From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 48) id 404EF385701D; Tue, 30 Jun 2020 12:27:43 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 404EF385701D DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org; s=default; t=1593520063; bh=Pmya05eNu1K5cvIVH2VfdjsO07FdoafUGG/zzHr+laU=; h=From:To:Subject:Date:In-Reply-To:References:From; b=jYLkGn0gfcQdxWWNTye8cQqAU2oeQa/EnuBBDe3Y7LHoWuRzOpLaavJoSYVKO6wZ0 dNa+qM7nDRKqc5T9OUPeoCIbWJuDwCOG65DZdWubeEksX47mwYiHbx+cMYNxX9LNL5 eQ4GUSeZmm5oVlMHS0k+rOBY5UB4A4fYQUrKHeqs= From: "jakub at gcc dot gnu.org" To: gcc-bugs@gcc.gnu.org Subject: [Bug c++/95976] [[no_unique_address]] on union members has the opposite-of-intended effect Date: Tue, 30 Jun 2020 12:27:43 +0000 X-Bugzilla-Reason: CC X-Bugzilla-Type: changed X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: gcc X-Bugzilla-Component: c++ X-Bugzilla-Version: 10.1.0 X-Bugzilla-Keywords: ABI, wrong-code X-Bugzilla-Severity: normal X-Bugzilla-Who: jakub at gcc dot gnu.org X-Bugzilla-Status: NEW X-Bugzilla-Resolution: X-Bugzilla-Priority: P3 X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org X-Bugzilla-Target-Milestone: --- X-Bugzilla-Flags: X-Bugzilla-Changed-Fields: cf_reconfirmed_on everconfirmed bug_status keywords cc Message-ID: In-Reply-To: References: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/ Auto-Submitted: auto-generated MIME-Version: 1.0 X-BeenThere: gcc-bugs@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc-bugs mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 30 Jun 2020 12:27:43 -0000 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95976 Jakub Jelinek changed: What |Removed |Added ---------------------------------------------------------------------------- Last reconfirmed| |2020-06-30 Ever confirmed|0 |1 Status|UNCONFIRMED |NEW Keywords| |ABI CC| |jason at gcc dot gnu.org, | |redi at gcc dot gnu.org --- Comment #2 from Jakub Jelinek --- One way to fix this would be: --- gcc/cp/class.c.jj 2020-06-27 12:35:11.935311848 +0200 +++ gcc/cp/class.c 2020-06-30 14:18:08.448067688 +0200 @@ -6575,7 +6575,9 @@ layout_class_type (tree t, tree *virtual /* We must also reset the DECL_MODE of the field. */ SET_DECL_MODE (field, TYPE_MODE (type)); } - else if (might_overlap && is_empty_class (type)) + else if (might_overlap + && is_empty_class (type) + && TREE_CODE (t) !=3D UNION_TYPE) { DECL_FIELD_ABI_IGNORED (field) =3D 1; layout_empty_base_or_field (rli, field, empty_base_offsets); Another might be not to change DECL_FIELD_OFFSET in layout_empty_base_or_fi= eld for TREE_CODE (rli->t) =3D=3D UNION_TYPE, etc. The question is if [[no_unique_address]] members of a union should affect a= ny ABI decisions (such as passing the union by value) or not. Looking at the config/*/* code, in most cases the targets ignore DECL_FIELD_ABI_IGNORED on UNION_TYPE members, but ia64 does not, and I think rs6000_special_round_type_align and darwin_rs6000_special_round_type_align don't ignore it either.=