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

On Wed, Nov 17 2021, Carlo Arenas wrote:

On Wed, Nov 17, 2021 at 3:18 PM brian m. carlson
[off-list ref] wrote:

quoted

On 2021-11-17 at 03:01:57, Jeff King wrote:

quoted

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.

quoted

quoted

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/ (local)

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/ (local)

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.