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Mon, 26 Oct 2026 15:53:54 +0000 Message-ID: Date: Mon, 26 Oct 2026 16:53:48 +0100 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] unroll: Make unrolled, unrolled_full and unrolled_none function-like To: David Laight CC: Tzu-Ching Yang , Eric Biggers , "Jason A. Donenfeld" , Ard Biesheuvel , "Jose E. Marchesi" , "Jakub Kicinski" , Andrew Morton , "Simon Horman" , Maciej Fijalkowski , , References: <20261016025613.2690485-1-shingekinocore@gmail.com> <0a38e0b0-2508-41c1-bd99-d15c27579307@intel.com> <20261022100140.101ddf52@pumpkin> <20261022110959.1173447-1-shingekinocore@gmail.com> <3c25921c-7bdf-4edd-ba06-838dd1765db6@intel.com> <20261023173845.2ca86a63@pumpkin> Content-Language: en-US From: Alexander Lobakin In-Reply-To: <20261023173845.2ca86a63@pumpkin> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: DB8P191CA0019.EURP191.PROD.OUTLOOK.COM (2603:10a6:10:130::29) To DS0PR11MB8718.namprd11.prod.outlook.com (2603:10b6:8:1b9::20) X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS0PR11MB8718:EE_|BN9PR11MB5244:EE_ X-MS-Office365-Filtering-Correlation-Id: 17565507-70ef-41a9-7778-08df337955fe X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|376014|1800799024|23010399003|366016|3023799007|261026140227099003|4143699003|11063799006|56012099006|5023799004|261022075727099003|261022105027099003|261021121427099003|10067099003|261026121527099003|18002099003|6133799003|22082099003; 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i < 8; i++, j--) >>> s += a[i] * b[j]; >>> >>> GCC 8 to 16 and Clang 17 to 23 unroll this fully, and the same works >>> with break and continue in the body, nested loops and a loop inside >>> another macro. The count form has the same shape: >>> >>> unrolled_count_for (4, int i = 0; i < n; i++) >>> s += a[i]; >>> >>> As a fourth option, the _for forms would replace the four unrolled* >>> macros: no clash with identifiers, and the loop reads like >>> for_each_*(); the cost is a new loop spelling at every user. Olek, >>> Eric, would you prefer this over options 1 to 3 from my earlier reply? >> >> I had considered this option in the past, but IIRC the unrolled >> attribute can also apply to `while` etc, this would limit it to >> `for` only. > > Is that really useful? > Partially unrolling a variable iteration count loop most likely just > trashes the I-cache. > (The unroll can't use simd instructions because they aren't normally > available in kernel - ok, some cpu have a 'load pair'.) In XSk xmit code, we have loops with the number of iterations being a variable (number of packets queued by the userspace program), but using unrolling by 8 packets helps there a lot if iterations are really independent from each other (except the counter itself). Pick a lower or bigger value and the performance (which can sometimes reach 90 Mpps if everything is optimized to the top) will drop, `unrolled_full` is not an option (not a compile-time constant), leave it without any manual unrolling and the compiler won't do anything ('cause not a compile-time constant). > The only exception is very short loops where unrolling once can help > because the cpu cant execute a single clock loop. > The compiler is likely to make a pigs breakfast of those loops > so they end up in asm anyway - look at the xor parity code. Thanks, Olek