Subject: Re: [PATCH 3/6] C-language equivalents of include/asm-*/bitops.h

From: Russell King <[email-masked]>
Date: Wed, 25 Jan 2006 20:02:50 +0000

On Wed, Jan 25, 2006 at 08:32:06PM +0900, Akinobu Mita wrote:
> +#ifndef HAVE_ARCH___FFS_BITOPS
> +
> +/**
> + * __ffs - find first bit in word.
> + * @word: The word to search
> + *
> + * Returns 0..BITS_PER_LONG-1
> + * Undefined if no bit exists, so code should check against 0 first.
> + */
> +static inline unsigned long __ffs(unsigned long word)
>  {
> -	int	mask;
> +	int b = 0, s;
>  
> -	addr += nr >> 5;
> -	mask = 1 << (nr & 0x1f);
> -	return ((mask & *addr) != 0);
> +#if BITS_PER_LONG == 32
> +	s = 16; if (word << 16 != 0) s = 0; b += s; word >>= s;
> +	s =  8; if (word << 24 != 0) s = 0; b += s; word >>= s;
> +	s =  4; if (word << 28 != 0) s = 0; b += s; word >>= s;
> +	s =  2; if (word << 30 != 0) s = 0; b += s; word >>= s;
> +	s =  1; if (word << 31 != 0) s = 0; b += s;
> +
> +	return b;
> +#elif BITS_PER_LONG == 64
> +	s = 32; if (word << 32 != 0) s = 0; b += s; word >>= s;
> +	s = 16; if (word << 48 != 0) s = 0; b += s; word >>= s;
> +	s =  8; if (word << 56 != 0) s = 0; b += s; word >>= s;
> +	s =  4; if (word << 60 != 0) s = 0; b += s; word >>= s;
> +	s =  2; if (word << 62 != 0) s = 0; b += s; word >>= s;
> +	s =  1; if (word << 63 != 0) s = 0; b += s;
> +
> +	return b;
> +#else
> +#error BITS_PER_LONG not defined
> +#endif

This code generates more expensive shifts than our (ARMs) existing C
version.  This is a backward step.

Basically, shifts which depend on a variable are more expensive than
constant-based shifts.

I've not really looked at the rest because I haven't figured out which
bits will be used on ARM and which won't - which I think is another
problem with this patch set.  I'll look again later tonight.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:  2.6 Serial core