linux-kernel - Re: [PATCH] kbuild: compute false-positive -Wmaybe-uninitialized cases in Kconfig

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Date:   Thu, 21 Feb 2019 18:45:05 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] kbuild: compute false-positive -Wmaybe-uninitialized
 cases in Kconfig

On Thu, Feb 21, 2019 at 6:23 PM Arnd Bergmann <arnd@...db.de> wrote:
>
> On Thu, Feb 21, 2019 at 5:14 AM Masahiro Yamada
> <yamada.masahiro@...ionext.com> wrote:
> >
> > Since -Wmaybe-uninitialized was introduced by GCC 4.7, we have patched
> > various false positives:
> >
> >  - commit e74fc973b6e5 ("Turn off -Wmaybe-uninitialized when building
> >    with -Os") turned off this option for -Os.
> >
> >  - commit 815eb71e7149 ("Kbuild: disable 'maybe-uninitialized' warning
> >    for CONFIG_PROFILE_ALL_BRANCHES") turned off this option for
> >    CONFIG_PROFILE_ALL_BRANCHES
> >
> >  - commit a76bcf557ef4 ("Kbuild: enable -Wmaybe-uninitialized warning
> >    for "make W=1"") turned off this option for GCC < 4.9
> >    Arnd provided more explanation in https://lkml.org/lkml/2017/3/14/903
> >
> > I think this looks better by shifting the logic from Makefile to Kconfig.
> >
> > Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
>
> I like how this cleans up the logic and documents it better.
> I've had my own plans to move this (and most other conditional warning
> options) into a header file using the KBUILD_WARN() infrastructure, but
> never got around to doing that. I still think that's what we want eventually,
> but in the meantime your patch seems fine as well.
>
> Also, I see that your patch keeps the existing logic, which is good,
> but we may want to refine this a little afterwards:
>
> - we may want to turn off -Wuninitialized for gcc-4.6 and gcc-4.7
>   instead of turning off -Wmaybe-uninitialized as we do in the later
>   compilers.

Ah, right.

I can do that if it is worthwhile.

But, I'd rather want to raise the minimum compiler version
than taking care of old compilers.
(We will do it eventually.)


> - the proposed CC_OPTIMIZE_FOR_DEBUGGING may need the
>   same workaround as CC_OPTIMZE_FOR_SIZE.


I know this.

First, my patch took care of CC_OPTIMIZE_FOR_DEBUGGING as well.
But, CC_OPTIMIZE_FOR_DEBUGGING was rejected.
(https://patchwork.kernel.org/patch/10744541/)

So, I dropped this.


> - For clang, we may want to /not/ turn off -Wmaybe-uninitialized
>   in all cases. I haven't tried build with with 'clang -Oz
>   -Wmaybe-uninitialized', but if the number of false positives is small
>   enough, we could decide to just leave that on.


Does clang recognize -Wmaybe-uninitialized?

I am using Clang I built a few days ago.


masahiro@pug:~$ clang --version
clang version 9.0.0 (http://llvm.org/git/clang.git
b492883ea31e2046d5725126318911465bc1374f)
(http://llvm.org/git/llvm.git
03a15eec9ef44798cd5c57b4ed6a63e62b2c1db6)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /home/masahiro/toolchains/clang-latest/bin

masahiro@pug:~$ clang -x c /dev/null  -Wmaybe-uninitialized -c -o  /dev/null
warning: unknown warning option '-Wmaybe-uninitialized'; did you mean
      '-Wuninitialized'? [-Wunknown-warning-option]
1 warning generated.




>       Arnd



-- 
Best Regards
Masahiro Yamada