Re: [PATCH treewide v2 1/3] bitfield: Add non-constant field_{prep,get}() helpers

[Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>]

Hi Jakub,

On Tue, 4 Feb 2025 at 16:30, Jakub Kicinski <kuba@xxxxxxxxxx> wrote:
> On Sun, 2 Feb 2025 17:26:04 +0900 Vincent Mailhol wrote:
> > On 31/01/2025 at 22:46, 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.
> >
> > Why is it that the existing FIELD_{GET,PREP}() macros must be limited to
> > compile time constants?
>
> Hard no, some high performance networking drivers use this on
> the fastpath. We want to make sure that the compiler doesn't
> do anything stupid, and decomposes the masks at build time.
>
> The macros work just fine for a *lot* of code:
>
> $ git grep -E 'FIELD_(PREP|GET)\(' | wc -l
> 22407

Indeed.

> BTW aren't u32_get_bits(), u32_replace_bits() etc. not what
> you need in the first place? I think people don't know about
> those, with all due respect the way they are coded up looks
> like an IOCCC submission..

These support only compile-time constants, too.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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