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SubjectRe: [PATCH] bitmap: Cleanup find_last_bit
On Feb. 17, 2009, 1:10 +0200, Rusty Russell <rusty@rustcorp.com.au> wrote:
> 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?

Yeah, I like that too.
I'm about to go on a flight
so the following was just tested to compile...

diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index 6182913..886ebf0 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -142,7 +142,7 @@ extern unsigned long find_first_zero_bit(const unsigned long *addr,
* @addr: The address to start the search at
* @size: The maximum size to search
*
- * Returns the bit number of the first set bit, or size.
+ * Returns the bit number of the last set bit, or size.
*/
extern unsigned long find_last_bit(const unsigned long *addr,
unsigned long size);
diff --git a/lib/find_last_bit.c b/lib/find_last_bit.c
index 5d202e3..6a35783 100644
--- a/lib/find_last_bit.c
+++ b/lib/find_last_bit.c
@@ -17,29 +17,31 @@

unsigned long find_last_bit(const unsigned long *addr, unsigned long size)
{
+ unsigned long final_bits = size;
unsigned long words;
- unsigned long tmp;
+ unsigned long word;

/* Start at final word. */
- words = size / BITS_PER_LONG;
+ words = final_bits / BITS_PER_LONG;

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

while (words) {
- tmp = addr[--words];
- if (tmp) {
-found:
- return words * BITS_PER_LONG + __fls(tmp);
- }
+ word = addr[--words];
+ if (word)
+ goto found;
}

/* Not found */
- return size;
+ return final_bits;
+
+found:
+ return words * BITS_PER_LONG + __fls(word);
}
EXPORT_SYMBOL(find_last_bit);
--
1.6.1.3

>
> Thanks!
> Rusty.


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