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

[Miguel Ojeda]

On Mon, Oct 22, 2018 at 7:50 PM 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.

I think Dan meant too simply not touch anything (i.e. not losing the
warning anywhere).

>
> 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).
>

Hm... that means they don't support (by default) the same regexps as
GCC; which is bad: it means that it is only equivalent to the most
relaxed level in gcc, 1:

    """
    -Wimplicit-fallthrough=1 matches .* regular expression, any
comment is used as fallthrough comment.
    """

    See https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html

i.e. any other level above will either not match Eclipse or not match gcc.

> >> 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.

It shouldn't according to the standard -- but who knows... :-)

> - 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 understanding [[fallthrough]] is very good, actually.

>   Eclipse "accepts" [[fallthrough]] only in C++ sources (and not in C
>   sources).

Bad, but I guess they will add it to C eventually, since it is
probably coming for C2x.

> - Neither gcc nor clang understand [[fallthrough]] (so it's probably a
>   no-go for the Kernel with C89 anyways).

clang does it if you enable -fdouble-square-bracket-attributes (please
see my other messages). gcc will at some point (if C2x gets the
attributes), but at the moment the C parser is different than the C++
parser and there is no support for it on trunk that I could see, so
they will have to copy the support; i.e. it will take a bit more time
than clang, likely.

Thanks a *lot* for taking a look at Eclipse!

Cheers,
Miguel