Re: [Xen-devel] [PATCH 4/4] bitmaps/bitops: Clarify tests for small constant size.

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 28.11.13 at 17:37, Tim Deegan <tim@xxxxxxx> wrote:
> No semantic changes, just makes the control flow a bit clearer.
> 
> I was looking at this bcause the (-!__builtin_constant_p(x) | x__)
> formula is too clever for Coverity, but in fact it always takes me a
> minute or two to understand it too. :)

I nevertheless like things like this... Anyway - did you check the
generated code is no worse with this change?

> -#define find_next_bit(addr, size, off) ({ \
> -    unsigned int r__ = (size); \
> -    unsigned int o__ = (off); \
> -    switch ( -!__builtin_constant_p(size) | r__ ) \
> -    { \
> -    case 0: (void)(addr); break; \

This dummy evaluation of addr ...

> -    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__; \
> +#define find_next_bit(addr, size, off) ({                                   \
> +    unsigned int r__;                                                       \
> +    unsigned int s__ = (size);                                              \
> +    unsigned int o__ = (off);                                               \
> +    if ( __builtin_constant_p(size) && s__ == 0 )                           \
> +        r__ = s__;                                                          \

... is being lost here.

> +    else if ( __builtin_constant_p(size) && s__ <= BITS_PER_LONG )          \
> +        r__ = o__ + __scanbit(*(const unsigned long *)(addr) >> o__, s__);  \
> +    else if ( __builtin_constant_p(off) && !o__ )                           \
> +        r__ = __find_first_bit(addr, s__);                                  \
> +    else                                                                    \
> +        r__ = __find_next_bit(addr, s__, o__);                              \
> +    r__;                                                                    \

Jan


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel