Re: [PATCH treewide v3 2/4] bitfield: Add non-constant field_{prep,get}() helpers

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi David,

On Fri, 14 Feb 2025 at 17:46, David Laight <david.laight.linux@gmail.com> wrote:
> On Fri, 14 Feb 2025 07:34:02 -0800
> Jakub Kicinski <kuba@kernel.org> wrote:
> > On Fri, 14 Feb 2025 14:55:51 +0100 Geert Uytterhoeven wrote:
> > > The existing FIELD_{GET,PREP}() macros are limited to compile-time
> > > constants.  However, it is very common to prepare or extract bitfield
> > > elements where the bitfield mask is not a compile-time constant.
> > >
> > > To avoid this limitation, the AT91 clock driver and several other
> > > drivers already have their own non-const field_{prep,get}() macros.
> > > Make them available for general use by consolidating them in
> > > <linux/bitfield.h>, and improve them slightly:
> > >   1. Avoid evaluating macro parameters more than once,
> > >   2. Replace "ffs() - 1" by "__ffs()",
> > >   3. Support 64-bit use on 32-bit architectures.
> > >
> > > This is deliberately not merged into the existing FIELD_{GET,PREP}()
> > > macros, as people expressed the desire to keep stricter variants for
> > > increased safety, or for performance critical paths.
> >
> > I really really think that people should just use the static inline
> > helpers if the field is not constant. And we should do something like
> > below so that people can actually find them.
>
> Especially since you really don't want to be calling ffs() on variables.

It is not that bad, as most temporary architectures have an instruction
for that.

> Much better to have saved the low bit and field width/mask.

While that would allow some space saving (only 10 or 12 bits needed to
store low + width), gcc would generate quite some code to create the
mask (even on PowerPC, where I expected a single instruction would
do ;-).


Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds