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Marchesi" To: Yonghong Song Cc: bpf@vger.kernel.org, david.faust@oracle.com, elena.zannoni@oracle.com, David Malcolm , Nick Desaulniers , Julia Lawall Subject: Re: Follow up from the btf_type_tag discussion in the BPF office hours References: <87o7s4ece1.fsf@oracle.com> <757e5dde-75ed-80e2-9a34-ff7c2259de78@meta.com> Date: Mon, 19 Dec 2022 18:27:32 +0100 In-Reply-To: <757e5dde-75ed-80e2-9a34-ff7c2259de78@meta.com> (Yonghong Song's message of "Fri, 16 Dec 2022 17:38:50 -0800") Message-ID: <87h6xrfgmz.fsf@oracle.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (gnu/linux) Content-Type: text/plain X-ClientProxiedBy: LO4P123CA0561.GBRP123.PROD.OUTLOOK.COM (2603:10a6:600:33b::10) To BYAPR10MB2888.namprd10.prod.outlook.com (2603:10b6:a03:88::32) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: BYAPR10MB2888:EE_|PH0PR10MB4501:EE_ X-MS-Office365-Filtering-Correlation-Id: c8f836d5-b42c-45be-91fd-08dae1e5bf23 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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Marchesi wrote: >> Of the two problems discussed: >> 1. DW_TAG_LLVM_annotation not being able to denote annotations to >> non-pointed based types. clang currently ignores these instances. >> We discussed two possible options to deal with this: >> 1.1 To continue ignoring these cases in the front-end, keep the dwarf >> expressiveness limitation, and document it. >> 1.2 To change DW_TAG_LLVM_annotation so it behaves like a qualifier >> DIE (like const, volatile, etc.) so it can apply to any type. > > Thanks for the detailed update. Yes, we do want to __tag behaving like > a qualifier. > > Today clang only support 'base_type *' style of code. > But we are open to support non-pointer style of tagging like > 'base_type global_var'. Because of this, the following > dwarf output should be adopted: > C: int __tag1 * __tag2 * p; > dwarf: ptr -> __tag2 --> ptr -> __tag1 -> int > or > C: int __tag1 g; > dwarf: var_g -> __tag1 --> int > > The above format *might* require particular dwarf tools to add support > for __tag attribute. But I think it is a good thing in the long run > esp. if we might add support to non-pointer types. In current > implementation, dwarf tools can simply ignore the children of ptr > which they may already do it. I wonder, since these annotations are atomic, is there a reason for not using an attribute instead of a DIE tag? Something like DW_AT_annotation. The attribute could then be used by any DIE (declaration, type, ...) and existing DWARF consumers that don't support the new attribute would happily just ignore it.