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

[Kuan-Wei Chiu <visitorckw@gmail.com>]

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.

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

---
This series has been tested on QEMU to verify that the correct GCD
implementation is used both with and without Zbb support.

v2 -> v3:
- Drop if (!a || !b) check in binary_gcd()
- Move DECLARE_STATIC_KEY_TRUE(efficient_ffs_key) to gcd.h
v1 -> v2:
- Use a static key to select the GCD implementation at runtime.

v2: https://lore.kernel.org/lkml/20250524155519.1142570-1-visitorckw@gmail.com/
v1: https://lore.kernel.org/lkml/20250217013708.1932496-1-visitorckw@gmail.com/

Kuan-Wei Chiu (3):
  lib/math/gcd: Use static key to select implementation at runtime
  riscv: Optimize gcd() code size when CONFIG_RISCV_ISA_ZBB is disabled
  riscv: Optimize gcd() performance on RISC-V without Zbb extension

 arch/riscv/Kconfig        |  1 +
 arch/riscv/kernel/setup.c |  5 +++++
 include/linux/gcd.h       |  3 +++
 lib/math/gcd.c            | 27 +++++++++++++++------------
 4 files changed, 24 insertions(+), 12 deletions(-)

-- 
2.34.1