From: Andrew Morton <akpm@linux-foundation.org>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: clang-built-linux@googlegroups.com, linux-kbuild@vger.kernel.org,
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>,
Steven Rostedt <rostedt@goodmis.org>,
Namhyung Kim <namhyung@kernel.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Alexander Shishkin <alexander.shishkin@linux.intel.com>,
Dan Williams <dan.j.williams@intel.com>,
Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4] lib/string.c: implement a basic bcmp
Date: Wed, 20 Mar 2019 19:11:35 -0700 [thread overview]
Message-ID: <20190320191135.e49e976a8258c8ac0bb428c9@linux-foundation.org> (raw)
In-Reply-To: <20190313211335.165605-1-ndesaulniers@google.com>
On Wed, 13 Mar 2019 14:13:31 -0700 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.
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
next prev parent reply other threads:[~2019-03-21 2:11 UTC|newest]
Thread overview: 32+ 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
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 [this message]
2019-03-21 17:02 ` Nick Desaulniers
2019-03-21 17:20 ` Nick Desaulniers
2019-03-21 21:05 ` Andrew Morton
2019-03-22 7:17 ` Rasmus Villemoes
2019-03-22 7:43 ` Sedat Dilek
2019-03-13 19:38 ` [PATCH] Makefile: Add '-fno-builtin-bcmp' to CLANG_FLAGS Arnd Bergmann
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20190320191135.e49e976a8258c8ac0bb428c9@linux-foundation.org \
--to=akpm@linux-foundation.org \
--cc=adhemerval.zanella@linaro.org \
--cc=alexander.shishkin@linux.intel.com \
--cc=andriy.shevchenko@linux.intel.com \
--cc=arnd@arndb.de \
--cc=clang-built-linux@googlegroups.com \
--cc=dan.j.williams@intel.com \
--cc=gregkh@linuxfoundation.org \
--cc=jyknight@google.com \
--cc=linux-kbuild@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux@rasmusvillemoes.dk \
--cc=namhyung@kernel.org \
--cc=natechancellor@gmail.com \
--cc=ndesaulniers@google.com \
--cc=rostedt@goodmis.org \
--cc=stable@vger.kernel.org \
--cc=yamada.masahiro@socionext.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).