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 7456C1E5B70 for ; Wed, 9 Sep 2026 00:13:02 +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=1788912783; cv=none; b=qqlhIf8keOGkoXKSl9rq9jFWSnUCbSUKhuvI2IHYpQzbE2sr6mGx84oRQ35QxvmgXxwYFClf0PcDZ0xga8DJp501bbBEKitckD0wN8UugYW/5DDpysTFvjlae+DbFy7ZneNfvXFkZ888XlQoKgExjWqYHkoWUUuxTfqxX1hje3w= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788912783; c=relaxed/simple; bh=1QsG25l8O2VLEuh25KrJgG3bL6h9kYPyJ9RRUufkS8U=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=GRmnYgzPCEL7lDkPbjarP11zlm1iPSryBCmRYRb+rIosrkMdj4itsHbZ8Xu3jLdllQyKXOy612jhqy7Vwa1TG2PKPNvwdSsyrnCzJh0CqNnI5yulTe7aNEjUl3ZuO0ro55i9HQBiI7S1jtQ12GpVWe2qgkZnY1md+zzZDzWf7NA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Flm3snO4; 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="Flm3snO4" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 799451F00A3E; Wed, 9 Sep 2026 00:13:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788912782; bh=WvJFOaQphIvH9VnjYWr9Zi/URbyT/0/TQJJh1ji7GNg=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=Flm3snO4XHFwJvNNSOJWDdMXgKq63xYrR0yCU9HmHWUYALOTLFFWDHr6+5aM1yHQA U1VlqaZ2VLvHQCmLMUIBUIn7rdggbvqEk1/dp9CjXobUS0XFEtqgL1tdtX0F4iBgUR 6ngnJ8Wp3hSI7NiPVbFnB4hoDuHSf6zWPCeZKkbMJOfRnGlDGG94j+Gm3P/cCuJUSE UjUmntKt7FIFvczywMzuhiAIhDB+Q5GyQbLcwdJEgS00FTPkKC/tCNZ2rBsNEEIUdD 6VzilnJSKQSK/AYm7KJ9qQSkxQUxUQbCkbiibrP9dN63CWjs4QTm8j6scXh5ZpqCtm D+a5gvxiNi0Wg== 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 v3 1/3] riscv: word-at-a-time: improve find_zero() for !RISCV_ISA_ZBB Date: Wed, 9 Sep 2026 07:53:10 +0800 Message-ID: <20260908235312.8440-2-jszhang@kernel.org> X-Mailer: git-send-email 2.51.0 In-Reply-To: <20260908235312.8440-1-jszhang@kernel.org> References: <20260908235312.8440-1-jszhang@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Current find_zero() heavily depends on fls64() for calculation. This bring non-optimal code when !RISCV_ISA_ZBB. 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. Before the patch: 0000000000000000 : 0: c529 beqz a0,4a <.L1> 2: 577d li a4,-1 4: 9301 srli a4,a4,0x20 6: 03f00793 li a5,63 a: 00a76463 bltu a4,a0,12 <.L3> e: 1502 slli a0,a0,0x20 10: 47fd li a5,31 0000000000000012 <.L3>: 12: 577d li a4,-1 14: 8341 srli a4,a4,0x10 16: 00a76463 bltu a4,a0,1e <.L4> 1a: 37c1 addiw a5,a5,-16 1c: 0542 slli a0,a0,0x10 000000000000001e <.L4>: 1e: 577d li a4,-1 20: 8321 srli a4,a4,0x8 22: 00a76463 bltu a4,a0,2a <.L5> 26: 37e1 addiw a5,a5,-8 28: 0522 slli a0,a0,0x8 000000000000002a <.L5>: 2a: 577d li a4,-1 2c: 8311 srli a4,a4,0x4 2e: 00a76463 bltu a4,a0,36 <.L6> 32: 37f1 addiw a5,a5,-4 34: 0512 slli a0,a0,0x4 0000000000000036 <.L6>: 36: 577d li a4,-1 38: 8309 srli a4,a4,0x2 3a: 00a76463 bltu a4,a0,42 <.L7> 3e: 37f9 addiw a5,a5,-2 40: 050a slli a0,a0,0x2 0000000000000042 <.L7>: 42: 00054563 bltz a0,4c <.L12> 46: 4037d51b sraiw a0,a5,0x3 000000000000004a <.L1>: 4a: 8082 ret 000000000000004c <.L12>: 4c: 2785 addiw a5,a5,1 4e: 4037d51b sraiw a0,a5,0x3 52: 8082 ret After the patch: 0000000000000000 : 0: 102037b7 lui a5,0x10203 4: 0792 slli a5,a5,0x4 6: 40578793 addi a5,a5,1029 # 10203405 <.L4+0x102033c5> a: 07c2 slli a5,a5,0x10 c: 60878793 addi a5,a5,1544 10: 02f50533 mul a0,a0,a5 14: 9161 srli a0,a0,0x38 16: 8082 ret 33 instructions vs 8 instructions! And this kind of instructions reducing dramatically improves the performance of below micro-benchmark: $ cat tt.c #inlcude #inlcude "word-at-a-time.h" // copy and modify, eg. remove other headers int main() { int i; unsigned long ret = 0; for (i = 0; i < 100000000; i++) ret |= find_zero(0xabcd123 + i); printf("%ld\n", ret); } $ gcc -O tt.c $ time ./a.out Per my test, the above micro-benchmark is improved by about 1150%! Signed-off-by: Jisheng Zhang --- arch/riscv/include/asm/word-at-a-time.h | 29 ++++++++++++++++++++++++- 1 file changed, 28 insertions(+), 1 deletion(-) diff --git a/arch/riscv/include/asm/word-at-a-time.h b/arch/riscv/include/asm/word-at-a-time.h index 3802cda71ab7..1dcac57a73dd 100644 --- a/arch/riscv/include/asm/word-at-a-time.h +++ b/arch/riscv/include/asm/word-at-a-time.h @@ -39,9 +39,36 @@ static inline unsigned long create_zero_mask(unsigned long bits) return bits >> 7; } +#ifdef CONFIG_64BIT +/* + * Jan Achrenius on G+: microoptimized version of + * the simpler "(mask & ONEBYTES) * ONEBYTES >> 56" + * that works for the bytemasks without having to + * mask them first. + */ +static inline long count_masked_bytes(unsigned long mask) +{ + return mask*0x0001020304050608ul >> 56; +} + +#else /* 32-bit case */ + +/* Carl Chatfield / Jan Achrenius G+ version for 32-bit */ +static inline long count_masked_bytes(long mask) +{ + /* (000000 0000ff 00ffff ffffff) -> ( 1 1 2 3 ) */ + long a = (0x0ff0001+mask) >> 23; + /* Fix the 1 for 00 case */ + return a & mask; +} +#endif + static inline unsigned long find_zero(unsigned long mask) { - return fls64(mask) >> 3; + if (IS_ENABLED(CONFIG_RISCV_ISA_ZBB) && IS_ENABLED(CONFIG_TOOLCHAIN_HAS_ZBB)) + return fls64(mask) >> 3; + + return count_masked_bytes(mask); } /* The mask we created is directly usable as a bytemask */ -- 2.53.0