Re: [PATCH v3 0/3] Optimize GCD performance on RISC-V by selecting implementation at runtime

[Andrew Morton <akpm@linux-foundation.org>]

On Fri,  6 Jun 2025 21:47:55 +0800 Kuan-Wei Chiu <visitorckw@gmail.com> wrote:

> The current implementation of gcd() selects between the binary GCD and
> the odd-even GCD algorithm at compile time, depending on whether
> CONFIG_CPU_NO_EFFICIENT_FFS is set. On platforms like RISC-V, however,
> this compile-time decision can be misleading: even when the compiler
> emits ctz instructions based on the assumption that they are efficient
> (as is the case when CONFIG_RISCV_ISA_ZBB is enabled), the actual
> hardware may lack support for the Zbb extension. In such cases, ffs()
> falls back to a software implementation at runtime, making the binary
> GCD algorithm significantly slower than the odd-even variant.
> 
> To address this, we introduce a static key to allow runtime selection
> between the binary and odd-even GCD implementations. On RISC-V, the
> kernel now checks for Zbb support during boot. If Zbb is unavailable,
> the static key is disabled so that gcd() consistently uses the more
> efficient odd-even algorithm in that scenario. Additionally, to further
> reduce code size, we select CONFIG_CPU_NO_EFFICIENT_FFS automatically
> when CONFIG_RISCV_ISA_ZBB is not enabled, avoiding compilation of the
> unused binary GCD implementation entirely on systems where it would
> never be executed.
> 
> This series ensures that the most efficient GCD algorithm is used in
> practice and avoids compiling unnecessary code based on hardware
> capabilities and kernel configuration.

I removed the v2 series from mm.git and added this, thanks.

v2 was in -next for a month, no issues of which I am aware.