Re: [RFC][PATCH] new byteorder primitives - ..._{replace,get}_bits()

[Al Viro]

On Tue, Dec 12, 2017 at 03:59:33PM -0800, Jakub Kicinski wrote:
> > +static __always_inline __##type type##_replace_bits(__##type old,	\
> > +					base val, base mask)		\
> > +{									\
> > +	__##type m = to(mask);						\
> > +        if (__builtin_constant_p(val) &&				\
> 
> Is the lack of a __builtin_constant_p(mask) test intentional?  Sometimes
> the bitfield is a packed array and people may have a helper to which
> only the mask is passed as non-constant and the value is implied by the
> helper, thus constant.

If the mask in non-constant, we probably shouldn't be using that at all;
could you show a real-world example where that would be the case?