From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 66083 invoked by alias); 29 Nov 2019 10:58:07 -0000 Mailing-List: contact libc-alpha-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: libc-alpha-owner@sourceware.org Received: (qmail 66045 invoked by uid 89); 29 Nov 2019 10:58:07 -0000 Authentication-Results: sourceware.org; auth=none X-Spam-SWARE-Status: No, score=-4.7 required=5.0 tests=AWL,BAYES_00 autolearn=ham version=3.3.1 spammy=tough X-HELO: us-smtp-delivery-1.mimecast.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1575025084; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=3rwGvYY2NLxZEksX9LUzOM2iyAPnZ3AR+6zxrdylrM0=; b=VgyQ44sunzWnu/E1YoheJEjWuZOmdkvSl/4jv7B6v7696OXhp5zjbGg4bmvOhEy4Wl5p+T evk7qJ2thECmo8Xjkgo8UAb+Yx+7Iq/u9FieF4F5sXSQpBt/WMfrJGnRw2AebNh5SHLX3I poumYZxC5l2jwyxAvEDzF7CPdlL5kcQ= Date: Fri, 29 Nov 2019 10:58:00 -0000 From: Jonathan Wakely To: Florian Weimer Cc: kamlesh kumar , libc-alpha@sourceware.org, Andreas Schwab , Brooks Moses Subject: Re: [PATCH] For Adding clang check Message-ID: <20191129105759.GX11522@redhat.com> References: <20191128184733.27377-1-kamleshbhalui@gmail.com> <87tv6n7s0t.fsf@mid.deneb.enyo.de> <87a78fay23.fsf@mid.deneb.enyo.de> MIME-Version: 1.0 In-Reply-To: <87a78fay23.fsf@mid.deneb.enyo.de> X-Clacks-Overhead: GNU Terry Pratchett User-Agent: Mutt/1.12.1 (2019-06-15) X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: quoted-printable Content-Disposition: inline X-SW-Source: 2019-11/txt/msg01015.txt.bz2 On 29/11/19 09:25 +0100, Florian Weimer wrote: >* kamlesh kumar: > >> It fixes this. >> https://bugs.llvm.org/show_bug.cgi?id=3D44169 > >What's the rationale for the condition? What's special about Clang >3.5? > >My understanding is that a compiler needs support for asm aliases >*and* the C++ library headers need to be compatible. Is there a way >to determine if libc++ is compatible? Libc++ already checks for this macro in its wrapper: https://github.com/llvm/llvm-project/blob/master/libcxx/include/string.h#L62 If the __CORRECT_ISO_CPP_STRING_H_PROTO macro is *not* defined after doing #include_next (to get the libc header) then libc++ makes use of a Clang extension to declare new overloads: https://github.com/llvm/llvm-project/blob/master/libcxx/include/string.h#L70 The _LIBCPP_PREFERRED_OVERLOAD macro is defined as: # define _LIBCPP_PREFERRED_OVERLOAD __attribute__ ((__enable_if__(true, = ""))) That Clang-only attribute means the compiler will use the new overloads in preference to the libc strchr. A non-standard extension is needed because according to the C++ rules the new overloads should be ambiguous with the one that was declared by libc's . >For libstdc++ with GCC, the >compiler version check covers libstdc++ implicity, but that does not >apply to Clang, or libc++ with either compiler. Libstdc++ with GCC already works. Libstdc++ with Clang needs this patch. If I'm reading the libc++ code right ... Libc++ with GCC should already work, because the __GNUC_PREREQ will pass and libc++ is already aware of the existence and effects of the __CORRECT_ISO_CPP_STRING_H_PROTO macro. (I doubt libc++ works with ancient GCC versions, but if it does they'll get the wrong signatures ... well tough luck, use a newer GCC). Libc++ with Clang doesn't need this patch, because it uses the Clang extension, but after this patch it would no longer need to use the extension. The right signatures would be declared by glibc. So of the four combinations, two already work and are unaffected by this patch. One already works and is affected, but not in a way users will notice (the correct signatures are already there, the patch just changes whether they come from glibc or libc++). And one doesn't currently work but is fixed by the patch. I think the patch is right.