Re: [PATCH 7/8] lib: add fast path for find_next_*_bit()

'Andy Shevchenko'
2021-02-01T16:24:23Z

On Mon, Feb 01, 2021 at 04:02:30PM +0000, David Laight wrote:
> From: Andy Shevchenko
> > Sent: 01 February 2021 13:49
> > On Sat, Jan 30, 2021 at 11:17:18AM -0800, Yury Norov wrote:
> > > Similarly to bitmap functions, find_next_*_bit() users will benefit
> > > if we'll handle a case of bitmaps that fit into a single word. In the
> > > very best case, the compiler may replace a function call with a
> > > single ffs or ffz instruction.
> >
> > Would be nice to have the examples how it reduces the actual code size (based
> > on the existing code in kernel, especially in widely used frameworks /
> > subsystems, like PCI).
>
> I bet it makes the kernel bigger but very slightly faster.
> But the fact that the wrappers end up in the i-cache may
> mean that inlining actually makes it slower for some calling
> sequences.

> If a bitmap fits in a single word (as a compile-time constant)
> then you should (probably) be using different functions if
> you care about performance.

Isn't this patch series exactly about it?

--
With Best Regards,
Andy Shevchenko