From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-13.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED,USER_AGENT_MUTT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 263CBC10F06 for ; Thu, 14 Mar 2019 08:33:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id F23792184C for ; Thu, 14 Mar 2019 08:33:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727340AbfCNId5 (ORCPT ); Thu, 14 Mar 2019 04:33:57 -0400 Received: from mga03.intel.com ([134.134.136.65]:55879 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726629AbfCNId4 (ORCPT ); Thu, 14 Mar 2019 04:33:56 -0400 X-Amp-Result: UNSCANNABLE X-Amp-File-Uploaded: False Received: from fmsmga006.fm.intel.com ([10.253.24.20]) by orsmga103.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 14 Mar 2019 01:33:55 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.58,477,1544515200"; d="scan'208";a="327206014" Received: from smile.fi.intel.com (HELO smile) ([10.237.72.86]) by fmsmga006.fm.intel.com with ESMTP; 14 Mar 2019 01:33:51 -0700 Received: from andy by smile with local (Exim 4.92) (envelope-from ) id 1h4LoD-0002G2-A6; Thu, 14 Mar 2019 10:33:49 +0200 Date: Thu, 14 Mar 2019 10:33:49 +0200 From: Andy Shevchenko To: Nick Desaulniers Cc: akpm@linux-foundation.org, clang-built-linux@googlegroups.com, linux-kbuild@vger.kernel.org, stable@vger.kernel.org, Nathan Chancellor , Adhemerval Zanella , Arnd Bergmann , James Y Knight , Masahiro Yamada , Rasmus Villemoes , Steven Rostedt , Namhyung Kim , Greg Kroah-Hartman , Alexander Shishkin , Dan Williams , linux-kernel@vger.kernel.org Subject: Re: [PATCH v4] lib/string.c: implement a basic bcmp Message-ID: <20190314083349.GT9224@smile.fi.intel.com> References: <7549EE7E-4172-467D-815A-63664A33D410@goodmis.org> <20190313211335.165605-1-ndesaulniers@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190313211335.165605-1-ndesaulniers@google.com> Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. Thanks! Reviewed-by: Andy Shevchenko > > 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 > Reported-by: Adhemerval Zanella > Suggested-by: Arnd Bergmann > Suggested-by: James Y Knight > Suggested-by: Masahiro Yamada > Suggested-by: Nathan Chancellor > Suggested-by: Rasmus Villemoes > Signed-off-by: Nick Desaulniers > Acked-by: Steven Rostedt (VMware) > --- > 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 > -- With Best Regards, Andy Shevchenko