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Patch series "Optimize GCD performance on RISC-V by selecting implementation at runtime", v3. 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. This patch (of 3): On platforms like RISC-V, the compiler may generate hardware FFS instructions even if the underlying CPU does not actually support them. Currently, the GCD implementation is chosen at compile time based on CONFIG_CPU_NO_EFFICIENT_FFS, which can result in suboptimal behavior on such systems. Introduce a static key, efficient_ffs_key, to enable runtime selection between the binary GCD (using ffs) and the odd-even GCD implementation. This allows the kernel to default to the faster binary GCD when FFS is efficient, while retaining the ability to fall back when needed. Link: https://lkml.kernel.org/r/20250606134758.1308400-1-visitorckw@gmail.com Link: https://lkml.kernel.org/r/20250606134758.1308400-2-visitorckw@gmail.com 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> Cc: Albert Ou <aou@eecs.berkeley.edu> Cc: Ching-Chun (Jim) Huang <jserv@ccns.ncku.edu.tw> Cc: Palmer Dabbelt <palmer@dabbelt.com> Cc: Paul Walmsley <paul.walmsley@sifive.com> Cc: Alexandre Ghiti <alexghiti@rivosinc.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> |
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| .. | ||
| tests | ||
| cordic.c | ||
| div64.c | ||
| gcd.c | ||
| int_log.c | ||
| int_pow.c | ||
| int_sqrt.c | ||
| Kconfig | ||
| lcm.c | ||
| Makefile | ||
| prime_numbers.c | ||
| prime_numbers_private.h | ||
| rational.c | ||
| reciprocal_div.c | ||
| test_div64.c | ||
| test_mul_u64_u64_div_u64.c | ||