linux-kernel - Re: [PATCH v4] lib/string.c: implement a basic bcmp

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Message-Id: <20190320191135.e49e976a8258c8ac0bb428c9@linux-foundation.org>
Date:   Wed, 20 Mar 2019 19:11:35 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     clang-built-linux@...glegroups.com, linux-kbuild@...r.kernel.org,
        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>,
        Steven Rostedt <rostedt@...dmis.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] lib/string.c: implement a basic bcmp

On Wed, 13 Mar 2019 14:13:31 -0700 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.

I guess we should backport this into -stable so that older kernels can
be built with newer Clang.

> ...
>
> --- a/lib/string.c
> +++ b/lib/string.c
> @@ -866,6 +866,26 @@ __visible int memcmp(const void *cs, const void *ct, size_t count)
>  EXPORT_SYMBOL(memcmp);
>  #endif
>  
> +#ifndef __HAVE_ARCH_BCMP
> +/**
> + * bcmp - returns 0 if and only if the buffers have identical contents.
> + * @a: pointer to first buffer.
> + * @b: pointer to second buffer.
> + * @len: size of buffers.
> + *
> + * The sign or magnitude of a non-zero return value has no particular
> + * meaning, and architectures may implement their own more efficient bcmp(). So
> + * while this particular implementation is a simple (tail) call to memcmp, do
> + * not rely on anything but whether the return value is zero or non-zero.
> + */
> +#undef bcmp

What is the undef for?

> +int bcmp(const void *a, const void *b, size_t len)
> +{
> +	return memcmp(a, b, len);
> +}
> +EXPORT_SYMBOL(bcmp);
> +#endif
> +
>  #ifndef __HAVE_ARCH_MEMSCAN
>  /**
>   * memscan - Find a character in an area of memory.
> -- 
> 2.21.0.360.g471c308f928-goog