From nobody Sun Feb 8 08:22:28 2026 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 627F538F24F for ; Tue, 13 Jan 2026 12:43:16 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768308196; cv=none; b=q25mcI1H7bHyOTDfXMNgxzgtjoN14fbqEYqMFJB3tN0ULDSesm8SODq6ATQIeXXs/DsVcp2CESCKXlKCJAIRUxnBxuGihw65Q0CF90P1ERVneArpWEcm0q6ka6CQJLxUqeza8t/Q9GA3FVS0Li5+GtEYcRpQrSZnmbUqHEr1VPk= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768308196; c=relaxed/simple; bh=QvvHlpNsMHZYg0o05xxg11E/+PQv6nRN4oyH5aP5cos=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=jqvDK+wi3/c6eFtgwhKPB5KhPWIEpzRlhEi/yf2FTDJ5SpBr0O+trQDjYh3uqEJlAM0P4L1ietyIkEAEryDp03ux6uOjUaH8hCsv0oSq0jjvKyDjCN2oXRpsFzs45ktmz8Ri1A9BYjpX/1RSO26RcX20dTkw5eEzf9Ufzk52uR4= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=KUCIo3TI; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="KUCIo3TI" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 59AEAC19422; Tue, 13 Jan 2026 12:43:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1768308195; bh=QvvHlpNsMHZYg0o05xxg11E/+PQv6nRN4oyH5aP5cos=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=KUCIo3TIElojfEs8jVB2YUPd6g5It9vKFoc0A9t0opz/APZbJkyMCorodh5fH/gdn jOvxHRlC9qM7sQvOPuQJmuLLuA8PlMt2UmEvvUAOEWc00NkWr/hBjxnABuWmHJZDEe ZcMSsJcEoQ7fhc20F1oT+H+xinFykt0JI9CRs/Tzx39v7yJd08ATZskU1hkbHHqENg ifB5omUCHdeQFCjZTS4KPtOgskFhzbdRnCRMNOXyxwakDpc5Ej9iOWT1cj0I3S57tK ulJkoGjBGRlk4Yi3vmFXMNJZQzN9fmxpoVvx7WYhEw8sbSEmQ2fcJ4khpJ2y5/SLwg iKo/gGKT5rNsA== 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 1/3] riscv: word-at-a-time: improve find_zero() for !RISCV_ISA_ZBB Date: Tue, 13 Jan 2026 20:24:55 +0800 Message-ID: <20260113122457.27507-2-jszhang@kernel.org> X-Mailer: git-send-email 2.51.0 In-Reply-To: <20260113122457.27507-1-jszhang@kernel.org> 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-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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 =3D 0; for (i =3D 0; i < 100000000; i++) ret |=3D 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 | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/arch/riscv/include/asm/word-at-a-time.h b/arch/riscv/include/a= sm/word-at-a-time.h index 3802cda71ab7..0c8a9b337f93 100644 --- a/arch/riscv/include/asm/word-at-a-time.h +++ b/arch/riscv/include/asm/word-at-a-time.h @@ -13,6 +13,9 @@ #include #include =20 +#if !(defined(CONFIG_RISCV_ISA_ZBB) && defined(CONFIG_TOOLCHAIN_HAS_ZBB)) +#include +#else struct word_at_a_time { const unsigned long one_bits, high_bits; }; @@ -47,6 +50,8 @@ static inline unsigned long find_zero(unsigned long mask) /* The mask we created is directly usable as a bytemask */ #define zero_bytemask(mask) (mask) =20 +#endif /* !(defined(CONFIG_RISCV_ISA_ZBB) && defined(CONFIG_TOOLCHAIN_HAS_= ZBB)) */ + #ifdef CONFIG_DCACHE_WORD_ACCESS =20 /* --=20 2.51.0