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([140.113.216.168]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-7482b0c1168sm1296798b3a.136.2025.06.06.06.48.04 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 06 Jun 2025 06:48:06 -0700 (PDT) From: Kuan-Wei Chiu To: paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu, alex@ghiti.fr, akpm@linux-foundation.org Cc: linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, jserv@ccns.ncku.edu.tw, Kuan-Wei Chiu , Yu-Chun Lin Subject: [PATCH v3 1/3] lib/math/gcd: Use static key to select implementation at runtime Date: Fri, 6 Jun 2025 21:47:56 +0800 Message-Id: <20250606134758.1308400-2-visitorckw@gmail.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20250606134758.1308400-1-visitorckw@gmail.com> References: <20250606134758.1308400-1-visitorckw@gmail.com> 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" On platforms like RISC-V, the compiler may generate hardware FFS instructions even if the underlying CPU does not actually support them. Currently, the GCD implementation is chosen at compile time based on CONFIG_CPU_NO_EFFICIENT_FFS, which can result in suboptimal behavior on such systems. Introduce a static key, efficient_ffs_key, to enable runtime selection between the binary GCD (using ffs) and the odd-even GCD implementation. This allows the kernel to default to the faster binary GCD when FFS is efficient, while retaining the ability to fall back when needed. Co-developed-by: Yu-Chun Lin Signed-off-by: Yu-Chun Lin Signed-off-by: Kuan-Wei Chiu --- include/linux/gcd.h | 3 +++ lib/math/gcd.c | 27 +++++++++++++++------------ 2 files changed, 18 insertions(+), 12 deletions(-) diff --git a/include/linux/gcd.h b/include/linux/gcd.h index cb572677fd7f..616e81a7f7e3 100644 --- a/include/linux/gcd.h +++ b/include/linux/gcd.h @@ -3,6 +3,9 @@ #define _GCD_H =20 #include +#include + +DECLARE_STATIC_KEY_TRUE(efficient_ffs_key); =20 unsigned long gcd(unsigned long a, unsigned long b) __attribute_const__; =20 diff --git a/lib/math/gcd.c b/lib/math/gcd.c index e3b042214d1b..62efca6787ae 100644 --- a/lib/math/gcd.c +++ b/lib/math/gcd.c @@ -11,22 +11,16 @@ * has decent hardware division. */ =20 +DEFINE_STATIC_KEY_TRUE(efficient_ffs_key); + #if !defined(CONFIG_CPU_NO_EFFICIENT_FFS) =20 /* If __ffs is available, the even/odd algorithm benchmarks slower. */ =20 -/** - * gcd - calculate and return the greatest common divisor of 2 unsigned lo= ngs - * @a: first value - * @b: second value - */ -unsigned long gcd(unsigned long a, unsigned long b) +static unsigned long binary_gcd(unsigned long a, unsigned long b) { unsigned long r =3D a | b; =20 - if (!a || !b) - return r; - b >>=3D __ffs(b); if (b =3D=3D 1) return r & -r; @@ -44,9 +38,15 @@ unsigned long gcd(unsigned long a, unsigned long b) } } =20 -#else +#endif =20 /* If normalization is done by loops, the even/odd algorithm is a win. */ + +/** + * gcd - calculate and return the greatest common divisor of 2 unsigned lo= ngs + * @a: first value + * @b: second value + */ unsigned long gcd(unsigned long a, unsigned long b) { unsigned long r =3D a | b; @@ -54,6 +54,11 @@ unsigned long gcd(unsigned long a, unsigned long b) if (!a || !b) return r; =20 +#if !defined(CONFIG_CPU_NO_EFFICIENT_FFS) + if (static_branch_likely(&efficient_ffs_key)) + return binary_gcd(a, b); +#endif + /* Isolate lsbit of r */ r &=3D -r; =20 @@ -80,6 +85,4 @@ unsigned long gcd(unsigned long a, unsigned long b) } } =20 -#endif - EXPORT_SYMBOL_GPL(gcd); --=20 2.34.1 From nobody Fri Dec 19 16:03:30 2025 Received: from mail-pf1-f180.google.com (mail-pf1-f180.google.com [209.85.210.180]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 0F65028980E for ; 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([140.113.216.168]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-7482b0c1168sm1296798b3a.136.2025.06.06.06.48.08 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 06 Jun 2025 06:48:09 -0700 (PDT) From: Kuan-Wei Chiu To: paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu, alex@ghiti.fr, akpm@linux-foundation.org Cc: linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, jserv@ccns.ncku.edu.tw, Kuan-Wei Chiu , Yu-Chun Lin Subject: [PATCH v3 2/3] riscv: Optimize gcd() code size when CONFIG_RISCV_ISA_ZBB is disabled Date: Fri, 6 Jun 2025 21:47:57 +0800 Message-Id: <20250606134758.1308400-3-visitorckw@gmail.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20250606134758.1308400-1-visitorckw@gmail.com> References: <20250606134758.1308400-1-visitorckw@gmail.com> 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" The binary GCD implementation depends on efficient ffs(), which on RISC-V requires hardware support for the Zbb extension. When CONFIG_RISCV_ISA_ZBB is not enabled, the kernel will never use binary GCD, as runtime logic will always fall back to the odd-even implementation. 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([140.113.216.168]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-7482b0c1168sm1296798b3a.136.2025.06.06.06.48.11 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 06 Jun 2025 06:48:12 -0700 (PDT) From: Kuan-Wei Chiu To: paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu, alex@ghiti.fr, akpm@linux-foundation.org Cc: linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, jserv@ccns.ncku.edu.tw, Kuan-Wei Chiu , Yu-Chun Lin Subject: [PATCH v3 3/3] riscv: Optimize gcd() performance on RISC-V without Zbb extension Date: Fri, 6 Jun 2025 21:47:58 +0800 Message-Id: <20250606134758.1308400-4-visitorckw@gmail.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20250606134758.1308400-1-visitorckw@gmail.com> References: <20250606134758.1308400-1-visitorckw@gmail.com> 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" The binary GCD implementation uses FFS (find first set), which benefits from hardware support for the ctz instruction, provided by the Zbb extension on RISC-V. Without Zbb, this results in slower software-emulated behavior. Previously, RISC-V always used the binary GCD, regardless of actual hardware support. This patch improves runtime efficiency by disabling the efficient_ffs_key static branch when Zbb is either not enabled in the kernel (config) or not supported on the executing CPU. This selects the odd-even GCD implementation, which is faster in the absence of efficient FFS. This change ensures the most suitable GCD algorithm is chosen dynamically based on actual hardware capabilities. Co-developed-by: Yu-Chun Lin Signed-off-by: Yu-Chun Lin Signed-off-by: Kuan-Wei Chiu Acked-by: Alexandre Ghiti --- arch/riscv/kernel/setup.c | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c index f7c9a1caa83e..785c7104fde7 100644 --- a/arch/riscv/kernel/setup.c +++ b/arch/riscv/kernel/setup.c @@ -21,6 +21,8 @@ #include #include #include +#include +#include =20 #include #include @@ -361,6 +363,9 @@ void __init setup_arch(char **cmdline_p) =20 riscv_user_isa_enable(); riscv_spinlock_init(); + + if (!IS_ENABLED(CONFIG_RISCV_ISA_ZBB) || !riscv_isa_extension_available(N= ULL, ZBB)) + static_branch_disable(&efficient_ffs_key); } =20 bool arch_cpu_is_hotpluggable(int cpu) --=20 2.34.1