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Thu, 29 Aug 2019 18:18:21 +0000 From: Wilco Dijkstra To: Alexander Monakov , Maxim Kuvyrkov CC: Richard Guenther , "gcc-patches@gcc.gnu.org" , nd Subject: Re: [PR91598] Improve autoprefetcher heuristic in haifa-sched.c Date: Thu, 29 Aug 2019 20:41:00 -0000 Message-ID: References: <09F25146-8361-4FB0-AE6B-E13BF8CF332F@gmail.com> , In-Reply-To: Authentication-Results-Original: spf=none (sender IP is ) smtp.mailfrom=Wilco.Dijkstra@arm.com; X-Microsoft-Antispam-Untrusted: BCL:0;PCL:0;RULEID:(2390118)(7020095)(4652040)(8989299)(4534185)(4627221)(201703031133081)(201702281549075)(8990200)(5600166)(711020)(4605104)(1401327)(4618075)(2017052603328)(7193020);SRVR:VI1PR0801MB1904; x-checkrecipientrouted: true x-ms-oob-tlc-oobclassifiers: OLM:8882;OLM:8882; X-Forefront-Antispam-Report-Untrusted: SFV:NSPM;SFS:(10009020)(4636009)(376002)(346002)(136003)(396003)(366004)(39860400002)(189003)(199004)(6506007)(486006)(52536014)(55016002)(6246003)(476003)(9686003)(478600001)(11346002)(8936002)(33656002)(25786009)(229853002)(6436002)(110136005)(26005)(186003)(7736002)(53936002)(54906003)(305945005)(446003)(5660300002)(99286004)(2906002)(102836004)(7696005)(76176011)(81166006)(81156014)(74316002)(4744005)(8676002)(64756008)(66556008)(66446008)(4326008)(76116006)(66946007)(14454004)(66476007)(86362001)(71190400001)(71200400001)(6116002)(3846002)(256004)(316002)(66066001);DIR:OUT;SFP:1101;SCL:1;SRVR:VI1PR0801MB1904;H:VI1PR0801MB2127.eurprd08.prod.outlook.com;FPR:;SPF:None;LANG:en;PTR:InfoNoRecords;MX:1;A:1; received-spf: None (protection.outlook.com: arm.com does not designate permitted sender hosts) X-MS-Exchange-SenderADCheck: 1 X-Microsoft-Antispam-Message-Info-Original: t08p+Y0pYFNLHjGipRD7w58aYrDpXcI0378BE86L3bqF7tHT00iBazoCJdLJ7cdO6GyFIQUH1wHDRzWGy8E2L2kUu+BE++VvmOOp0f582zxVx54L/0dnLDNKd3XE0DMPedq5KY8eM6HnsbkumDh6rScsGIpkNH5YDevSnZTXe+7goV+xJjAkfpVJX46hIQBTNc0PaOANJZlAJSGaa2TFg22NJtnIDGUhGIsP0VRBoy9DZIl7Q5GHRbWwm+TQmxm/ByFCsNbqWcAWTRmDSTrbkH3l4E75xGbk7x71TlYwRUfOiOUAzyo7+HFEx0Lw0Bbx1qPFrv7KKqy8uLRCAYHES/yM5vhT4FPYHyMhu+VDYcowrO6mkd5S4GwOGV2m9dFtx8e8r1DlNCmiu9aiQkVzutDHNPX17DyC7K0Bph2XRaw= x-ms-exchange-transport-forked: True Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 Original-Authentication-Results: spf=none (sender IP is ) smtp.mailfrom=Wilco.Dijkstra@arm.com; Return-Path: Wilco.Dijkstra@arm.com X-MS-Exchange-Transport-CrossTenantHeadersStripped: VE1EUR03FT015.eop-EUR03.prod.protection.outlook.com X-MS-Office365-Filtering-Correlation-Id-Prvs: dc0418d9-f8f8-4544-3cbb-08d72cad461e X-SW-Source: 2019-08/txt/msg02015.txt.bz2 Hi Alexander, =20 > So essentially the main issue is not a hardware peculiarity, but rather t= he > bad schedule being totally wrong (it could only make sense if loads had 1= -cycle > latency, which they do not). The scheduling is only bad because the specific intrinsics used are mapped onto asm statements, so they are ignored by the scheduler and modelled with zero latencies. > I think this highlights how implementing this autoprefetch heuristic via = the > dfa_lookahead_guard interface looks questionable in the first place, but = the > patch itself makes sense to me. Yes I'm still not sure what this autoprefetch heuristic is trying to accomp= lish... We could try disabling it and see whether it actually helps. Wilco