From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6EEB63F483C for ; Mon, 17 Aug 2026 15:37:57 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786981078; cv=none; b=Tn7T70rkmhe35CK84tthHS+XJWVIYeguYyPjU3nHXoGNzFj9uT4yj5G4JLLIRZpj31DMlEgLTbCqwFRXby2Vi6x1gy3BJ34dTIMGkLNGAUK/oCUs5SNJyVsEP/1w3hYnKL5GrIS+NF5MkvI3x/dtVq2q+dMIQoXnNKpzuyp81lo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786981078; c=relaxed/simple; bh=+5arUmHf/2/qFlWEzjVoIweswzOhSgm2G4xAM9CUGuE=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=MRtewdb0QsKIGC/vfkeFH8OkREEfMHgTbAnJaIl1TlTfzEqXXuOrgTkWTQ8c55Jzgh2SQ/2AfQy8T59SwcsDg1Dg/nlYOfsfj9/Zb2MZvzI7WccrvqGPjhlK1eGik7hJLOyxbRwq7cjnaezUjv0JvABPS+WqqMbuVzYU1Wh3ynk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=lQSS5Yo+; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="lQSS5Yo+" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2F3EC1F000E9; Mon, 17 Aug 2026 15:37:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1786981077; bh=EQYv+Pegy1JUyIfOBX3dvwkCCsiDghlnpEv268jTriI=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=lQSS5Yo+sm0YlDB1xoxbKEdEQE1sQpPLGIRutIidNwheE+LQSHnqFu5xAUas5NC7D DULA4Z3qeortr85tjZy/MuOVWn6u+8sL9AQinv4ncR55nJTeoTF7oViwCZbKU9CWRk qHP1V9tSUtmdmDJMzuaedLfRHKv171olpUMf6ZEQFxHJdNLXwAxzbIM1Plpm1+FO6C lqtUAKfcha1U671GiUQQE0l+61UBG92jZMiimGz8qE0MokNx2Sj/3i4dM/FPMMH57Z l9opBdF1P9Vcus8A7tP7q40mCcAScBhzSyVE6+Fi3XGJ7BWU13CUrGf2zFBA+mxJzB 0fM5XFQIzrVyA== Date: Mon, 17 Aug 2026 23:18:16 +0800 From: Jisheng Zhang To: Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexandre Ghiti Cc: linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 0/3] riscv: word-at-a-time: improve find_zero() Message-ID: References: <20260113122457.27507-1-jszhang@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20260113122457.27507-1-jszhang@kernel.org> On Tue, Jan 13, 2026 at 08:24:54PM +0800, Jisheng Zhang wrote: > 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. Soft ping. No feedback for more than 6 months. Fortunately, this series can still be cleanly applied. Thanks > > 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 | 47 +++++++++++++++++++++++-- > 1 file changed, 44 insertions(+), 3 deletions(-) > > -- > 2.51.0 > > > _______________________________________________ > linux-riscv mailing list > linux-riscv@lists.infradead.org > http://lists.infradead.org/mailman/listinfo/linux-riscv