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

From: Miguel Ojeda
Date: Mon Oct 22 2018 - 17:34:33 EST

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.


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
> 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!