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From: Richard Biener <richard.guenther@gmail.com>
To: Florian Weimer <fweimer@redhat.com>
Cc: gcc@gcc.gnu.org, c-std-porting@lists.linux.dev
Subject: Re: More C type errors by default for GCC 14
Date: Tue, 9 May 2023 17:16:19 +0200	[thread overview]
Message-ID: <97CCB305-9ECD-4B3C-B13E-4F2790FE0543@gmail.com> (raw)
In-Reply-To: <877cth66qb.fsf@oldenburg.str.redhat.com>

> Am 09.05.2023 um 14:16 schrieb Florian Weimer via Gcc <gcc@gcc.gnu.org>:
> TL;DR: This message is about turning implicit-int,
> implicit-function-declaration, and possibly int-conversion into errors
> for GCC 14.

I suppose the goal is to not need to rely on altering CFLAGS but change the default behavior with still being able to undo this using -Wno-error= or -Wno-?

I think instead of hard-coding a set of changes would it be possible to alter the default setting of diagnostic options during GCC configuration?  And maybe even note that when diagnosing?


> A few of you might remember that I've been looking into turning some
> type errors from warnings into errors by default.  Mainly I've been
> looking at implicit function declarations because in too many cases, the
> synthesized declaration does not match the prototype used at function
> definition and can cause subtle ABI issues.
> To recap, the main challenge is that GCC has to serve disparate groups
> of users: some use GCC for writing programs themselves, but others just
> need GCC to build sources that they have obtained from somewhere.  The
> first group benefits from more type errors because they catch errors
> earlier during development (experience shows that compiler warnings are
> easy to miss in a long build log).  The second group might find these
> errors challenging because the sources they have no longer build.
> To see how large the impact is on that second group, we've mostly
> removed implicit function declarations and implicit ints from Fedora:
>  <https://fedoraproject.org/wiki/Changes/PortingToModernC>
>  <https://fedoraproject.org/wiki/Toolchain/PortingToModernC>
> Roughly 870 packages out of ~14,500 that have GCC present during the
> build needed fixing (or flagging that they can't be built with the
> additional errors), so 6%.  Most of the changes are mechanical in
> nature, like adding additional headers to configure checks.  For about
> ~150 packages, automated patching could be used to rewrite problematic
> built-in checks generated by long-obsolete autoconf versions.
> Some of these changes prevent the compiler behavior for altering the
> build results silently because the new errors changed the outcome of
> autoconf checks.  (We had one of those in libstdc++, changing the ABI on
> GNU/Linux because futex support oculd no longer be detected.)
> Unfortunately, I did not record numbers about them, but think those were
> quite rare; most autoconf problems were also accompanied with other
> problems, or the incorrect results from autoconf led to build failures
> later.  So it seems to me that the risk that the second group mentioned
> above would silently get unexpected build results is fairly low.
> Where possible, we tried to upstream patches, to simplify sharing across
> distributions and to help those who compile upstream sources directly.
> We also benefited from other distributions upstreaming changes along
> similar lines (notably Gentoo for their Clang 16 project, but also from
> Homebrew to a lesser extent).
> An area we started exploring only recently for Fedora is implicit
> conversions between integers and pointers (covered by -Wint-conversion).
> These add another ~170 packages, but some of those are false positives
> (due to our instrumented compiler approach) that do not change the build
> outcome at all.  I'm still negotiating whether we have the capacity to
> develop fixes for these packages proactively.
> I brought up the matter with distributions, and the feedback was neutral
> (not overly negative, as in “this would be the end of the world for
> us”).
>  <https://discourse.nixos.org/t/rfc-more-c-errors-by-default-in-gcc-14/27390>
>  <https://lists.debian.org/debian-gcc/2023/04/msg00015.html>
>  <https://lists.opensuse.org/archives/list/factory@lists.opensuse.org/thread/5OL76NH5AX75WOTZ43O3ZF2JOS3ABBXL/#QZLCA5YPN5CWSS7BCC6TNBC6N7RFTW7J>
> (I tried to contact Arch, but my message didn't make it past the
> moderators, it seems.)
> All in all, the whole situation is not great, but it still seems
> manageable to me.
> Anyway, thanks for reading this far.
> I would like to suggest to turn implicit-int,
> implicit-function-declaration, and possibly int-conversion from warnings
> into errors for GCC 14.  This would give upstream projects roughly
> another year to make new releases with compatibility fixes that have
> been contributed so far.  I do not think waiting longer, until GCC 15,
> would make a meaningful difference because any upstream project that
> does not release within the next 12 months is not likely to release in
> the next 24 months, either.
> Regarding mechanics of the necessary opt out facility, Clang used
> -Werror=… by default, but that seems a bit hackish to me.  Presently, we
> cannot use -std=gnu89 etc. to opt out because there are packages which
> require both C89-only language features and C99-style inlining, which is
> currently not a combination supported by GCC (but maybe that could be
> changed).  Some build systems do not treat CFLAGS and CXXFLAGS as fully
> separate, and a flag approach that works for C and C++ without
> introducing any new warnings would be most convenient.  So maybe we
> could use -fpermissive for C as well.
> One fairly big GCC-internal task is to clear up the C test suite so that
> it passes with the new compiler defaults.  I already have an offer of
> help for that, so I think we can complete this work in a reasonable time
> frame.
> I have no data how big the compatibility impact of turning
> incompatible-pointer-types into errors will be.  For now, int-conversion
> has higher priority.  However, it looks like another change that could
> benefit developers (the first group).
> I do not plan to work specifically on C2X compatibility fixes for now
> (to support bool-as-keyword, for example), but I could imagine a similar
> project a few years from now, fewer than 25 hopefully, that would enable
> GCC to make the switch to C2X by default.
> Thanks,
> Florian

  reply	other threads:[~2023-05-09 15:16 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-05-09 12:15 More C type errors by default for GCC 14 Florian Weimer
2023-05-09 15:16 ` Richard Biener [this message]
2023-05-09 16:05   ` Jakub Jelinek
2023-05-09 16:11     ` Sam James
2023-05-09 16:59   ` Florian Weimer
2023-05-09 17:07     ` Sam James
2023-05-09 17:35       ` Florian Weimer
     [not found] ` <CAGWvnykoEtT0BQtuN70DN6tWohtf=Mm7SW55psZvVmj3OvKUEg@mail.gmail.com>
2023-05-09 15:07   ` Sam James
2023-05-09 15:14   ` Jonathan Wakely
     [not found]     ` <20230509102201.6aa2a7d14fdb2f1e7abff449@killthe.net>
     [not found]       ` <87r0rp5uf8.fsf@aarsen.me>
     [not found]         ` <83ttwla1ep.fsf@gnu.org>
     [not found]           ` <CAH6eHdS-7USHzX3rPJaR66sEdjG+-nHp7mDb0gD7ceBnp+Fmpg@mail.gmail.com>
     [not found]             ` <83lehx9vix.fsf@gnu.org>
     [not found]               ` <ZFqZ25xKm4w4IkqF@tucnak>
     [not found]                 ` <83fs859unu.fsf@gnu.org>
     [not found]                   ` <CAGWvny=xi++Ghyo2Ez5MJdYy5h3yH16gVpviKC99Ufu_bUmdYQ@mail.gmail.com>
     [not found]                     ` <CAH6eHdRAVwBcCmBgiuPyQNWGx3x33diAgQU8UG3s_NrJxK3ucQ@mail.gmail.com>
     [not found]                       ` <CAGWvnyk_MkQOrNNh1C=ubPj8qzg5x-p8CBfgWNqqtDVJ1mbhMg@mail.gmail.com>
     [not found]                         ` <CAF9ehCWsq-N9+U1+mTQahKT7cXEqLTptLb=b3Yq58gsWFEnxgA@mail.gmail.com>
     [not found]                           ` <CAH6eHdS_sQ37VTpJHL7cqtgSdCSL3i4=tqMoY65wep9G56cf5g@mail.gmail.com>
     [not found]                             ` <CAMfHzOsR=gY6QTss2R029Q-uMOTpC-RDV09bEF2FO7x5jpy5gg@mail.gmail.com>
2023-05-10 10:45                               ` Sam James
2023-05-10 10:56                                 ` Neal Gompa
2023-05-10 12:06                                   ` Eli Zaretskii
2023-05-10 12:10                                     ` Neal Gompa
2023-05-10 12:41                                       ` Eli Zaretskii
2023-05-09 18:22   ` Florian Weimer
2023-05-11 21:32     ` Segher Boessenkool
2023-05-12  9:33 ` Martin Jambor
2023-05-12 12:30   ` Jakub Jelinek
2023-05-15 12:46     ` Michael Matz
2023-05-15 13:14     ` Richard Earnshaw (lists)
     [not found] <CAGWvnynMpa83AduLhRhf1PxkVujngDmmwE7Z4vk1z6+UqqAuKA@mail.gmail.com>
     [not found] ` <BBE9950C-28AA-4A1C-A4C5-7F486538004E@gmail.com>
2023-05-09 16:44   ` Florian Weimer
2023-05-09 16:58     ` Ian Lance Taylor
2023-05-09 17:08     ` Jason Merrill
2023-05-09 17:16       ` Sam James

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