linux-kernel - Re: [PATCH] include/uapi/linux/swab.h: add __u16 cast to _

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Date:   Tue, 7 Jun 2022 15:27:44 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Justin Stitt <jstitt007@...il.com>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH] include/uapi/linux/swab.h: add __u16 cast to __swab16
 conditional

On Tue,  7 Jun 2022 15:20:06 -0700 Justin Stitt <jstitt007@...il.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?