Re: [Xen-devel] [PATCH] bitmap_*() should cope with zero size bitmaps

[Tim Deegan <tim@xxxxxxx>]

At 14:33 +0100 on 03 Jun (1370270004), Jan Beulich wrote:
> ... to match expectations set by memset()/memcpy().
> 
> Similarly for find_{first,next}_{,zero_}_bit() on x86.
> 
> __bitmap_shift_{left,right}() would also need fixing (they more
> generally can't cope with the shift count being larger than the bitmap
> size, and they perform undefined operations by possibly shifting an
> unsigned long value by BITS_PER_LONG bits), but since these functions
> aren't really used anywhere I wonder if we wouldn't better simply get
> rid of them.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> --- a/xen/include/asm-x86/bitops.h
> +++ b/xen/include/asm-x86/bitops.h
> @@ -327,10 +327,7 @@ static inline unsigned int __scanbit(uns
>   * Returns the bit-number of the first set bit, not the number of the byte
>   * containing a bit.
>   */
> -#define find_first_bit(addr,size)                               \
> -((__builtin_constant_p(size) && (size) <= BITS_PER_LONG ?       \
> -  (__scanbit(*(const unsigned long *)addr, size)) :             \
> -  __find_first_bit(addr,size)))
> +#define find_first_bit(addr, size) find_next_bit(addr, size, 0)
>  
>  /**
>   * find_next_bit - find the first set bit in a memory region
> @@ -338,10 +335,24 @@ static inline unsigned int __scanbit(uns
>   * @offset: The bitnumber to start searching at
>   * @size: The maximum size to search
>   */
> -#define find_next_bit(addr,size,off)                                     \
> -((__builtin_constant_p(size) && (size) <= BITS_PER_LONG ?                \
> -  ((off) + (__scanbit((*(const unsigned long *)addr) >> (off), size))) : \
> -  __find_next_bit(addr,size,off)))
> +#define find_next_bit(addr, size, off) ({ \
> +    unsigned int r__ = (size); \
> +    unsigned int o__ = (off); \
> +    switch ( -!__builtin_constant_p(size) | r__ ) \

Mmmm, binary trickery. :)  But doesn't leave us with the old behaviour
in cases where 'size' is a non-compile-time-constant zero?

Tim.

> +    { \
> +    case 0: (void)(addr); break; \
> +    case 1 ... BITS_PER_LONG: \
> +        r__ = o__ + __scanbit(*(const unsigned long *)(addr) >> o__, r__); \
> +        break; \
> +    default: \
> +        if ( __builtin_constant_p(off) && !o__ ) \
> +            r__ = __find_first_bit(addr, r__); \
> +        else \
> +            r__ = __find_next_bit(addr, r__, o__); \
> +        break; \
> +    } \
> +    r__; \
> +})


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