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    Subject[patch] compiler: introduce __used and __maybe_unused
    __used is defined to be __attribute__((unused)) for all pre-3.4 gcc
    compilers to suppress warnings for unused functions because perhaps they
    are referenced only in inline assembly. It is defined to be
    __attribute__((used)) for gcc 3.4 and later so that the code is still
    emitted for such functions.

    There was a mistake in the current implementation of __attribute_used__
    whereas it would be defined to be __attribute__((used)) incorrectly for
    gcc 3.3 and later. The unit-at-a-time compilation scheme was only
    introduced in gcc 3.4 and later versions as specified in
    http://www.gnu.org/software/gcc/gcc-3.4/changes.html.

    __maybe_unused is defined to be __attribute__((unused)) for both function
    and variable use if it could possibly be unreferenced due to the
    evaluation of preprocessor macros. Function prototypes shall be marked
    with __maybe_unused if the actual definition of the function is dependant
    on preprocessor macros.

    No update to compiler-intel.h is necessary because ICC supports both
    __attribute__((used)) and __attribute__((unused)) as specified by the
    gcc manual.

    __attribute_used__ is deprecated and will be removed once all current
    code is converted to using __used.

    Cc: Rusty Russell <rusty@rustcorp.com.au>
    Cc: Andrian Bunk <bunk@stusta.de>
    Signed-off-by: David Rientjes <rientjes@google.com>
    ---
    include/linux/compiler-gcc.h | 1 +
    include/linux/compiler-gcc3.h | 13 ++++++-------
    include/linux/compiler-gcc4.h | 3 ++-
    include/linux/compiler.h | 17 ++++++++++++++---
    4 files changed, 23 insertions(+), 11 deletions(-)

    diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
    --- a/include/linux/compiler-gcc.h
    +++ b/include/linux/compiler-gcc.h
    @@ -37,3 +37,4 @@
    #define noinline __attribute__((noinline))
    #define __attribute_pure__ __attribute__((pure))
    #define __attribute_const__ __attribute__((__const__))
    +#define __maybe_unused __attribute__((unused))
    diff --git a/include/linux/compiler-gcc3.h b/include/linux/compiler-gcc3.h
    --- a/include/linux/compiler-gcc3.h
    +++ b/include/linux/compiler-gcc3.h
    @@ -3,14 +3,13 @@
    /* These definitions are for GCC v3.x. */
    #include <linux/compiler-gcc.h>

    -#if __GNUC_MINOR__ >= 3
    -# define __attribute_used__ __attribute__((__used__))
    -#else
    -# define __attribute_used__ __attribute__((__unused__))
    -#endif
    -
    #if __GNUC_MINOR__ >= 4
    -#define __must_check __attribute__((warn_unused_result))
    +# define __used __attribute__((__used__))
    +# define __attribute_used__ __used /* deprecated */
    +# define __must_check __attribute__((warn_unused_result))
    +#else
    +# define __used __attribute__((__unused__))
    +# define __attribute_used__ __used /* deprecated */
    #endif

    #define __always_inline inline __attribute__((always_inline))
    diff --git a/include/linux/compiler-gcc4.h b/include/linux/compiler-gcc4.h
    --- a/include/linux/compiler-gcc4.h
    +++ b/include/linux/compiler-gcc4.h
    @@ -12,7 +12,8 @@
    # define __inline __inline __attribute__((always_inline))
    #endif

    -#define __attribute_used__ __attribute__((__used__))
    +#define __used __attribute__((__used__))
    +#define __attribute_used__ __used /* deprecated */
    #define __must_check __attribute__((warn_unused_result))
    #define __compiler_offsetof(a,b) __builtin_offsetof(a,b)
    #define __always_inline inline __attribute__((always_inline))
    diff --git a/include/linux/compiler.h b/include/linux/compiler.h
    --- a/include/linux/compiler.h
    +++ b/include/linux/compiler.h
    @@ -108,15 +108,26 @@ extern void __chk_io_ptr(const void __iomem *);
    * Allow us to avoid 'defined but not used' warnings on functions and data,
    * as well as force them to be emitted to the assembly file.
    *
    - * As of gcc 3.3, static functions that are not marked with attribute((used))
    - * may be elided from the assembly file. As of gcc 3.3, static data not so
    + * As of gcc 3.4, static functions that are not marked with attribute((used))
    + * may be elided from the assembly file. As of gcc 3.4, static data not so
    * marked will not be elided, but this may change in a future gcc version.
    *
    * In prior versions of gcc, such functions and data would be emitted, but
    * would be warned about except with attribute((unused)).
    + *
    + * Mark functions that are referenced only in inline assembly as __used so
    + * the code is emitted even though it appears to be unreferenced.
    */
    #ifndef __attribute_used__
    -# define __attribute_used__ /* unimplemented */
    +# define __attribute_used__ /* deprecated */
    +#endif
    +
    +#ifndef __used
    +# define __used /* unimplemented */
    +#endif
    +
    +#ifndef __maybe_unused
    +# define __maybe_unused /* unimplemented */
    #endif

    /*
    -
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