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To: Julian P Samaroo , References: From: Yonghong Song Message-ID: <75772f13-b366-d1f7-07a1-c43666e512d1@fb.com> Date: Wed, 2 Jun 2021 10:19:21 -0700 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.10.2 In-Reply-To: Content-Type: text/plain; charset="utf-8"; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [2620:10d:c090:400::5:80b] X-ClientProxiedBy: SJ0PR13CA0041.namprd13.prod.outlook.com (2603:10b6:a03:2c2::16) To SN6PR1501MB2064.namprd15.prod.outlook.com (2603:10b6:805:d::27) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [IPv6:2620:10d:c085:21e8::1354] (2620:10d:c090:400::5:80b) by SJ0PR13CA0041.namprd13.prod.outlook.com (2603:10b6:a03:2c2::16) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4195.9 via Frontend Transport; Wed, 2 Jun 2021 17:19:23 +0000 X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: edb12432-d523-4a0c-34b3-08d925ea915e X-MS-TrafficTypeDiagnostic: SN6PR1501MB2029: X-Microsoft-Antispam-PRVS: X-FB-Source: Internal X-MS-Oob-TLC-OOBClassifiers: OLM:4941; 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In my particular case that generated this code, my frontend doesn't > have the notion of packed structs, so I can't workaround this by making the > struct packed. > > My belief is that for unpacked structs, LLVM should emit these stores as 64-bit > stores, which should be OK since the padding bytes are going to be zero (from > my limited understanding of LLVM structs). Does this seem like a reasonable Your assumption about padding bytes to be zero is not correct. Except explicitly requesting to fill padding bytes with zero e.g., using __builtin_memset(), the compiler doesn't need to write to padding bytes. So this is not a compiler bug. The best approach is to do manual padding or using __builtin_memset() before assigning values to each individual field. > change to make? I'm also unable to test this on LLVM 12 (my language hasn't yet > updated to support that version), so this could have possibly already been > fixed; please let me know if so! > > Julian >