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Mon, 19 Oct 2020 09:43:43 +0000 From: =?ISO-8859-1?Q?J=E9r=F4me?= Pouiller To: 'Joe Perches' , Colin King , Greg Kroah-Hartman , "devel@driverdev.osuosl.org" , David Laight Cc: "kernel-janitors@vger.kernel.org" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH] staging: wfx: make a const array static, makes object smaller Date: Mon, 19 Oct 2020 11:43:36 +0200 Message-ID: <1800171.ETUgnxhFMb@pc-42> Organization: Silicon Labs In-Reply-To: <048fded745634e369fa2646f87a05ec4@AcuMS.aculab.com> References: <20201016223303.687278-1-colin.king@canonical.com> <09cd7e609324d460afdf14829baf3c2f1a9cb9cd.camel@perches.com> <048fded745634e369fa2646f87a05ec4@AcuMS.aculab.com> Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="iso-8859-1" X-Originating-IP: [37.71.187.125] X-ClientProxiedBy: PR0P264CA0137.FRAP264.PROD.OUTLOOK.COM (2603:10a6:100:1a::29) To SN6PR11MB2718.namprd11.prod.outlook.com (2603:10b6:805:63::18) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from pc-42.localnet (37.71.187.125) by PR0P264CA0137.FRAP264.PROD.OUTLOOK.COM (2603:10a6:100:1a::29) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3477.21 via Frontend Transport; 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X-MS-Exchange-AntiSpam-MessageData: 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 X-OriginatorOrg: silabs.com X-MS-Exchange-CrossTenant-Network-Message-Id: ce29d0c8-ff23-461a-7516-08d874137743 X-MS-Exchange-CrossTenant-AuthSource: SN6PR11MB2718.namprd11.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 19 Oct 2020 09:43:42.9454 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 54dbd822-5231-4b20-944d-6f4abcd541fb X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: G5tUAaWh30IY0+TtvgaImVcp3E4YiM4z/RqHK8KGQaNTHy2LFA51VPWmsAFMDgFCroHCcUMfFNhpxqKa/JlG8A== X-MS-Exchange-Transport-CrossTenantHeadersStamped: SN6PR11MB3040 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Monday 19 October 2020 10:09:19 CEST David Laight wrote: > From: Joe Perches > > Sent: 17 October 2020 01:12 > > > > On Fri, 2020-10-16 at 23:33 +0100, Colin King wrote: > > > From: Colin Ian King > > > > > > Don't populate const array filter_ies on the stack but instead > > > make it static. Makes the object code smaller by 261 bytes. > > > > > > Before: > > > text data bss dec hex filename > > > 21674 3166 448 25288 62c8 drivers/staging/wfx/sta.o > > > > > > After: > > > text data bss dec hex filename > > > 21349 3230 448 25027 61c3 drivers/staging/wfx/sta.o > > > > Thanks. > > > > It's odd to me it's so large a change as it's only > > 24 bytes of initialization. (3 entries, each 8 bytes) >=20 > Perhaps the 'stack protector' crap? >=20 > Interestingly, loading the data from the 'readonly' section > is probably a data cache miss. > Which might end up being slower than the extra code to > update the on-stack data. > The extra code might get prefetched... I had never realized the difference between "const" and "static const" in this case. With my gcc fro arm, the output of "objdump -h sta.o" gives: Before: 0 .text 000019fc 00000000 00000000 00000034 2**2 CONTENTS, ALLOC, LOAD, READONLY, CODE [...] 7 .rodata 00000015 00000000 00000000 00001e78 2**2 CONTENTS, ALLOC, LOAD, READONLY, DATA After: 0 .text 00001974 00000000 00000000 00000034 2**2 CONTENTS, ALLOC, LOAD, READONLY, CODE [...] 7 .rodata 0000002d 00000000 00000000 00001dd4 2**2 CONTENTS, ALLOC, LOAD, READONLY, DATA The difference of .rodata is exactly what is expected (24 bytes) and we save 115 bytes of code. Reviewed-by: J=E9r=F4me Pouiller --=20 J=E9r=F4me Pouiller