linux-kernel - Re: [PATCH] Makefile: Add '-fno-builtin-bcmp' to CLANG

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Message-ID: <CAK8P3a344k=MDAX-d21U4+H+mv7Vcd9YhAN4NPEJm1CXv76tkw@mail.gmail.com>
Date:   Wed, 13 Mar 2019 14:48:49 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        James Y Knight <jyknight@...gle.com>,
        clang-built-linux@...glegroups.com,
        "# 3.4.x" <stable@...r.kernel.org>
Subject: Re: [PATCH] Makefile: Add '-fno-builtin-bcmp' to CLANG_FLAGS

On Wed, Mar 13, 2019 at 2:44 PM Nathan Chancellor
<natechancellor@...il.com> wrote:
> On Wed, Mar 13, 2019 at 09:13:11AM +0100, Rasmus Villemoes wrote:
> > Wouldn't it be better to just define bcmp as an alias for memcmp? They
> > seem to have compatible prototypes, and then somebody might someday sit
> > down and implement some word-at-a-time version of bcmp making use of the
> > weaker guarantees about the return value to gain some performance. But I
> > suppose that can also be done later.
>
> Thank you much for the review, I didn't even realize this was possible :)
>
> I'd certainly like to explore it as that is what glibc does. How would
> you suggest going about it here?

I suggested a possible implementation (likely contains bugs) and
an alias for architectures that require strict alignment, see
https://bugs.llvm.org/show_bug.cgi?id=41035#c11

We could start out with just the alias.

       Arnd