Re: [PATCH 1/2] Compiler Attributes: add support for __fallthrough (gcc >= 7.1)

[Nick Desaulniers]

On Mon, Oct 22, 2018 at 10:50 AM Bernd Petrovitsch
<bernd@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx> 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