linux-kernel - Re: [PATCH v3 2/3] riscv: Optimize gcd() code size when CONFIG_RISCV_ISA

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Message-ID: <b74f5717-bab6-4d8c-8afb-fe3fcb6e29ff@ghiti.fr>
Date: Thu, 12 Jun 2025 14:59:24 +0200
From: Alexandre Ghiti <alex@...ti.fr>
To: Kuan-Wei Chiu <visitorckw@...il.com>, paul.walmsley@...ive.com,
 palmer@...belt.com, aou@...s.berkeley.edu, akpm@...ux-foundation.org
Cc: linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
 jserv@...s.ncku.edu.tw, Yu-Chun Lin <eleanor15x@...il.com>
Subject: Re: [PATCH v3 2/3] riscv: Optimize gcd() code size when
 CONFIG_RISCV_ISA_ZBB is disabled

Hi Kuan-Wei,

On 6/6/25 15:47, Kuan-Wei Chiu wrote:
> 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)


In v2, Andrew asked if he could merge it, so:

Acked-by: Alexandre Ghiti <alexghiti@...osinc.com>

Thanks,

Alex