From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-x534.google.com (mail-ed1-x534.google.com [IPv6:2a00:1450:4864:20::534]) by sourceware.org (Postfix) with ESMTPS id C7F9A3858405 for ; Wed, 30 Mar 2022 11:00:55 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org C7F9A3858405 Received: by mail-ed1-x534.google.com with SMTP id h1so23967344edj.1 for ; Wed, 30 Mar 2022 04:00:55 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=3VvSe7caTIbMNVBe9JGKCvifVWURqy1z4AN0kmwNjVI=; b=DlNY21EsnAC8Hi+IovHdNukWt7HoW0sCD7dmAdzgXMkCPH/KuC68+Uooufp/TDSz4b kQEM92OxWTng6vJoDLOrb3FnQwPbNSM6idIW8r5GpzbWdNtgXFBsshjuP2yz0b+4l4qj mskhKG//xVLyOhZZyy0Xd9Oykn7S7JFQXWRyI1cs5dQNaFVy6DyNmh9h5ohd/REJuX8O bsjwNvmq9NLrlDKnIyx2S9icYcFziHA+sThZQQ+LFSWxA3Ma6WGvvcQJ5RQTtW/110eQ 8ZiAlsgU9p9/oQLJqEfVF7gl8aSwqN9SBCRDLSkqyCR//Qu7CJYQCpo/dA1H0DB7tgrN oheQ== X-Gm-Message-State: AOAM533MJhWH52+Ug8XTPxNs7DBBvCnGHLc3h10YhLUkgxBrVf9Gpce7 JVxI/zD4FOliQ/KOnoCB4Ij90aqPyszyhIHgFVk= X-Google-Smtp-Source: ABdhPJwW7QI1fNAnYgwZa65R5HW/O6HL+JbDjSiOVijaqoeqHpIaL8jzlSX3ej+bjGx2r2H68Se3KxWczywrpNbvKms= X-Received: by 2002:a50:a41a:0:b0:419:d2b:8391 with SMTP id u26-20020a50a41a000000b004190d2b8391mr9984835edb.390.1648638054383; Wed, 30 Mar 2022 04:00:54 -0700 (PDT) MIME-Version: 1.0 References: In-Reply-To: From: Richard Biener Date: Wed, 30 Mar 2022 13:00:43 +0200 Message-ID: Subject: Re: [wwwdocs] Document zero width bit-field passing ABI changes in gcc-12/changes.html [PR104796] To: Jakub Jelinek Cc: GCC Patches Content-Type: text/plain; charset="UTF-8" X-Spam-Status: No, score=-8.0 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, FREEMAIL_FROM, GIT_PATCH_0, KAM_SHORT, RCVD_IN_DNSWL_NONE, SPF_HELO_NONE, SPF_PASS, 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 11:00:58 -0000 On Wed, Mar 30, 2022 at 12:08 PM 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. "containing zero width bit-fields"? > + 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. > +
    • Otherwise LGTM. The case with float a; int :0; float b; looks quite artificial - are there cases where { int a0 : 24; int a1 : 8; int :0; int b0 : 24; int b1 : 8; } are affected? Thus cases where people might actually use :0 which is inbetween bitfields? At least I can't convince GCC on x86_64 to pass those differently, struct X { long a0 : 24; long a1 : 8; long :0; long b0 : 24; long b1 : 8; }; struct X foo (struct X x) { return x; } seem to pass in %rsi/%rdi and return in %rax/%rdx with both GCC 11 and trunk. Richard. >
  • > C: > Computed gotos require a pointer type now. > > Jakub >