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Marchesi" To: Eduard Zingerman Cc: Yonghong Song , bpf@vger.kernel.org, Yonghong Song , Alexei Starovoitov , david.faust@oracle.com, cupertino.miranda@oracle.com Subject: Re: [PATCH bpf-next] bpf: abstract loop unrolling pragmas in BPF selftests In-Reply-To: <7d2b05bf2e7ae7c95807ac4b2a9664f203facbfe.camel@gmail.com> (Eduard Zingerman's message of "Thu, 08 Feb 2024 17:53:48 +0200") References: <20240207101253.11420-1-jose.marchesi@oracle.com> <87h6ijfayj.fsf@oracle.com> <87wmrfdsk7.fsf@oracle.com> <4ad9dad64b38ae90e4a050ce5181ced750913b23.camel@gmail.com> <87o7crdmjn.fsf@oracle.com> <87il2zdl43.fsf@oracle.com> <7d2b05bf2e7ae7c95807ac4b2a9664f203facbfe.camel@gmail.com> Date: Thu, 08 Feb 2024 17:51:49 +0100 Message-ID: <871q9mew62.fsf@oracle.com> User-Agent: Gnus/5.13 (Gnus v5.13) Content-Type: text/plain X-ClientProxiedBy: AS4P195CA0001.EURP195.PROD.OUTLOOK.COM (2603:10a6:20b:5e2::8) To DM6PR10MB3113.namprd10.prod.outlook.com (2603:10b6:5:1a7::12) Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DM6PR10MB3113:EE_|CH2PR10MB4360:EE_ X-MS-Office365-Filtering-Correlation-Id: 16375156-3b1c-44c0-31fa-08dc28c640ce X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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Marchesi wrote: > [...] > >> If the compiler generates assembly code the same code for profile2.c for >> before and after, that means that the loop does _not_ get unrolled when >> profiler.inc.h is built with -O2 but without #pragma unroll. >> >> But what if #pragma unroll is used? If it unrolls then, that would mean >> that the pragma does something more than -funroll-loops/-O2. >> >> Sorry if I am not making sense. Stuff like this confuses me to no end >> ;) > > Sorry, I messed up while switching branches :( > Here are the correct stats: > > | File | insn # | insn # | > | | before | after | > |-----------------+--------+--------| > | profiler1.bpf.o | 16716 | 4813 | This means: - With both `#pragma unroll' and -O2 we get 16716 instructions. - Without `#pragma unroll' and with -O2 we get 4813 instructions. Weird. > | profiler2.bpf.o | 2088 | 2050 | - Without `#pragma unroll' and with -O2 we get 2088 instructions. - With `#pragma loop unroll(disable)' and with -O2 we get 2050 instructions. Also surprising. > | profiler3.bpf.o | 4465 | 1690 |