From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id E8247C433EF for ; Wed, 20 Oct 2021 07:30:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BCE8B61378 for ; Wed, 20 Oct 2021 07:30:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229919AbhJTHcO (ORCPT ); Wed, 20 Oct 2021 03:32:14 -0400 Received: from szxga01-in.huawei.com ([45.249.212.187]:29916 "EHLO szxga01-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229771AbhJTHcN (ORCPT ); Wed, 20 Oct 2021 03:32:13 -0400 Received: from dggemv711-chm.china.huawei.com (unknown [172.30.72.54]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4HZ2Fb3VL3zbnKC; Wed, 20 Oct 2021 15:25:23 +0800 (CST) Received: from dggpemm500006.china.huawei.com (7.185.36.236) by dggemv711-chm.china.huawei.com (10.1.198.66) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2308.15; Wed, 20 Oct 2021 15:29:56 +0800 Received: from [10.174.178.55] (10.174.178.55) by dggpemm500006.china.huawei.com (7.185.36.236) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2308.15; Wed, 20 Oct 2021 15:29:56 +0800 Subject: Re: [PATCH v2 2/2] iommu/arm-smmu-v3: Simplify useless instructions in arm_smmu_cmdq_build_cmd() To: Will Deacon CC: Robin Murphy , Joerg Roedel , linux-arm-kernel , iommu , , "John Garry" References: <20210818080452.2079-1-thunder.leizhen@huawei.com> <20210818080452.2079-3-thunder.leizhen@huawei.com> <20211004120714.GD27373@willie-the-truck> From: "Leizhen (ThunderTown)" Message-ID: <68742c41-8610-9b3f-49b7-7ae04dc87ec2@huawei.com> Date: Wed, 20 Oct 2021 15:29:44 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: <20211004120714.GD27373@willie-the-truck> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.178.55] X-ClientProxiedBy: dggems706-chm.china.huawei.com (10.3.19.183) To dggpemm500006.china.huawei.com (7.185.36.236) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021/10/4 20:07, Will Deacon wrote: > On Wed, Aug 18, 2021 at 04:04:52PM +0800, Zhen Lei wrote: >> Although the parameter 'cmd' is always passed by a local array variable, >> and only this function modifies it, the compiler does not know this. The >> compiler almost always reads the value of cmd[i] from memory rather than >> directly using the value cached in the register. This generates many >> useless instruction operations and affects the performance to some extent. >> >> To guide the compiler for proper optimization, 'cmd' is defined as a local >> array variable, marked as register, and copied to the output parameter at >> a time when the function is returned. >> >> The optimization effect can be viewed by running the "size arm-smmu-v3.o" >> command. >> >> Before: >> text data bss dec hex >> 26954 1348 56 28358 6ec6 >> >> After: >> text data bss dec hex >> 26762 1348 56 28166 6e06 >> >> Signed-off-by: Zhen Lei >> --- >> drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 11 ++++++++--- >> 1 file changed, 8 insertions(+), 3 deletions(-) >> >> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c >> index 01e95b56ffa07d1..7cec0c967f71d86 100644 >> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c >> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c >> @@ -234,10 +234,12 @@ static int queue_remove_raw(struct arm_smmu_queue *q, u64 *ent) >> } >> >> /* High-level queue accessors */ >> -static int arm_smmu_cmdq_build_cmd(u64 *cmd, struct arm_smmu_cmdq_ent *ent) >> +static int arm_smmu_cmdq_build_cmd(u64 *out_cmd, struct arm_smmu_cmdq_ent *ent) >> { >> - memset(cmd, 0, 1 << CMDQ_ENT_SZ_SHIFT); >> - cmd[0] |= FIELD_PREP(CMDQ_0_OP, ent->opcode); >> + register u64 cmd[CMDQ_ENT_DWORDS]; > > 'register' is pretty badly specified outside of a handful of limited uses in > conjunction with inline asm, so I really don't think we should be using it > here. OK, I think I was overly aggressive in the beginning, and I just tried to remove the register modifier and get the same optimization. > >> + cmd[0] = FIELD_PREP(CMDQ_0_OP, ent->opcode); > > This generates a compiler warning about taking the address of a 'register' > variable. > > Will > . >