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

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Date:   Wed, 13 Mar 2019 20:15:30 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     akpm@...ux-foundation.org, clang-built-linux@...glegroups.com,
        linux-kbuild@...r.kernel.org, stable@...r.kernel.org,
        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, Mar 13, 2019 at 02:13:31PM -0700, Nick Desaulniers 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>
> Acked-by: Steven Rostedt (VMware) <rostedt@...dmis.org>

Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
Tested-by: Nathan Chancellor <natechancellor@...il.com>

Thanks for fixing this better than I did (or tried to)!

> ---
> Changes V3 -> V4:
> * Include the entirety of Rasmus' sugguestion, as per Steven.
> * Change formal parameter identifiers to match the comment.
> Changes V2 -> V3:
> * Adjust comment as per Steven to Rasmus' sugguestion.
> * Pick up Steven's Ack.
> Changes V1 -> V2:
> * Add declaration to include/linux/string.h.
> * Reword comment above bcmp.
> 
>  include/linux/string.h |  3 +++
>  lib/string.c           | 20 ++++++++++++++++++++
>  2 files changed, 23 insertions(+)
> 
> diff --git a/include/linux/string.h b/include/linux/string.h
> index 7927b875f80c..6ab0a6fa512e 100644
> --- a/include/linux/string.h
> +++ b/include/linux/string.h
> @@ -150,6 +150,9 @@ extern void * memscan(void *,int,__kernel_size_t);
>  #ifndef __HAVE_ARCH_MEMCMP
>  extern int memcmp(const void *,const void *,__kernel_size_t);
>  #endif
> +#ifndef __HAVE_ARCH_BCMP
> +extern int bcmp(const void *,const void *,__kernel_size_t);
> +#endif
>  #ifndef __HAVE_ARCH_MEMCHR
>  extern void * memchr(const void *,int,__kernel_size_t);
>  #endif
> diff --git a/lib/string.c b/lib/string.c
> index 38e4ca08e757..3ab861c1a857 100644
> --- 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
> +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
>