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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 1422FCA5FB1 for ; Wed, 30 Sep 2026 13:01:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:MIME-Version:Message-ID:Date:Subject:Cc :To:From:Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:In-Reply-To:References: List-Owner; bh=AfKoysl94rUoSVNlBng0a3BlXOT9U2YTs/O4okST/3g=; b=lnzynq7jaueFry PDO5gxjba77WbrbtgPmTy62+1RVsEv6VKLH9RLHmfiVNrEc331YPJOc6/56fs3caJhTh+j8525Ezb xcsJu/njNB5FSg8I+1sLxCqP1wiJJGNKheg0hraO7HhG+w0lMI7cgxVgWSELoVx4fJFeZmpQRRttW QWI6V0rCmpcWsuFEa4Cvx96CrogXSj8oS0GvZ7u1Fi1W4O5YNMHIjzAKtFSHeyrYqVE/i46GEaBJz gdC0d8ZZwfF7XWhbJCwj6lt2PhCvegkK8KfcXmKeaSS2J1zFz4R4ghK+bbpPxanx45xOn7fekjUEg XHSz8kqhpfaYL52pxn7g==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1xBtvY-000000064ja-0B1n; Wed, 30 Sep 2026 13:00:52 +0000 Received: from sea.source.kernel.org ([2600:3c0a:e001:78e:0:1991:8:25]) by bombadil.infradead.org with esmtps (Exim 4.99.1 #2 (Red Hat Linux)) id 1xBtvW-000000064jQ-2v5D for linux-riscv@lists.infradead.org; Wed, 30 Sep 2026 13:00:50 +0000 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id 18A2543E9A; Wed, 30 Sep 2026 13:00:50 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9739C1F000FF; Wed, 30 Sep 2026 13:00:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790773250; bh=BN61Chre73hrYLj0CBq/u0yA/QVkhU4K+myvgwugnmI=; h=From:To:Cc:Subject:Date; b=U4geHWF3sGD8zeH7k7va2WH/jQHrLWud0syMY6PYrIvnSyPkMEss2x8l8EaAwp0hd L0ZezYAAPC+z/QbPC2f4clQlgbXV6nZmULk6yu7zhxV0dDXS6piq2yHu/ioZOFyCmK SIZp28bMtWH00nHKwGklpPI/L2TfQGUbxSscpT8hC22vgQn7LsXxPBlaRu7JnAICWQ +tCmO0aly6FhOvTnRxYCNzFrxkgIAZn4os4Zc1d9crzttq14Wd75M9sFKhpPDQ0noa P1XTxu+G6T2SiIUBnGX6B1eWyEYzy0+uRbK8H+PcgfJttwNbrkrKhpKWkbX+4/sMrw HhxHNIc3+KMeg== From: Jisheng Zhang To: Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexandre Ghiti , Nam Cao Cc: linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org Subject: [PATCH v4 0/3] riscv: word-at-a-time: improve find_zero() Date: Wed, 30 Sep 2026 20:40:49 +0800 Message-ID: <20260930124052.21986-1-jszhang@kernel.org> X-Mailer: git-send-email 2.51.0 MIME-Version: 1.0 X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org Currently, there are two problems with riscv find_zero(): 1. When !RISCV_ISA_ZBB, the generic fls64() bring non-optimal code. But in word-at-a-time case, we don't have to go with fls64() code path, instead, we can fallback to the generic word-at-a-time implementaion. What's more, the fls64() brings non-necessary zero bits couting for RV32. In fact, fls() is enough. 2. Similar as 1, the generic fls64() also brings non-optimal code when RISCV_ISA_ZBB=y but HW doesn't support Zbb. So this series tries to improve find_zero() by falling back to generic word-at-a-time implementaion where necessary. We dramatically reduce the instructions of find_zero() from 33 to 8! Also testing with the micro-benchamrk in patch1 shows that the performance is improved by about 1150%! After that, we improve find_zero() for Zbb further by applying similar optimization as Linus did in commit f915a3e5b018 ("arm64: word-at-a-time: improve byte count calculations for LE"), so that we share the similar improvements: "The difference between the old and the new implementation is that "count_zero()" ends up scheduling better because it is being done on a value that is available earlier (before the final mask). But more importantly, it can be implemented without the insane semantics of the standard bit finding helpers that have the off-by-one issue and have to special-case the zero mask situation." On RV64 w/ Zbb, the new "find_zero()" ends up just "ctz" plus the shift right that then ends up being subsumed by the "add to final length". Reduce the total instructions from 7 to 3! But I have no HW platform which supports Zbb, so I can't get the performance improvement numbers by the last patch, only built and tested the patch on QEMU. Since v3: - fold part of patch2 into patch1 and make the RV32 improvement as patch2. Since v2: - fix wrong version(PATCH vs "PATCH v2" in cover letter) Since v1: - rebase on the latest rc1 - Use if (IS_ENABLED(CONFIG_RISCV_ISA_ZBB) && IS_ENABLED(CONFIG_TOOLCHAIN_HAS_ZBB) && riscv_has_extension_likely(RISCV_ISA_EXT_ZBB)) Jisheng Zhang (3): riscv: word-at-a-time: improve find_zero() for !RISCV_ISA_ZBB or no Zbb riscv: word-at-a-time: improve find_zero() for RV32 riscv: word-at-a-time: improve find_zero() for Zbb arch/riscv/include/asm/word-at-a-time.h | 46 +++++++++++++++++++++++-- 1 file changed, 43 insertions(+), 3 deletions(-) -- 2.51.0 _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv