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

[Bernd Petrovitsch]

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).
-- 
"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

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