linux-kernel - Re: [PATCH] bitmap: Cleanup find_last

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Date:	Tue, 17 Feb 2009 09:40:59 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Benny Halevy <bhalevy@...asas.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bitmap: Cleanup find_last_bit

On Monday 16 February 2009 23:28:40 Benny Halevy wrote:
> Fix cut & paste error in header comment.
> Simplify implementation a bit.

Hi Benny,

  Nice catch!  But I disagree with one change:

>  	/* Partial final word? */
> -	if (size & (BITS_PER_LONG-1)) {
> -		tmp = (addr[words] & (~0UL >> (BITS_PER_LONG
> -					 - (size & (BITS_PER_LONG-1)))));
> +	tmp = size & (BITS_PER_LONG-1);
> +	if (tmp) {
> +		tmp = addr[words] & (~0UL >> (BITS_PER_LONG - tmp));
>  		if (tmp)
>  			goto found;
>  	}

The overloading of tmp to the remainder size here is not really a cleanup: it
makes it into a dual-use var.  I know it's called tmp, but that's a sign of
poor code too :)

I'd prefer a new final_bits var: gcc will almost certainly just slap it in
a register anyway, but it's clearer.

tmp could then be called something like... word?

Thanks!
Rusty.
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