linux-kernel - [PATCH v2 2/3] riscv: Optimize gcd() code size when CONFIG_RISCV_ISA

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Message-Id: <20250524155519.1142570-3-visitorckw@gmail.com>
Date: Sat, 24 May 2025 23:55:18 +0800
From: Kuan-Wei Chiu <visitorckw@...il.com>
To: paul.walmsley@...ive.com,
	palmer@...belt.com,
	aou@...s.berkeley.edu,
	alex@...ti.fr,
	akpm@...ux-foundation.org
Cc: linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	jserv@...s.ncku.edu.tw,
	Kuan-Wei Chiu <visitorckw@...il.com>,
	Yu-Chun Lin <eleanor15x@...il.com>
Subject: [PATCH v2 2/3] riscv: Optimize gcd() code size when CONFIG_RISCV_ISA_ZBB is disabled

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=384, After=110, chg -71.35%

Co-developed-by: Yu-Chun Lin <eleanor15x@...il.com>
Signed-off-by: Yu-Chun Lin <eleanor15x@...il.com>
Signed-off-by: Kuan-Wei Chiu <visitorckw@...il.com>
---
 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_FTRACE)
-- 
2.34.1