linux-kernel - Re: [PATCH v3] include/uapi/linux/swab.h: move explicit cast outside ternary

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Date:   Thu, 9 Jun 2022 13:31:40 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Justin Stitt <jstitt007@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [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@...il.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@...iv.linux.org.uk>
> Suggested-by: Nathan Chancellor <nathan@...nel.org>
> Suggested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Signed-off-by: Justin Stitt <jstitt007@...il.com>

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

Reviewed-by: Nathan Chancellor <nathan@...nel.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
> 
>