Re: [PATCH] bitmap: Cleanup find_last_bit

[Benny Halevy <bhalevy@panasas.com>]

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.