From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by sourceware.org (Postfix) with ESMTP id B99FA3858405 for ; Wed, 30 Mar 2022 12:13:48 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org B99FA3858405 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 695CF23A; Wed, 30 Mar 2022 05:13:48 -0700 (PDT) Received: from [10.57.40.209] (unknown [10.57.40.209]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id E7EA83F7D8; Wed, 30 Mar 2022 05:13:47 -0700 (PDT) Message-ID: Date: Wed, 30 Mar 2022 13:13:46 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.5.0 Subject: Re: [wwwdocs] Document zero width bit-field passing ABI changes in gcc-12/changes.html [PR104796] Content-Language: en-GB From: Richard Earnshaw To: Jakub Jelinek , gcc-patches@gcc.gnu.org References: In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Spam-Status: No, score=-3495.6 required=5.0 tests=BAYES_00, BODY_8BITS, GIT_PATCH_0, KAM_DMARC_STATUS, KAM_LAZY_DOMAIN_SECURITY, KAM_SHORT, NICE_REPLY_A, SPF_HELO_NONE, SPF_NONE, TXREP, T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.4 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on server2.sourceware.org X-BeenThere: gcc-patches@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc-patches mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 30 Mar 2022 12:13:50 -0000 Never mind, just spotted it. :) On 30/03/2022 13:10, Richard Earnshaw wrote: > Doesn't this need the anchor that the compiler links to? > #zero_width_bitfields > > R. > > On 30/03/2022 11:07, Jakub Jelinek via Gcc-patches wrote: >> Hi! >> >> This patch documents the PR102024 ABI changes. >> The x86-64, ARM and AArch64 backends refer to this in their -Wpsabi >> diagnostics. >> Ok for wwwdocs? >> >> diff --git a/htdocs/gcc-12/changes.html b/htdocs/gcc-12/changes.html >> index 689feeba..dc0e4074 100644 >> --- a/htdocs/gcc-12/changes.html >> +++ b/htdocs/gcc-12/changes.html >> @@ -28,6 +28,31 @@ a work-in-progress.

>>   >>  

Caveats

>>  
    >> + 
  • >> +    An ABI incompatibility between >> C and >> +    C++ when passing or returning by value certain aggregates with zero >> +    width bit-fields has been discovered on various targets. >> +    As mentioned in PR102024, >> +    since the PR42217 fix in >> +    GCC 4.5 the C++ front-end has been removing zero width bit-fields >> +    from the internal representation of the aggregates after the >> layout of those >> +    aggregates, but the C front-end kept them, so passing e.g. >> +    struct S { float a; int : 0; float b; } or >> +    struct T { float c; int : 0; } by value could differ >> +    between C and C++.  Starting with GCC 12 the C++ front-end no longer >> +    removes those bit-fields from the internal representation and >> +    per clarified psABI some targets have been changed, so that they >> +    either ignore those bit-fields in the argument passing by value >> +    decisions in both C and C++, or they always take them into account. >> +    x86-64, ARM and AArch64 will always ignore them (so there is >> +    a C ABI incompatibility between GCC 11 and earlier with GCC 12 or >> +    later), PowerPC64 ELFv2 and S/390 always take them into account >> +    (so there is a C++ ABI incompatibility, GCC 4.4 and earlier >> compatible >> +    with GCC 12 or later, incompatible with GCC 4.5 through GCC 11). >> +    RISC-V has changed the handling of these already starting with >> GCC 10. >> +    GCC 12 on the above targets will report such incompatibilities as >> +    warnings or other diagnostics unless -Wno-psabi is >> used. >> + 
    • >>    
  • >>       C: >>       Computed gotos require a pointer type now. >> >>     Jakub >>