FEATURES etc. we can just compile a C
program as part of our Makefile auto-configuration. See the direction
suggested in:
https://lore.kernel.org/git/87bl6aypke.fsf@evledraar.gmail.com/

That example is ad-hoc, but the right way to do this is:

 1. Stick a C program somewhere, maybe git-autoconf/compiler.c 
 2. (Try to) Compile that unconditionally
 3. Emit its output to a generated file that we then "include", which
    likewise if it fails indicate that in something the Makefile can
    "include".

Since we set up that file-based dependency relationship we'll only do
that auto-detection on the first build.

This is really much simpler than fiddling with the version parsing
shellscript, i.e. we can just compile a program with -std=c99 or
whatever and see if it works, and if it does we stick that flag in
CFLAGS or equivalent.