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CTRY:IE; LANG:en; SCL:1; SRV:; IPV:CAL; SFV:NSPM; H:64aa7808-outbound-1.mta.getcheckrecipient.com; PTR:ec2-63-35-35-123.eu-west-1.compute.amazonaws.com; CAT:NONE; SFS:(4636009)(396003)(39860400002)(346002)(136003)(376002)(46966006)(36840700001)(70206006)(81166007)(8886007)(26005)(44832011)(55016002)(356005)(4326008)(5660300002)(2906002)(36756003)(6862004)(82740400003)(70586007)(7696005)(956004)(33656002)(2616005)(86362001)(6636002)(316002)(8936002)(82310400003)(336012)(36860700001)(186003)(4744005)(37006003)(1076003)(478600001)(83380400001)(8676002)(47076005)(54906003); DIR:OUT; SFP:1101; X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 02 Aug 2021 14:50:12.5785 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 69592139-acaa-4a0a-ef60-08d955c4d512 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d; Ip=[63.35.35.123]; Helo=[64aa7808-outbound-1.mta.getcheckrecipient.com] X-MS-Exchange-CrossTenant-AuthSource: AM5EUR03FT048.eop-EUR03.prod.protection.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DB8PR08MB5322 X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , From: Szabolcs Nagy via Libc-alpha Reply-To: Szabolcs Nagy Cc: 'GNU C Library' Errors-To: libc-alpha-bounces+e=80x24.org@sourceware.org Sender: "Libc-alpha" The 08/02/2021 14:38, Wilco Dijkstra via Libc-alpha wrote: > > We discussed how should be defined BTI_C macro before, at that time conclusion > > was "NOP" rather than empty unless HAVE_AARCH64_BTI. > > Now the above code defines BTI_C as empty unconditionally. > > A64FX doesn't support BTI, so this code is OK. > > But I'm just interested in the reason why it is changed. > > We changed to NOP in the generic code, so that works for all string functions. > In this specific case removing the initial NOP as well allows all performance critical > code for <= 512 bytes to be perfectly aligned to 16-byte fetch blocks. yes, this makes sense: originally BTI_C was always hint 34, but since that can be slow it was changed for !HAVE_AARCH64_BTI. We don't want the layout of asm code to change based on toolchain configuration so BTI_C is defined as a place holder nop then. but in a64fx specific code bti is never needed so we also don't need the place holder nop, BTI_C can be unconditionally empty.