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=-19.6 required=3.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED, USER_IN_DEF_DKIM_WL autolearn=ham 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 174E1C43381 for ; Wed, 13 Mar 2019 18:11:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D738A21019 for ; Wed, 13 Mar 2019 18:11:44 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="AWsPH1VG" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726649AbfCMSLn (ORCPT ); Wed, 13 Mar 2019 14:11:43 -0400 Received: from mail-pf1-f195.google.com ([209.85.210.195]:37284 "EHLO mail-pf1-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726142AbfCMSLm (ORCPT ); Wed, 13 Mar 2019 14:11:42 -0400 Received: by mail-pf1-f195.google.com with SMTP id s22so1945453pfh.4 for ; Wed, 13 Mar 2019 11:11:42 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=T643AOotQu5+CznGzie/WvJ4IouEOakV1gIrCGRusns=; b=AWsPH1VGoVtomMV6b/wshDYrfx2K+j5wDf1QAFKROe53Tpi9mUg8rXpu6e1PuOoLRF sQGQjwpkDuO7RFy7WrqAY77FWg6QC5642dZ8EYGfIHO5F/WnDktBp7fnonTnqzjmYDG9 OA1YDRrkLN8cwhC07zoaw2VwmdPDuGQb1d09wCytCJRsiVppzm9xCPG79/irp/i/HcOw bA0hKCmbPy8ZP9XMjL/Cn9H88eWOpHejiEBwPH0UTSm07ObXYdrsW1zsR3vTUoDL/Cbn Xo5mHWF8m7IB07V80nJPDfmmOxOFoFkun8N7myq9Zba8kn5Yi/KeG1y6T/iAqkqNGul2 y3jA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=T643AOotQu5+CznGzie/WvJ4IouEOakV1gIrCGRusns=; b=hY8olClNnYHEh78ac5LFpPoFduvizoNTDlxPBcIeuivljw7J9Hpitwpyqwg+DTeX2j pvky17WwuLbzvOXm34etZ7OS6I64XnmdKIAzyYf2jfQg+T3cClNYoqyxAGmoy/uatnd9 UwCLtbEdwYdw5FsOa/nbpTd/1xwD/yWmlD9ghc8fonHYVe5WitnMfXgTzu/h1rBv6XoG iYwzu9K98grn0brP68jUWcIhvW90z+Co1jZiNEXL0Lu8Lype/MilEWeMs9U7MzFAaIk1 HFU1U9u44Gv2yTZep2Hb99vAEPn92LAqwjYujftoTirUuoI7/I+KR8WO2UW+1+itfDTp 7uJA== X-Gm-Message-State: APjAAAV72RHdR/+bei7MiP6sBwXsxJsEVesNnIICGI+FZ4uoP+dY8UD7 1BclAnrsx0wxx5Ez17NslQcgGSVUc4c20nKzcmaxmg== X-Google-Smtp-Source: APXvYqzWf3o2X4lomAdzayBb9eMJ47Y3fc24aOdnCLG1gAcipSwUUxL9RznxQuWWsXaRgPG/7BQgLVNOM5O1Ze/BCj0= X-Received: by 2002:a17:902:8e82:: with SMTP id bg2mr46987986plb.217.1552500701263; Wed, 13 Mar 2019 11:11:41 -0700 (PDT) MIME-Version: 1.0 References: <20190313180239.261938-1-ndesaulniers@google.com> In-Reply-To: <20190313180239.261938-1-ndesaulniers@google.com> From: Nick Desaulniers Date: Wed, 13 Mar 2019 11:11:30 -0700 Message-ID: Subject: Re: [PATCH] lib/string.c: implement a basic bcmp To: Andrew Morton Cc: clang-built-linux@googlegroups.com, Linux Kbuild mailing list , "# 3.4.x" , Nathan Chancellor , Adhemerval Zanella , Arnd Bergmann , James Y Knight , Masahiro Yamada , Rasmus Villemoes , Greg Kroah-Hartman , Alexander Shishkin , Andy Shevchenko , LKML 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 Wed, Mar 13, 2019 at 11:02 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 > --- > 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