Argh, please drop - Stephen just hit a build fail in linux-next.
On Thu, 05 May 2016 16:22:39 -0700 akpm@xxxxxxxxxxxxxxxxxxxx wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
> Subject: byteswap: try to avoid __builtin_constant_p gcc bug
>
> 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.
>
> The regression got introduced due to the combination of two separate
> workarounds (e3bde9568d99 ("include/linux/unaligned: force inlining of
> byteswap operations") and ef3fb2422ffe ("scsi: fc: use get/put_unaligned64
> for wwn access")) that each try to sidestep distinct problems with gcc
> behavior (code growth and increased stack usage). Unfortunately after
> both have been applied, a more serious gcc bug has been uncovered, leading
> to incorrect object code that discards part of a function and causes
> undefined behavior.
>
> As part of this 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.
>
> Both of the original workarounds were only merged in the 4.6 kernel, and
> the bug that is fixed by this patch should only appear if both are there,
> so we probably don't need to backport the fix. On the other hand, it
> works by simplifying the code path and should not have any negative
> effects.
>
> [arnd@xxxxxxxx: fix older gcc warnings]
> (http://lkml.kernel.org/r/12243652.bxSxEgjgfk@wuerfel)
> Link: https://lkml.org/lkml/headers/2016/4/12/1103
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66122
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70232
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70646
> Fixes: e3bde9568d99 ("include/linux/unaligned: force inlining of byteswap operations")
> Fixes: ef3fb2422ffe ("scsi: fc: use get/put_unaligned64 for wwn access")
> Link: http://lkml.kernel.org/r/1780465.XdtPJpi8Tt@wuerfel
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> Reviewed-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Tested-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> # on gcc-5.3
> Tested-by: Quinn Tran <quinn.tran@xxxxxxxxxx>
> Cc: Martin Jambor <mjambor@xxxxxxx>
> Cc: "Martin K. Petersen" <martin.petersen@xxxxxxxxxx>
> Cc: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
> Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
> Cc: Thomas Graf <tgraf@xxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Himanshu Madhani <himanshu.madhani@xxxxxxxxxx>
> Cc: Jan Hubicka <hubicka@xxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> ---
>
> include/uapi/linux/swab.h | 24 +++++++++++++++---------
> 1 file changed, 15 insertions(+), 9 deletions(-)
>
> diff -puN include/uapi/linux/swab.h~byteswap-try-to-avoid-__builtin_constant_p-gcc-bug-v2 include/uapi/linux/swab.h
> --- a/include/uapi/linux/swab.h~byteswap-try-to-avoid-__builtin_constant_p-gcc-bug-v2
> +++ a/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
>
> 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
>
> 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
> * __swab16 - return a byteswapped 16-bit value
> * @x: value to byteswap
> */
> +#ifdef __HAVE_BUILTIN_BSWAP16__
> +#define __swab16(x) (__u16)__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) (__u32)__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) (__u64)__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
> _
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