Re: [PATCH] include/uapi/linux/swab.h: add __u16 cast to __swab16 conditional

[Nick Desaulniers <ndesaulniers@google.com>]

On Tue, Jun 7, 2022 at 3:27 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Tue,  7 Jun 2022 15:20:06 -0700 Justin Stitt <jstitt007@gmail.com> wrote:
>
> > if __HAVE_BUILTIN_BSWAP16__ is defined then __swab16 utilizes a __u16 cast.
> > This same cast should be used if __HAVE_BUILTIN_BSWAP16__ is not defined as
> > well. This should fix loads (at least a few) clang -Wformat warnings
> > specifically with `ntohs()`
> >
> > ...
> >
> > --- a/include/uapi/linux/swab.h
> > +++ b/include/uapi/linux/swab.h
> > @@ -102,7 +102,7 @@ static inline __attribute_const__ __u32 __fswahb32(__u32 val)
> >  #define __swab16(x) (__u16)__builtin_bswap16((__u16)(x))
> >  #else
> >  #define __swab16(x)                          \
> > -     (__builtin_constant_p((__u16)(x)) ?     \
> > +     (__u16)(__builtin_constant_p((__u16)(x)) ?      \
> >       ___constant_swab16(x) :                 \
> >       __fswab16(x))
> >  #endif
>
> More explanation, please?  Both ___constant_swab16() and __fswab16()
> return __u16, so why does this patch have any effect?
>

See this example:
https://godbolt.org/z/fzE73jn13
And the ImplicitCastExpr nodes adding to the AST:
https://godbolt.org/z/oYeYxYdKW

Both the second and third operand are promoted to int.

C11: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf

6.5.15/5
>> If both the second and third operands have arithmetic type, the result type that would be determined by the usual arithmetic conversions, were they applied to those two operands, is the type of the result.
6.3.1.8/1
>> Otherwise, the integer promotions are performed on both operands.
6.3.1.1/2
>> If an int can represent all values of the original type (as restricted by the width, for a bit-field), the value is converted to an int; otherwise, it is converted to an unsigned int. These are called the integer promotions.
-- 
Thanks,
~Nick Desaulniers