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Lu" Cc: Haochen Jiang , wwwhhhyyy , binutils@sourceware.org References: <20221014091248.4920-1-haochen.jiang@intel.com> <20221014091248.4920-2-haochen.jiang@intel.com> <863655db-f202-477f-c638-00773c25886c@suse.com> From: Jan Beulich In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: AM6PR01CA0051.eurprd01.prod.exchangelabs.com (2603:10a6:20b:e0::28) To VE1PR04MB6560.eurprd04.prod.outlook.com (2603:10a6:803:122::25) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: VE1PR04MB6560:EE_|AS8PR04MB9048:EE_ X-MS-Office365-Filtering-Correlation-Id: 19e5f233-2021-4b46-0293-08daaf413394 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; X-Microsoft-Antispam-Message-Info: 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 X-Forefront-Antispam-Report: CIP:255.255.255.255;CTRY:;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:VE1PR04MB6560.eurprd04.prod.outlook.com;PTR:;CAT:NONE;SFS:(13230022)(376002)(136003)(366004)(39850400004)(396003)(346002)(451199015)(6916009)(54906003)(31686004)(5660300002)(66556008)(66946007)(66476007)(316002)(478600001)(4326008)(6506007)(41300700001)(53546011)(8676002)(6512007)(186003)(6486002)(2906002)(8936002)(2616005)(83380400001)(36756003)(6666004)(38100700002)(31696002)(86362001)(43740500002)(45980500001);DIR:OUT;SFP:1101; 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Lu wrote: > On Fri, Oct 14, 2022 at 2:52 AM Jan Beulich wrote: >> >> On 14.10.2022 11:12, Haochen Jiang wrote: >>> From: wwwhhhyyy >>> >>> x86: Support Intel AVX-IFMA >>> >>> Intel AVX IFMA instructions are marked with CpuVEX_PREFIX, which is >>> cleared by default. Without {vex} pseudo prefix, Intel IFMA instructions >>> are encoded with EVEX prefix. {vex} pseudo prefix will turn on VEX >>> encoding for Intel IFMA instructions. >> >> I firmly object to the proliferation of this mis-feature. As expressed >> before for AVX-VNNI, as long as the user has disabled AVX512 (or >> respective sub-features thereof), there should be no need to use {vex} in >> the source code. There's also no reason at all to make the disassembler >> print {vex} prefixes - we don't do so for any other insns (apart from >> AVX-VNNI) where an ambiguity exists between their VEX and EVEX encodings >> (when none of the EVEX-specific features is used). > > The {vex} prefix is used with AVX-IFMA instructions so that IFMA instructions > without a prefix, which are generated by compilers or handwritten, will be > always encoded with EVEX. So again: Why is this necessary when a programmer disabled AVX512? I fully agree we need to pick the EVEX encoding by default if available, but I see no reason whatsoever to insist on a {vex} prefix when the EVEX variant is unavailable anyway. As you said back at the time for AVX-VNNI - this was a design decision taken at Intel. Which is fine for a draft implementation. But decisions for an open source project should be taken in the open, and opinions of others should not simply be put off. Jan