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

Yury Norov
2022-09-01T03:51:49Z

On Wed, Aug 31, 2022 at 01:54:01AM -0700, Yury Norov wrote:
> 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.

Also, what about fls? Is there any difference with ffs/ffz wrt compile
time optimizations? If not, would be great if the series will take
care of it too.

Thanks,
Yury