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=-12.0 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED 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 51739C4360F for ; Thu, 14 Mar 2019 05:01:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 187EA217F5 for ; Thu, 14 Mar 2019 05:01:24 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=nifty.com header.i=@nifty.com header.b="ZA8xNhq7" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727022AbfCNFBW (ORCPT ); Thu, 14 Mar 2019 01:01:22 -0400 Received: from conssluserg-02.nifty.com ([210.131.2.81]:24677 "EHLO conssluserg-02.nifty.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726204AbfCNFBV (ORCPT ); Thu, 14 Mar 2019 01:01:21 -0400 X-Greylist: delayed 64920 seconds by postgrey-1.27 at vger.kernel.org; Thu, 14 Mar 2019 01:01:19 EDT Received: from mail-vk1-f180.google.com (mail-vk1-f180.google.com [209.85.221.180]) (authenticated) by conssluserg-02.nifty.com with ESMTP id x2E516tS028942; Thu, 14 Mar 2019 14:01:07 +0900 DKIM-Filter: OpenDKIM Filter v2.10.3 conssluserg-02.nifty.com x2E516tS028942 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=nifty.com; s=dec2015msa; t=1552539667; bh=Uo4E4mUuPURHNRf7HqEPeFupT8psefdD/4LAQaEeYgg=; h=References:In-Reply-To:From:Date:Subject:To:Cc:From; b=ZA8xNhq7E8zd3cIGx6zNmYa+Uu2wFjFQ3wrwWPhG4NGhcadgli1YjWh4HVU8RzX0W H5fOCJQOFICrQT8nh4hoYCpfxVwxuXabHriR3ZMZjn0gWr7lZo/o6AerBBhftTT729 XNftp/yAmRejUIWPeVepNYpvS4vrH4FHTFV5XK9q6hK3SfAxae6yw/nEo0VHo9Tyu3 IptNQZ3sdndV0unCThGMVJX89Y/IR5h48flFrwpursnKUtFI3o9Vb070LQ6GZ4w380 1BQtyCB9Td6vpURZ1Q248A+bSGTvFbyVxaek0KeXFFwv0ZYU9eL9i8z9k4QXrXBCGY JFH9rDRLlEevw== X-Nifty-SrcIP: [209.85.221.180] Received: by mail-vk1-f180.google.com with SMTP id r189so1076219vkb.0; Wed, 13 Mar 2019 22:01:07 -0700 (PDT) X-Gm-Message-State: APjAAAUhWDDC+gPX2zPffYM/NHpVCBI9EQ61Br3jNF65a6u3FSnhab0R pYK4hjledI7wvWLGDXDOGFgQKDRDZKETCdDHr/c= X-Google-Smtp-Source: APXvYqw4fUUMOGM6rJhNqJIc3smfephevtXH7cZOhpfLHdO5pQP3EdMjch9ysVSuxxAs1v4mdZRUlA4D8BznnmwslPM= X-Received: by 2002:a1f:ae07:: with SMTP id x7mr2779005vke.74.1552539666061; Wed, 13 Mar 2019 22:01:06 -0700 (PDT) MIME-Version: 1.0 References: <7549EE7E-4172-467D-815A-63664A33D410@goodmis.org> <20190313211335.165605-1-ndesaulniers@google.com> In-Reply-To: <20190313211335.165605-1-ndesaulniers@google.com> From: Masahiro Yamada Date: Thu, 14 Mar 2019 14:00:29 +0900 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v4] lib/string.c: implement a basic bcmp To: Nick Desaulniers Cc: Andrew Morton , clang-built-linux@googlegroups.com, Linux Kbuild mailing list , stable , Nathan Chancellor , Adhemerval Zanella , Arnd Bergmann , James Y Knight , Rasmus Villemoes , Steven Rostedt , Namhyung Kim , Greg Kroah-Hartman , Alexander Shishkin , Dan Williams , Andy Shevchenko , Linux Kernel Mailing List Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 14, 2019 at 6:13 AM 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@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) Reviewed-by: Masahiro Yamada > --- > 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 > -- Best Regards Masahiro Yamada