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SubjectRe: [PATCH v3] include/uapi/linux/swab.h: move explicit cast outside ternary
On Wed, Jun 08, 2022 at 03:35:39PM -0700, Justin Stitt wrote:
> From: Justin Stitt <jstitt007@gmail.com>
>
> A cast inside __builtin_constant_p doesn't do anything since it should
> evaluate as constant at compile time irrespective of this cast. Instead,
> I moved this cast outside the ternary to ensure the return type is as
> expected.
>
> Additionally, if __HAVE_BUILTIN_BSWAP16__ was not defined then __swab16
> is actually returning an `int` not a `u16` due to integer promotion.
>
> As Al Viro notes:
> You *can't* get smaller-than-int out of ? :, same as you can't get it
> out of addition, etc.
>
> This also fixes some clang -Wformat warnings involving default
> argument promotion.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/378
> Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
> Suggested-by: Nathan Chancellor <nathan@kernel.org>
> Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
> Signed-off-by: Justin Stitt <jstitt007@gmail.com>

Thanks for the patch and sticking with it through all the review!

Reviewed-by: Nathan Chancellor <nathan@kernel.org>

> ---
> diff from v2 -> v3:
> * re-insert respective (u16, u32, u64) cast to __builtin_bswap as per
> Nick's suggestion
> * added note from Al Viro
>
> include/uapi/linux/swab.h | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/include/uapi/linux/swab.h b/include/uapi/linux/swab.h
> index 7272f85d6d6a..0723a9cce747 100644
> --- 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(x) ? \
> ___constant_swab16(x) : \
> __fswab16(x))
> #endif
> @@ -115,7 +115,7 @@ static inline __attribute_const__ __u32 __fswahb32(__u32 val)
> #define __swab32(x) (__u32)__builtin_bswap32((__u32)(x))
> #else
> #define __swab32(x) \
> - (__builtin_constant_p((__u32)(x)) ? \
> + (__u32)(__builtin_constant_p(x) ? \
> ___constant_swab32(x) : \
> __fswab32(x))
> #endif
> @@ -128,7 +128,7 @@ static inline __attribute_const__ __u32 __fswahb32(__u32 val)
> #define __swab64(x) (__u64)__builtin_bswap64((__u64)(x))
> #else
> #define __swab64(x) \
> - (__builtin_constant_p((__u64)(x)) ? \
> + (__u64)(__builtin_constant_p(x) ? \
> ___constant_swab64(x) : \
> __fswab64(x))
> #endif
> --
> 2.36.1.476.g0c4daa206d-goog
>
>

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