Re: [PATCH 01/17] bitfield: Add non-constant field_{prep,get}() helpers

On Mon, 22 Nov 2021 17:32:43 +0100 Johannes Berg wrote:

On Mon, 2021-11-22 at 16:53 +0100, Geert Uytterhoeven wrote:

quoted

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.

I'm not sure it's really a good idea to add a third API here?

+1

We have the upper-case (constant) versions, and already
{u32,...}_get_bits()/etc.

Also, you're using __ffs(), which doesn't work for 64-bit on 32-bit
architectures (afaict), so that seems a bit awkward.

Maybe we can make {u32,...}_get_bits() be doing compile-time only checks
if it is indeed a constant? The __field_overflow() usage is already only
done if __builtin_constant_p(v), so I guess we can do the same with
__bad_mask()?

Either that or add decomposition macros. Are compilers still really bad
at passing small structs by value?