Re: [PATCH] bitfield: Fix FIELD_PREP_CONST() with __GENMASK_ULL() on gcc<14

[David Laight]

On Thu, 9 Apr 2026 15:57:53 +0100
Matt Coster <matt.coster@xxxxxxxxxx> wrote:

> There is a bug in gcc<14[1] that causes the following minimal example to
> not be treated as a constant:
> 
>   int main() {
>       sizeof(struct {
>           int t : !(__builtin_ffsll(~0ULL) + 1 < 0);
>       });
>   }
> 
>   test.c: In function 'main':
>   test.c:3:21: error: bit-field 't' width not an integer constant
>       3 |                 int t : !(__builtin_ffsll(~0ULL) + 1 < 0);
>         |                     ^
> 
> The result of this bug is a bizarre interaction between FIELD_PREP_CONST()
> and __GENMASK_ULL(). Note that this does not occur with GENMASK_ULL() since
> that has not been based on the UAPI variant since commit 104ea1c84b91
> ("bits: unify the non-asm GENMASK*()").

Can you give the actual code that is failing.
I can only find the old version - that is pretty horrid itself.
It really isn't necessary to use GENMASK_ULL(15, 0) to get 0xffffu.

I've bumped into that file before.
It gave a false positive for FIELD_GET() returning a constant value
for non-constant input - which would be a nice check for invalid usage.

Someone went over the top on getting the pre-processor to bloat the
intermediate file by using the longest way possible to perform the
simplest of code patterns.
I suspect they've made it worse.

> 
> The underlying compiler bug has been fixed in gcc since 14.1.0, but the fix
> appears to have been incidental in a larger change[2].


> 
> [1]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124699
> [2]: https://gcc.gnu.org/cgit/gcc/commit/?id=0d00385eaf72ccacff17935b0d214a26773e095f
> 
> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> Closes: https://lore.kernel.org/oe-kbuild-all/202603222211.A2XiR1YU-lkp@xxxxxxxxx/
> Signed-off-by: Matt Coster <matt.coster@xxxxxxxxxx>
> ---
> Some below-the-dash thoughts:
> 
> This is the most minimal workaround I could find. I'm not sure what a
> "real" fix would look like here so I'm open to suggestions.

Using a completely different expression instead of ffs() and shifts.
For constants it doesn't matter what you do.
Multiply/divide by mask & (~mask + 1) will be the same.
(I think it is used lower down the file.)

But passing the output of GENMASK() (any variant) into FIELD_PREP()
(any current variant) is a good way to generate pre-processor output
lines that are many kb long.

Oh, in my experiments, the value passed to ffsll() must be unsigned
and have the top bit set.

	David

> 
> The reproduction case is amazing because changing almost any subtle
> detail of it will result in the expression correctly being parsed as
> constant (even raising the construct to file scope; the use of
> BUILD_BUG_ON_ZERO() is part of the bizarre confluence required to
> trigger the bug).
> 
> The complexity of the associated change in gcc makes it difficult to
> trace what actually changed to fix the underlying bug. If I had more
> time, I'd have dug in further and (tried to) come up with an answer.
> 
> In reality, the (long long) cast could probably just be univerally
> applied without the conditional compilation. My only concern with that
> approach is that it risks turning the workaround into an arcane
> incantation whose meaning will eventually have been lost to time.
> ---
>  include/linux/bitfield.h | 30 +++++++++++++++++++++++++++---
>  1 file changed, 27 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
> index 54aeeef1f0ec7..12f5c5a3c8d72 100644
> --- a/include/linux/bitfield.h
> +++ b/include/linux/bitfield.h
> @@ -46,6 +46,30 @@
>  
>  #define __bf_shf(x) (__builtin_ffsll(x) - 1)
>  
> +#if defined(GCC_VERSION) && (GCC_VERSION < 140000)
> +/*
> + * This workaround is required for gcc<14. The issue is an interaction between
> + * FIELD_PREP_CONST() and __GENMASK_ULL() that can be boiled down this MCVE,
> + * which fails to compile as GCC doesn't recognize the expression as constant:
> + *
> + *   int main() {
> + *       sizeof(struct {
> + *           int t : !(__builtin_ffsll(~0ULL) + 1 < 0);
> + *       });
> + *   }
> + *
> + * The underlying issue was inadvertently "fixed" (or perhaps sidestepped) in
> + * commit 0d00385eaf7 ("wide-int: Allow up to 16320 bits wide_int and change
> + * widest_int precision to 32640 bits [PR102989]"), which first appeared in
> + * GCC 14.1.
> + *
> + * See: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124699
> + */
> +#define __const_bf_shf(x) __bf_shf((long long)(x))
> +#else
> +#define __const_bf_shf(x) __bf_shf(x)
> +#endif
> +
>  #define __scalar_type_to_unsigned_cases(type)				\
>  		unsigned type:	(unsigned type)0,			\
>  		signed type:	(unsigned type)0
> @@ -157,11 +181,11 @@
>  		/* mask must be non-zero */				\
>  		BUILD_BUG_ON_ZERO((_mask) == 0) +			\
>  		/* check if value fits */				\
> -		BUILD_BUG_ON_ZERO(~((_mask) >> __bf_shf(_mask)) & (_val)) + \
> +		BUILD_BUG_ON_ZERO(~((_mask) >> __const_bf_shf(_mask)) & (_val)) + \
>  		/* check if mask is contiguous */			\
> -		__BF_CHECK_POW2((_mask) + (1ULL << __bf_shf(_mask))) +	\
> +		__BF_CHECK_POW2((_mask) + (1ULL << __const_bf_shf(_mask))) + \
>  		/* and create the value */				\
> -		(((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask))	\
> +		(((typeof(_mask))(_val) << __const_bf_shf(_mask)) & (_mask)) \
>  	)
>  
>  /**
> 
> ---
> base-commit: b20a9b5f9c4baeae0b2e143046b195b910c59714
> change-id: 20260324-field-prep-fix-fdfc4eb68156
> 
>