linux-ext4 - Re: [PATCH 1/2] Compiler Attributes: add support for _

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Message-ID: <CANiq72k2z78uae1GWjECo+OB7GC90b8irt0SHYtnSXT8F2Grrw@mail.gmail.com>
Date:   Mon, 22 Oct 2018 23:17:15 +0200
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Dan <dan.carpenter@...cle.com>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Olof Johansson <olof@...m.net>,
        Konstantin Ryabitsev <konstantin@...uxfoundation.org>,
        David Miller <davem@...emloft.net>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Paul Lawrence <paullawrence@...gle.com>,
        Sandipan Das <sandipan@...ux.vnet.ibm.com>,
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        Philippe Ombredanne <pombredanne@...b.com>,
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        David Rientjes <rientjes@...gle.com>, Willy Tarreau <w@....eu>,
        Martin Sebor <msebor@...il.com>,
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        Stefan Agner <stefan@...er.ch>,
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Subject: Re: [PATCH 1/2] Compiler Attributes: add support for __fallthrough
 (gcc >= 7.1)

On Mon, Oct 22, 2018 at 7:36 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> On Sun, Oct 21, 2018 at 10:14 AM Miguel Ojeda
> <miguel.ojeda.sandonis@...il.com> wrote:
> >
> > of its kind) to deal with this: [[fallthrough]] is meant to be
> > a new control keyword in the form of an extension.
>
> I think we can leave the details about the [[]] notation out. IIUC,
> it's only applicable to C++.

No, because C++ is the driving force for the standard attributes
syntax; i.e. C2x is adding them because of the syntax published by
C++17; and possibly gcc 7.1 added support for fallthrough (and comment
parsing) due to C++17 too.

Basically, it is a small paragraph explaining how this came to be.

> > +#if __has_attribute(__fallthrough__)
> > +# define __fallthrough                  __attribute__((__fallthrough__))
> > +#else
> > +# define __fallthrough
>
> While this is in the correct format as the other attributes in this
> file, I think this particular attribute is a special case, because of
> the variety of fallbacks and differing support for them.  I'd like to

No, is it the correct format because we cannot add support for the
other syntax in gcc; so the best way to proceed is to simply wait
until clang supports the GNU attribute in C mode.

The tooling, of course, is another matter and independent of this.

> see in the commit message maybe a list of tools we'd like to support

Yes, I already said I would write it in one of the other threads.

> and links to the feature requests/bug reports for them.  I acknowledge
> it's more work to file bugs, but it's being a good open source
> citizen, IMO.

Who said we aren't going to do it? :-)

I was actually in the process of checking which OSS tools supported
what and how easy it was to fix in each of them (gcc's [[...]] syntax,
clang's GNU and C++ attrs in C mode, cppcheck's fallthrough
support...), but it takes time; I prefer to do the research
beforehand; so that the submitted bug reports are better/more
precise/more helpful, etc.

However, you already sent the LLVM report (without mentioning this
thread or me, nor the -fdouble-square-bracket-attributes clang flag
that I mentioned). That is a bit rude :-) Please take things a little
easier, there is no need to rush stuff. If I didn't have submitted the
LLVM bug report is because I hadn't finish looking at the issue. In
general, I think it is polite (and also more productive to avoid
duplicating efforts) to first ask whoever is working on something
before you rush to do it...

>
> I'm also curious which is the first version of GCC to support the
> comment format?

gcc 7.1 started everything. It is stated in the commit message and
several messages/threads already. Again, please pause, relax and read
a bit before sending stuff around :-)

Cheers,
Miguel