From: Nick Desaulniers <ndesaulniers@google.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: clang-built-linux@googlegroups.com,
Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
"# 3.4.x" <stable@vger.kernel.org>,
Nathan Chancellor <natechancellor@gmail.com>,
Adhemerval Zanella <adhemerval.zanella@linaro.org>,
Arnd Bergmann <arnd@arndb.de>,
James Y Knight <jyknight@google.com>,
Masahiro Yamada <yamada.masahiro@socionext.com>,
Rasmus Villemoes <linux@rasmusvillemoes.dk>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Alexander Shishkin <alexander.shishkin@linux.intel.com>,
Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] lib/string.c: implement a basic bcmp
Date: Wed, 13 Mar 2019 11:11:30 -0700 [thread overview]
Message-ID: <CAKwvOdmdEH1U9vOmVKo2Fn+hMPF9AB1enZ2rkpJcU9XHgR8Wng@mail.gmail.com> (raw)
In-Reply-To: <20190313180239.261938-1-ndesaulniers@google.com>
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
next prev parent reply other threads:[~2019-03-13 18:11 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-12 21:52 [PATCH] Makefile: Add '-fno-builtin-bcmp' to CLANG_FLAGS Nathan Chancellor
2019-03-12 22:55 ` Nick Desaulniers
2019-03-13 8:13 ` Rasmus Villemoes
2019-03-13 10:58 ` Masahiro Yamada
2019-03-13 13:44 ` Nathan Chancellor
2019-03-13 13:48 ` Arnd Bergmann
2019-03-13 15:32 ` Nathan Chancellor
2019-03-13 17:21 ` Nick Desaulniers
2019-03-13 17:27 ` Nick Desaulniers
2019-03-13 18:02 ` [PATCH] lib/string.c: implement a basic bcmp Nick Desaulniers
2019-03-13 18:11 ` Nick Desaulniers [this message]
2019-03-13 18:17 ` [PATCH v2] " Nick Desaulniers
2019-03-13 18:40 ` Steven Rostedt
2019-03-13 18:51 ` Nick Desaulniers
2019-03-13 19:01 ` Steven Rostedt
2019-03-13 19:34 ` Rasmus Villemoes
2019-03-13 20:12 ` Steven Rostedt
2019-03-13 20:37 ` [PATCH v3] " Nick Desaulniers
2019-03-13 21:03 ` Steven Rostedt
2019-03-13 21:13 ` [PATCH v4] " Nick Desaulniers
2019-03-14 3:15 ` Nathan Chancellor
2019-03-14 5:00 ` Masahiro Yamada
2019-03-14 8:33 ` Andy Shevchenko
2019-03-14 9:57 ` David Laight
2019-03-14 11:07 ` Rasmus Villemoes
2019-03-21 2:11 ` Andrew Morton
2019-03-21 17:02 ` Nick Desaulniers
2019-03-21 17:20 ` Nick Desaulniers
2019-03-21 21:05 ` Andrew Morton
2019-03-13 19:38 ` [PATCH] Makefile: Add '-fno-builtin-bcmp' to CLANG_FLAGS Arnd Bergmann
[not found] ` <20190325003834.2F24E2133F@mail.kernel.org>
2019-03-25 14:02 ` Nathan Chancellor
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