Re: [PATCH v6 0/2] x86/asm/bitops: optimize ff{s,z} functions for constant expressions

Yury Norov
2022-08-31T08:54:38Z

On Wed, Aug 31, 2022 at 04:57:40PM +0900, Vincent Mailhol wrote:
> The compilers provide some builtin expression equivalent to the ffs(),
> __ffs() and ffz() functions of the kernel. The kernel uses optimized
> assembly which produces better code than the builtin
> functions. However, such assembly code can not be folded when used
> with constant expressions.
>
> This series relies on __builtin_constant_p to select the optimal solution:
>
> * use kernel assembly for non constant expressions
>
> * use compiler's __builtin function for constant expressions.
>
>
> ** Statistics **
>
> Patch 1/2 optimizes 26.7% of ffs() calls and patch 2/2 optimizes 27.9%
> of __ffs() and ffz() calls (details of the calculation in each patch).

Hi Vincent,

Can you please add a test for this? We've recently added a very similar
test_bitmap_const_eval() in lib/test_bitmap.c.

dc34d5036692c ("lib: test_bitmap: add compile-time optimization/evaluations
assertions")

Would be nice to have something like this for ffs() and ffz() in
lib/test_bitops.c.

Please keep me in loop in case of new versions.

Thanks,
Yury