[Sheepdog] expected bit size of __ffs

[Steven Dake]

On Tue, 2010-05-18 at 08:45 +0900, FUJITA Tomonori wrote:
> On Mon, 17 May 2010 09:34:37 -0700
> Steven Dake <sdake at redhat.com> wrote:
> 
> > Is __ffs expecting 64 bit parameters?  At line 18 of bitops.h, there is
> > a >> by 32, which with a 32 bit parameter results in the value 0.

> I think that 'if (BITS_PER_LONG == 64)' trick works on 32-bit archs.
>  

That patch looks good.

Regards
-steve

> #define BITS_PER_LONG (BITS_PER_BYTE * sizeof(long))
> 
> static inline unsigned long __ffs(unsigned long word)
> {
> 	int num = 0;
> 
> 	if (BITS_PER_LONG == 64) {
> 		if ((word & 0xffffffff) == 0) {
> 			num += 32;
> 			word >>= 32;
> 		}
> 	}
> 
> So we don't hit "word >>= 32" on 32-bit archs. We get a warning like
> "right shift count >= width of type" on this though.
> 
> The following patch kills the warning with gcc's built-in ffs().
> 
> =
> From: FUJITA Tomonori <fujita.tomonori at lab.ntt.co.jp>
> Subject: [PATCH] use gcc built-in ffs()
> 
> Signed-off-by: FUJITA Tomonori <fujita.tomonori at lab.ntt.co.jp>
> ---
>  include/bitops.h |   34 +---------------------------------
>  1 files changed, 1 insertions(+), 33 deletions(-)
> 
> diff --git a/include/bitops.h b/include/bitops.h
> index 6c85d5e..0e0f402 100644
> --- a/include/bitops.h
> +++ b/include/bitops.h
> @@ -3,43 +3,11 @@
>  #define BITS_TO_LONGS(nr)	DIV_ROUND_UP(nr, BITS_PER_BYTE * sizeof(long))
>  #define DECLARE_BITMAP(name,bits) \
>  	unsigned long name[BITS_TO_LONGS(bits)]
> -
>  #define BITS_PER_LONG (BITS_PER_BYTE * sizeof(long))
>  
> +#define __ffs(x)  (x ? __builtin_ffsl(x) - 1 : 0)
>  #define ffz(x)  __ffs(~(x))
>  
> -static inline unsigned long __ffs(unsigned long word)
> -{
> -	int num = 0;
> -
> -	if (BITS_PER_LONG == 64) {
> -		if ((word & 0xffffffff) == 0) {
> -			num += 32;
> -			word >>= 32;
> -		}
> -	}
> -
> -	if ((word & 0xffff) == 0) {
> -		num += 16;
> -		word >>= 16;
> -	}
> -	if ((word & 0xff) == 0) {
> -		num += 8;
> -		word >>= 8;
> -	}
> -	if ((word & 0xf) == 0) {
> -		num += 4;
> -		word >>= 4;
> -	}
> -	if ((word & 0x3) == 0) {
> -		num += 2;
> -		word >>= 2;
> -	}
> -	if ((word & 0x1) == 0)
> -		num += 1;
> -	return num;
> -}
> -
>  #define BITOP_WORD(nr)		((nr) / BITS_PER_LONG)
>  
>  static inline unsigned long find_next_zero_bit(const unsigned long *addr, unsigned long size,