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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Wed, 23 Feb 2022 21:48:59 +0100
Message-ID: <CAK8P3a2b_RtXkhQ2pwqbZ1zz6QtjaWwD4em_MCF_wGXRwZirKA@mail.gmail.com>
Subject: Re: [RFC PATCH 03/13] usb: remove the usage of the list iterator
 after the loop
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Arnd Bergmann <arnd@arndb.de>, Jakob <jakobkoschel@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        Mike Rapoport <rppt@kernel.org>,
        "Gustavo A. R. Silva" <gustavo@embeddedor.com>,
        Brian Johannesmeyer <bjohannesmeyer@gmail.com>,
        Cristiano Giuffrida <c.giuffrida@vu.nl>,
        "Bos, H.J." <h.j.bos@vu.nl>, Nathan Chancellor <nathan@kernel.org>
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On Wed, Feb 23, 2022 at 9:43 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Wed, Feb 23, 2022 at 12:25 PM Arnd Bergmann <arnd@arndb.de> wrote:
> >
> > I looked at the gcc documentation for this flag, and it tells me that
> > it's default-enabled for std=c99 or higher. Turning it on for --std=gnu89
> > shows the same warning, so at least it doesn't sound like the actual
> > behavior changed, only the warning output. clang does not warn
> > for this code at all, regardless of the --std= flag.
>
> Ok, so we should be able to basically convert '--std=gnu89' into
> '--std=gnu11 -Wno-shift-negative-value' with no expected change of
> behavior.

Yes, I think that is correct.

> Of course, maybe we need to make -Wno-shift-negative-value be
> conditional on the compiler supporting it in the first place?

I think they all do. I discussed this with Nathan Chancellor on IRC, to
see what clang does, and he points out that the warning was made
conditional on -fwrapv there a while ago:

https://github.com/llvm/llvm-project/commit/59802321785b4b9fc31b10456c62ba3a06d3a631

So the normal behavior on clang is to always warn about it, but since
we explicitly ask for sane signed integer behavior, it doesn't warn for
the kernel.

        Arnd