linux-kernel - Re: [PATCH] lib/string.c: implement a basic bcmp

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Message-ID: <CAKwvOdmdEH1U9vOmVKo2Fn+hMPF9AB1enZ2rkpJcU9XHgR8Wng@mail.gmail.com>
Date:   Wed, 13 Mar 2019 11:11:30 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     clang-built-linux@...glegroups.com,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        "# 3.4.x" <stable@...r.kernel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Adhemerval Zanella <adhemerval.zanella@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        James Y Knight <jyknight@...gle.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lib/string.c: implement a basic bcmp

On Wed, Mar 13, 2019 at 11:02 AM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> A recent optimization in Clang (r355672) lowers comparisons of the
> return value of memcmp against zero to comparisons of the return value
> of bcmp against zero.  This helps some platforms that implement bcmp
> more efficiently than memcmp. glibc simply aliases bcmp to memcmp, but
> an optimized implementation is in the works.
>
> This results in linkage failures for all targets with Clang due to the
> undefined symbol.  For now, just implement bcmp as a tailcail to memcmp
> to unbreak the build.  This routine can be further optimized in the
> future.
>
> Other ideas discussed:
> * A weak alias was discussed, but breaks for architectures that define
> their own implementations of memcmp since aliases to declarations are
> not permitted (only definitions).  Arch-specific memcmp implementations
> typically declare memcmp in C headers, but implement them in assembly.
> * -ffreestanding also is used sporadically throughout the kernel.
> * -fno-builtin-bcmp doesn't work when doing LTO.
>
> Link: https://bugs.llvm.org/show_bug.cgi?id=41035
> Link: https://code.woboq.org/userspace/glibc/string/memcmp.c.html#bcmp
> Link: https://github.com/llvm/llvm-project/commit/8e16d73346f8091461319a7dfc4ddd18eedcff13
> Link: https://github.com/ClangBuiltLinux/linux/issues/416
> Cc: stable@...r.kernel.org
> Reported-by: Nathan Chancellor <natechancellor@...il.com>
> Reported-by: Adhemerval Zanella <adhemerval.zanella@...aro.org>
> Suggested-by: Arnd Bergmann <arnd@...db.de>
> Suggested-by: James Y Knight <jyknight@...gle.com>
> Suggested-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
> Suggested-by: Nathan Chancellor <natechancellor@...il.com>
> Suggested-by: Rasmus Villemoes <linux@...musvillemoes.dk>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> ---
>  lib/string.c | 15 +++++++++++++++

ah, looks like I should provide a declaration in
include/linux/string.h, too, in case others within the kernel would
like to use bcmp.  WIll send a v2.

>  1 file changed, 15 insertions(+)
>
> diff --git a/lib/string.c b/lib/string.c
> index 38e4ca08e757..5b2377d7143f 100644
> --- a/lib/string.c
> +++ b/lib/string.c
> @@ -866,6 +866,21 @@ __visible int memcmp(const void *cs, const void *ct, size_t count)
>  EXPORT_SYMBOL(memcmp);
>  #endif
>
> +#ifndef __HAVE_ARCH_BCMP
> +/**
> + * bcmp - Like memcmp but the return code simply indicates a non-match.
> + * @cs: One area of memory.
> + * @ct: Another area of memory.
> + * @count: The size of the areas.
> + */
> +#undef bcmp
> +int bcmp(const void *cs, const void *ct, size_t count)
> +{
> +       return memcmp(cs, ct, count);
> +}
> +EXPORT_SYMBOL(bcmp);
> +#endif
> +
>  #ifndef __HAVE_ARCH_MEMSCAN
>  /**
>   * memscan - Find a character in an area of memory.
> --
> 2.21.0.360.g471c308f928-goog
>


-- 
Thanks,
~Nick Desaulniers