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

Nick Desaulniers
2019-03-13T18:11:45Z

On Wed, Mar 13, 2019 at 11:02 AM Nick Desaulniers
<ndesaulniers@google.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@vger.kernel.org
> Reported-by: Nathan Chancellor <natechancellor@gmail.com>
> Reported-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
> Suggested-by: Arnd Bergmann <arnd@arndb.de>
> Suggested-by: James Y Knight <jyknight@google.com>
> Suggested-by: Masahiro Yamada <yamada.masahiro@socionext.com>
> Suggested-by: Nathan Chancellor <natechancellor@gmail.com>
> Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.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