Re: [PATCH v2 0/3] detect-compiler: clang updates

From: Jeff King <peff@peff.net>
Date: Aug 7, 2021, 14:13 UTC
Message-ID: <YQ6VJW0AwkouDnDe@coredump.intra.peff.net>
In-Reply-To: <875ywiyn4y.fsf@evledraar.gmail.com>

On Sat, Aug 07, 2021 at 04:56:04AM +0200, Ævar Arnfjörð Bjarmason wrote:

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> > On Sat, Aug 07, 2021 at 04:02:45AM +0200, Ævar Arnfjörð Bjarmason wrote:
> >
> >> Perhaps I've missed some obvious reason not to do this, but why are we
> >> parsing the --version output of two modern compilers, as opposed to just
> >> asking them what type/version they are via their usual macro facilities?
> >> I.e. something like the below:
> >
> > That would probably work OK in practice, but it actually seems more
> > complex to me (how do other random compilers react to "-E -"?
> 
> We only care about gcc and clang in that script, which I think have
> supported that form of "-E" on stdin input for any version we're likely
> to care about for the purposes of config.mak.dev. It seems unlikely that
> we'll care about non-modern compilers in config.mak.dev, so using more
> modern features there seems fine (it's all for opting us into even more
> modern warning flags and the like...).

Yeah, but we don't find out what we have until we run the script in question. I guess it is OK as long as we redirect stderr, ignore the exit code, and only look for a positive outcome in the output (your patch does the latter two already).

I also wondered how this might interact with CC="ccache gcc" (where caching might fail to notice version changes). But from some quick testing, it looks like it doesn't cache in this case (neither stdin, nor with -E).

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> > Is it possible for us to get other output from the preprocessor that
> > would confuse an eval?).
> 
> Probably, I just meant that as a POC. We could pipe it into some
> awk/grep/cut/perl or whatever that would be more strict.

That would probably be better. I would be curious to hear from somebody with a mac if this technique gives more sensible version numbers for the Apple-clang compiler.

-Peff