[PATCH v3] RISC-V: Use __builtin_ffs when zbb is enabled

[Adhemerval Zanella Netto]

On 27/01/25 10:11, Anton Blanchard wrote:
> Using __builtin_ffs when zbb is enabled takes __ffs from 26
> instructions to 5.

We already have a similar patch [1] which also cover __riscv_xtheadbb.

[1] https://patchwork.sourceware.org/project/glibc/patch/20241108134143.327-1-julian.oerv@isrc.iscas.ac.cn/

> ---
>  sysdeps/riscv/math-use-builtins-ffs.h | 9 +++++++++
>  1 file changed, 9 insertions(+)
>  create mode 100644 sysdeps/riscv/math-use-builtins-ffs.h
> 
> diff --git a/sysdeps/riscv/math-use-builtins-ffs.h b/sysdeps/riscv/math-use-builtins-ffs.h
> new file mode 100644
> index 0000000000..c2b50b0b62
> --- /dev/null
> +++ b/sysdeps/riscv/math-use-builtins-ffs.h
> @@ -0,0 +1,9 @@
> +/* Expansion of __builtin_ffs using zbb instructions was not implemented
> +   until gcc 12, so we need to check the compiler version.  */
> +#if __GNUC_PREREQ (12, 0) && defined __riscv_zbb
> +#define USE_FFS_BUILTIN   1
> +#define USE_FFSLL_BUILTIN 1
> +#else
> +#define USE_FFS_BUILTIN   0
> +#define USE_FFSLL_BUILTIN 0
> +#endif