On 04/01/2011 12:18 PM, Maksym Planeta wrote:
> Loop was repalaced with __builtin_clz(). This still allows to precompute
> constants, but on some architectures it uses special instruction to
> calculate order.
>
> Signed-off-by: Maksym Planeta <mcsim.planeta@xxxxxxxxx>
> ---
> include/asm-generic/getorder.h | 8 +++-----
> 1 files changed, 3 insertions(+), 5 deletions(-)
>
> diff --git a/include/asm-generic/getorder.h b/include/asm-generic/getorder.h
> index 67e7245..fe8020c 100644
> --- a/include/asm-generic/getorder.h
> +++ b/include/asm-generic/getorder.h
> @@ -11,11 +11,9 @@ static inline __attribute_const__ int get_order(unsigned long size)
> int order;
>
> size = (size - 1) >> (PAGE_SHIFT - 1);
> - order = -1;
> - do {
> - size >>= 1;
> - order++;
> - } while (size);
> + order = (__builtin_clzl(size) ^ (BITS_PER_LONG - 1));
> + if (size == 0)
> + order = 0;
> return order;
> }
>
You need to guard this with __GNUC__ >= 4; there are still laggards
using gcc 3. Furthermore, on some platforms __builtin_clz*() does a
libgcc call which may be undesirable.
For the generic case, one can do something like this instead of a loop:
static inline unsigned int __clzl(unsigned long v)
{
unsigned int p;
#if BITS_PER_LONG == 64
p = 63;
if (v & 0xffffffff00000000UL) {
p -= 32;
v >>= 32;
}
#else
p = 31;
#endif
if (v & 0xffff0000) {
p -= 16;
v >>= 16;
}
if (v & 0xff00) {
p -= 8;
v >>= 8;
}
if (v & 0xf0) {
p -= 4;
v >>= 4;
}
if (v & 0xc) {
p -= 2;
v >>= 2;
}
if (v & 0x2) {
p -= 1;
v >>= 1;
}
return p;
}
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