Re: [PATCH 1/9] bitfield: add FIELD_GET_SIGNED()

David Laight
2026-04-17T19:43:58Z

On Fri, 17 Apr 2026 13:36:12 -0400
Yury Norov <ynorov@nvidia.com> wrote:

> The bitfields are designed in assumption that fields contain unsigned
> integer values, thus extracting the values from the field implies
> zero-extending.
>
> Some drivers need to sign-extend their fields, and currently do it like:
>
> dc_re += sign_extend32(FIELD_GET(0xfff000, tmp), 11);
> dc_im += sign_extend32(FIELD_GET(0xfff, tmp), 11);
>
> It's error-prone because it relies on user to provide the correct
> index of the most significant bit and proper 32 vs 64 function flavor.
>
> Thus, introduce a FIELD_GET_SIGNED() macro, which is the more
> convenient and compiles (on x86_64) to just a couple instructions:
> shl and sar.
>
> Signed-off-by: Yury Norov <ynorov@nvidia.com>
> ---
> include/linux/bitfield.h | 16 ++++++++++++++++
> 1 file changed, 16 insertions(+)
>
> diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
> index 54aeeef1f0ec..35ef63972810 100644
> --- a/include/linux/bitfield.h
> +++ b/include/linux/bitfield.h
> @@ -178,6 +178,22 @@
> __FIELD_GET(_mask, _reg, "FIELD_GET: "); \
> })
>
> +/**
> + * FIELD_GET_SIGNED() - extract a signed bitfield element
> + * @mask: shifted mask defining the field's length and position
> + * @reg: value of entire bitfield
> + *
> + * Returns the sign-extended field specified by @_mask from the
> + * bitfield passed in as @_reg by masking and shifting it down.
> + */
> +#define FIELD_GET_SIGNED(mask, reg) \
> + ({ \
> + __BF_FIELD_CHECK(mask, reg, 0U, "FIELD_GET_SIGNED: "); \
> + ((__signed_scalar_typeof(mask))((long long)(reg) << \
> + __builtin_clzll(mask) >> (__builtin_clzll(mask) + \
> + __builtin_ctzll(mask))));\

Have you looked at what that generates on a typical 32bit architecture?

It really a bad idea to use __signed_scalar_typeof() on anything that isn't
a simple variable.
The bloat from all this when 'mask' is an expansion of GENMASK() is horrid.
Indeed both signed_scalar_typeof() and unsigned_scalar_typeof() should
really not be used - there are generally much better ways.

In this case you can just write:
({
auto _mask = mask;
unsigned int __sl = __builtin_clzll(_mask);
unsigned int __sr = __sl + __builtin_ctzll(_mask);
__builtin_chose_expr(sizeof(_mask) <= 4,
(int)(reg) << __sl - 32 >> __sr - 32,
((long long)(reg) << __sl >> __sr)
})
and let the compiler do any more integer promotions (etc).

I'm also not convinced that the checks __BF_FIELD_CHECK() does
on 'reg' are in any sense worth the effort.

I have tried some simpler alternatives, eg:
!__builtin_constant_p(reg) && statically_true((reg & mask) == 0)
however that throws up some false positives due to some of weird ways
people have used FIELD_GET() where it is nothing like the simplest
(or most obvious) way to do things.
That might have been the code that split a 32bit value into bytes
in a printf with:
FIELD_GET(GENMASK(7, 0), val), FIELD_GET(GENMASK(15, 8), val),
FIELD_GET(GENMASK(23, 16), val), FIELD_GET(GENMASK(31, 24), val),

David

> + })
> +
> /**
> * FIELD_MODIFY() - modify a bitfield element
> * @_mask: shifted mask defining the field's length and position