From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 27EAD606A32F for ; Sat, 7 Nov 2026 14:06:39 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.a=rsa-sha256 header.s=bombadil.20210309 header.b=vbp5zhVa; dkim=fail reason="signature verification failed" (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=gsBrWpqV 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-Unsubscribe-Post:List-Owner; bh=Qo5ZpZwwn6ee1D34usCF+uZDb+MZU1PIEOBRqCTJzDQ=; b=vbp5zhVaMRh4xDNSKTriaV8JTc OTQybU6iiKHcH57hnOBCo6B/sokq5Xcn+RjLijctG9oM7gx3hMACCplr5HfTBZlhKTQgqMgofO06r 63OUgZo4r8+WUw41i4wcHUacTpFTGxyjsUgh3Zl8bH31NqtSHNI0ILrn+AScEX5XpQQD0fRdrGr/8 9Tm/Iy4Xe8d+r3Bnm0p98gedIGxvVtuyt5E3BfbdLPyu2xvWZcq6j1XULlCScGrRpWJPLeQGZeDzt vhUQoPMGBwJme6+lKME5b3uRpDRm7CishHAeentkBjw8S9Do7F0EYup5WG1QqXXDIyLu1Ws/IE2nn Kmvh9NoA==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.100.1 #2 (Red Hat Linux)) id 1xPh3q-0000000Emqz-3Mt9; Sat, 07 Nov 2026 14:06:26 +0000 Received: from sea.source.kernel.org ([2600:3c0a:e001:78e:0:1991:8:25]) by bombadil.infradead.org with esmtps (Exim 4.100.1 #2 (Red Hat Linux)) id 1xPh3o-0000000EmqF-3yrD for linux-riscv@lists.infradead.org; Sat, 07 Nov 2026 14:06:24 +0000 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id 98CE9417A9; Sat, 7 Nov 2026 14:06:24 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id D414D1F008A6; Sat, 7 Nov 2026 14:06:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794060384; bh=NFvj71R+I3vUUj1f/B2usaeSFh/yQzsOo/5M2fDlCjM=; h=From:To:Cc:Subject:Date; b=gsBrWpqVvPeea8UW8W1X6wu8nhFQdtpfOO6XMRyJz6fkKK4OixBknWQDAI5sMXEG4 qmcx0tvJJCWHEfn9tnI3ZgZXO7aZH+aqDMR4yIoBVgbhzzrzEwG50t1zyvgAUsH8jA Y0aPGxZIb4oPEA5ygJAjUYUyABNYb2JbxrqxcoeUqFDpGxJab87PywX1AV6fprp5Su hHs9Z28fmFQZ0ZraXroaHixm4iHB6obwxWZ4u76AlRthDVII5POQf3VxhDZlvzNLQR rvfkZgR9BDpM3C1XW/lj6ked+l2OprwYB7PmmbcpSREprTBXXvDvN6vNADbOLUX1Vi y+du8uKXJG9sA== From: Jisheng Zhang To: Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexandre Ghiti Cc: linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org Subject: [PATCH RESEND v5 0/3] riscv: word-at-a-time: improve find_zero() Date: Sat, 7 Nov 2026 21:46:10 +0800 Message-ID: <20261107134613.17830-1-jszhang@kernel.org> X-Mailer: git-send-email 2.55.0 X-Archived-By: MIME-Version: 1.0 X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-riscv" X-Spam-Status: No, score=-11.9 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_EF,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_GIT autolearn=ham autolearn_force=no version=4.0.1 X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 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 v4: - fix commit msg of patch1 - collect Reviewed-by tag 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