[parisc-linux] a fast fls also for 2.6?

[Grant Grundler]

On Fri, Sep 05, 2003 at 01:00:35PM +0000, Joel Soete wrote:
> remember me that I also suggest a __fls() in: 
> <http://lists.parisc-linux.org/pipermail/parisc-linux/2003-August/020628.html>

sorry - I missed that.
 
> Without any remark, I don't know if you could also be interested to 
> include it in 2.6.

no - becuase fls() and ffs() return the same values for given input.
(I see comments in include/asm-ppc/bitops.h to that effect).

parisc __ffs costs the same number of cycles regardless of input.
(2 cycles per 3 instructions about on PA 2.0 machines).
ie there is no advantage to "searching from the top" vs "searching
from the bottom" which is what I think the intent of fls() vs ffs().
For generic implementations, this intent is important.

What we could do is redefine fls() to also use ffs() and add
my comment above. Patch appended. Please test/review and tell me
if that should be committed. I haven't tested it yet and the comments
in PPC bitops.h could be wrong.

thanks,
grant


Index: include/asm-parisc/bitops.h
===================================================================
RCS file: /var/cvs/linux-2.6/include/asm-parisc/bitops.h,v
retrieving revision 1.2
diff -u -p -r1.2 bitops.h
--- include/asm-parisc/bitops.h	1 Sep 2003 00:30:42 -0000	1.2
+++ include/asm-parisc/bitops.h	5 Sep 2003 18:11:21 -0000
@@ -281,9 +281,14 @@ static __inline__ int ffs(int x)
 
 /*
  * fls: find last bit set.
+ *
+ * parisc __ffs costs the same number of cycles regardless of input.
+ * A similar implementation for __fls() would have no advantage.
+ * ie there is no advantage to "searching from the top" vs "searching 
+ * from the bottom" which is the intent of fls() vs ffs().
  */
 
-#define fls(x) generic_fls(x)
+#define fls(x) ffs(x)
 
 /*
  * hweightN: returns the hamming weight (i.e. the number