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Date:   Mon, 22 Oct 2018 10:54:02 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     bernd@...rovitsch.priv.at
Cc:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        dan.carpenter@...cle.com, "Theodore Ts'o" <tytso@....edu>,
        Greg KH <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>, adilger.kernel@...ger.ca,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>, rostedt@...dmis.org,
        mchehab+samsung@...nel.org, olof@...m.net,
        Konstantin Ryabitsev <konstantin@...uxfoundation.org>,
        "David S. 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@...ux.vnet.ibm.com,
        Andrey Konovalov <andreyknvl@...gle.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Will Deacon <will.deacon@....com>,
        Philippe Ombredanne <pombredanne@...b.com>,
        paul.burton@...s.com, David Rientjes <rientjes@...gle.com>,
        Willy Tarreau <w@....eu>, msebor@...il.com, sparse@...isli.org,
        Jonathan Corbet <corbet@....net>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>, joe@...ches.com,
        Arnd Bergmann <arnd@...db.de>, asmadeus@...ewreck.org,
        Stefan Agner <stefan@...er.ch>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-doc@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-sparse@...r.kernel.org,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>
Subject: Re: [PATCH 1/2] Compiler Attributes: add support for __fallthrough
 (gcc >= 7.1)

On Mon, Oct 22, 2018 at 10:50 AM Bernd Petrovitsch
<bernd@...rovitsch.priv.at> wrote:
>
> Hi all!
>
> On 22/10/18 13:07, Miguel Ojeda wrote:
> > On Mon, Oct 22, 2018 at 12:54 PM Dan Carpenter <dan.carpenter@...cle.com> wrote:
> >>
> >> Doing both is super ugly.  Let's just do comments until Eclipse gets
> >> updated.
>
> Yes, "Eclipse" as the IDE.
>
> And yes but IMHO better super ugly than loosing the warning - YMMV.
>
> For the archives: I have Eclipse Photon/June 2016 here. And "no break"
> is the (default) string in a comment used by Eclipse (it can be
> customized and is actually a regexp but it must be in a comment).
>
> >> I had wanted to move to the attribute because that would simplify things
> >> in Smatch but it's not a huge deal to delay for another year.
> >
> > I can re-send them later on, no problem. On the other hand, doing the
> > changes will push tools to get updated sooner ;-)
> >
> > If tools were doing something as fancy as comment parsing for
> > diagnostics, they should have been updated with the attribute support
> > (either gcc's or C++17's) -- it has been more than a year now since
> > gcc 7.1 and the C++17 final draft. (Note that this does not apply for
> > things like clang, since they weren't doing comment parsing to begin
> > with.)
>
> That would be nice. And if they agree on the same texts (or accept per
> default all somewhat widely used and/or old ones).
>
> After stumbling over
> https://stackoverflow.com/questions/16935935/how-do-i-turn-off-a-static-code-analysis-warning-on-a-line-by-line-warning-in-cd,
> looking into Eclipses Window -> Preferences -> C/C++ -> Code Analysis ->
> "No break at the end of case" screen (that's the screenshot there) and I
> tried various things:
>
> Preface:
> I have
> ----  snip  ----
> #define __fallthrough __attribute__((fallthrough))
> ----  snip  ----
> for gcc >= 7 (because clang doesn't know it and I had also older
> gcc's in use before).
>
> So:
> - Adding a comment to the #define doesn't change anything for Eclipse.
> - Eclipse looks *only* in comments for the string/regexp given
>   the warnings configuration (and that comment must be on the line
>   directly before the "case").
> - Eclipse understands [[fallthrough]] out-of-the-box though (which
>   is C++11 AFAIK) as does g++-7 (I use -std=gnu++17 - most of the
>   sources are C++, but not all) and clang++-6 (all the current standard
>   Ubuntu-18.06/Bionic packages).
>   Eclipse "accepts" [[fallthrough]] only in C++ sources (and not in C
>   sources).
> - Neither gcc nor clang understand [[fallthrough]] (so it's probably a
>   no-go for the Kernel with C89 anyways).
>
> MfG,
>         Bernd
>
> PS: clang++ errors with "fallthrough annotation in unreachable code" if
>     [[fallthrough]] is after an assert(). clang-devs there, please, the
>     fallthrough doesn't really generated code (I hope;-).
>     I have lots of switch()es which catch undefined values (for enums
>     et. al.) with "default"+assert() and fall through to the most safe
>     case (for the deployed version).

Can you send me a link to a simple reproducer in godbolt (godbolt.org)
and we'll take a look?

> --
> "I dislike type abstraction if it has no real reason. And saving
> on typing is not a good reason - if your typing speed is the main
> issue when you're coding, you're doing something seriously wrong."
>     - Linus Torvalds



-- 
Thanks,
~Nick Desaulniers

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