Re: [PATCH, RFT] byteswap: try to avoid __builtin_constant_p gcc bug

[Josh Poimboeuf]

On Thu, Apr 28, 2016 at 12:00:36AM +0200, Arnd Bergmann wrote:
> This is another attempt to avoid a regression in wwn_to_u64()
> after that started using get_unaligned_be64(), which in turn
> ran into a bug on gcc-4.9 through 6.1.
> 
> As part of the problem is how __builtin_constant_p gets evaluated
> on an argument passed by reference into an inline function, this
> avoids the use of __builtin_constant_p() for all architectures
> that set CONFIG_ARCH_USE_BUILTIN_BSWAP. Most architectures do not
> set ARCH_SUPPORTS_OPTIMIZED_INLINING, which means they probably
> do not suffer from the problem in the qla2xxx driver, but they
> might still run into it elsewhere.
> 
> I have not been able to reproduce the original problem, so I don't
> know if this patch solves it, but at least it leads to simpler
> code doing the same thing, so at least there should be no downsides.
> 
> Please test.
> 
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>

Nice patch.  I can confirm it fixes the issue with gcc 5.3.1.

Tested-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Reviewed-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>

> diff --git a/include/uapi/linux/swab.h b/include/uapi/linux/swab.h
> index 3f10e5317b46..de56fd54428d 100644
> --- a/include/uapi/linux/swab.h
> +++ b/include/uapi/linux/swab.h
> @@ -45,9 +45,7 @@
>  
>  static inline __attribute_const__ __u16 __fswab16(__u16 val)
>  {
> -#ifdef __HAVE_BUILTIN_BSWAP16__
> -	return __builtin_bswap16(val);
> -#elif defined (__arch_swab16)
> +#if defined (__arch_swab16)
>  	return __arch_swab16(val);
>  #else
>  	return ___constant_swab16(val);
> @@ -56,9 +54,7 @@ static inline __attribute_const__ __u16 __fswab16(__u16 val)
>  
>  static inline __attribute_const__ __u32 __fswab32(__u32 val)
>  {
> -#ifdef __HAVE_BUILTIN_BSWAP32__
> -	return __builtin_bswap32(val);
> -#elif defined(__arch_swab32)
> +#if defined(__arch_swab32)
>  	return __arch_swab32(val);
>  #else
>  	return ___constant_swab32(val);
> @@ -67,9 +63,7 @@ static inline __attribute_const__ __u32 __fswab32(__u32 val)
>  
>  static inline __attribute_const__ __u64 __fswab64(__u64 val)
>  {
> -#ifdef __HAVE_BUILTIN_BSWAP64__
> -	return __builtin_bswap64(val);
> -#elif defined (__arch_swab64)
> +#if defined (__arch_swab64)
>  	return __arch_swab64(val);
>  #elif defined(__SWAB_64_THRU_32__)
>  	__u32 h = val >> 32;
> @@ -102,28 +96,40 @@ static inline __attribute_const__ __u32 __fswahb32(__u32 val)
>   * __swab16 - return a byteswapped 16-bit value
>   * @x: value to byteswap
>   */
> +#ifdef __HAVE_BUILTIN_BSWAP16__
> +#define __swab16(x) __builtin_bswap16((__u16)(x))
> +#else
>  #define __swab16(x)				\
>  	(__builtin_constant_p((__u16)(x)) ?	\
>  	___constant_swab16(x) :			\
>  	__fswab16(x))
> +#endif
>  
>  /**
>   * __swab32 - return a byteswapped 32-bit value
>   * @x: value to byteswap
>   */
> +#ifdef __HAVE_BUILTIN_BSWAP32__
> +#define __swab32(x) __builtin_bswap32((__u32)(x))
> +#else
>  #define __swab32(x)				\
>  	(__builtin_constant_p((__u32)(x)) ?	\
>  	___constant_swab32(x) :			\
>  	__fswab32(x))
> +#endif
>  
>  /**
>   * __swab64 - return a byteswapped 64-bit value
>   * @x: value to byteswap
>   */
> +#ifdef __HAVE_BUILTIN_BSWAP64__
> +#define __swab64(x) __builtin_bswap64((__u64)(x))
> +#else
>  #define __swab64(x)				\
>  	(__builtin_constant_p((__u64)(x)) ?	\
>  	___constant_swab64(x) :			\
>  	__fswab64(x))
> +#endif
>  
>  /**
>   * __swahw32 - return a word-swapped 32-bit value
> 

-- 
Josh