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 71805C79FA7 for ; Mon, 7 Sep 2026 15:31:24 +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=cTi0IYPPKyOwKXAuZ9T9XKbadnkNKxO0/QqSa1dZAIw=; b=oaymxmHOsmQAWx ZII/YYRb5E1kfsl0anHH6CAzjOe/05zFxI2FIbrUK7OnN/5G5L6WV8m+cMcYbZnp3t/8Ogy7SqJ02 eKGCefqsjpAGJ8ChHy5kL9X//UurBzawREoj7ItYuoaGNlcWOcn1BJDD9aYm9nCsW7kCu3rY969bf f+1qNRBUNV96NLI9WQh5e0bcWukdhNAL3SipUH6nPzqqepu9JYUIbKHp61aLD+8Tl201/ODhPWN/Y 1SeInFony/eJjj9w9Yk/dvcLDwz68+JQb7PYsZXpLdV1boI4awNuCjf5k3Z16OPPVzkgrxegcbLoe pig6xvqh1EgS+ZyaVHxQ==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1x3bJG-000000079pj-0QQj; Mon, 07 Sep 2026 15:31:02 +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 1x3bJB-000000079oK-3CaH for linux-riscv@lists.infradead.org; Mon, 07 Sep 2026 15:30:59 +0000 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id 26CA240768; Mon, 7 Sep 2026 15:30:57 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id D6EC81F00A3A; Mon, 7 Sep 2026 15:30:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788795057; bh=AygeBGTDZ2oWU8QHGA0f9XVLcVWKrxlMH2XdKkG3oEQ=; h=From:To:Cc:Subject:Date; b=DcgTUS2meJa+TVgPacExIr5BNxRJRngNvDBsf5ZXl4lPjBM2b16GssU4s4z/Y65LI 1lxWIKrYgtV+98SkXO1Lwsc1C2/jOwbrAyJ+Pb50GDTtZVH8cF4iIFEGb5Rdn2slTJ hbU0K4KG+9kR76zm+WXvN+iv0E/LKwijKKi9RkSLA7ECVatBCbBfmfV/C2YSdki59M I5NAZUppX3IEHfej7KX/ZBqwuKRU+YDa7Lkhd67BVFqq3XgYJDVx2RMhqnHCWj3ZcW kLxHivF6L41wGUwqnT6rFOY83TgBpaPjoUqiGhoCBPz5PzoJUh68bcJT/4k/vkF3Mk pfwVyRwMAQFEA== 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 0/3] riscv: word-at-a-time: improve find_zero() Date: Mon, 7 Sep 2026 23:11:05 +0800 Message-ID: <20260907151108.7538-1-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 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 riscv: word-at-a-time: improve find_zero() without Zbb 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(-) -- 2.53.0 _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv