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([140.113.216.168]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-234396eaf62sm1200745ad.9.2025.05.24.08.55.41 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 24 May 2025 08:55:43 -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 v2 2/3] riscv: Optimize gcd() code size when CONFIG_RISCV_ISA_ZBB is disabled Date: Sat, 24 May 2025 23:55:18 +0800 Message-Id: <20250524155519.1142570-3-visitorckw@gmail.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20250524155519.1142570-1-visitorckw@gmail.com> References: <20250524155519.1142570-1-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. To avoid compiling unused code and reduce code size, select CONFIG_CPU_NO_EFFICIENT_FFS when CONFIG_RISCV_ISA_ZBB is not set. $ ./scripts/bloat-o-meter ./lib/math/gcd.o.old ./lib/math/gcd.o.new add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-274 (-274) Function old new delta gcd 360 86 -274 Total: Before=3D384, After=3D110, chg -71.35% Co-developed-by: Yu-Chun Lin Signed-off-by: Yu-Chun Lin Signed-off-by: Kuan-Wei Chiu --- arch/riscv/Kconfig | 1 + 1 file changed, 1 insertion(+) diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig index bbec87b79309..f085adc6f573 100644 --- a/arch/riscv/Kconfig +++ b/arch/riscv/Kconfig @@ -95,6 +95,7 @@ config RISCV select CLINT_TIMER if RISCV_M_MODE select CLONE_BACKWARDS select COMMON_CLK + select CPU_NO_EFFICIENT_FFS if !RISCV_ISA_ZBB select CPU_PM if CPU_IDLE || HIBERNATION || SUSPEND select EDAC_SUPPORT select FRAME_POINTER if PERF_EVENTS || (FUNCTION_TRACER && !DYNAMIC_FTRAC= E) --=20 2.34.1