Geert Uytterhoeven [off-list ref] writes:

Hi Johannes,

On Mon, Nov 22, 2021 at 5:33 PM Johannes Berg [off-list ref] wrote:

quoted

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?

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

These don't work for non-const masks.

quoted

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

That's a valid comment. Can be fixed by using a wrapper macro
that checks if typeof(mask) == u64, and uses an __ffs64() version when
needed.

quoted

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()?

Are all compilers smart enough to replace the division by
field_multiplier(field) by a shift?

It looks like the answer is no as few weeks back I received a comment
internally that a team is seeing a slow down with u32_get_bits():

"Time taken for executing both the macros/inline function (in terms of microseconds)
(out of 3 Trails)
FIELD_GET	: 32, 31, 32
u32_get_bits	: 6379, 6664, 6558"

Sadly I didn't realise to ask what compiler they were using. But I still
prefer {u32,...}_get_bits() over FIELD_GET(), they are just so much
cleaner to use.

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