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SubjectRe: [PATCH] bitmap: optimize bitmap_remap()
On Thu, Aug 17, 2023 at 07:21:59AM -0700, Yury Norov wrote:
> On Thu, Aug 17, 2023 at 12:38:28PM +0300, Andy Shevchenko wrote:
> > On Thu, Aug 17, 2023 at 12:37:05PM +0300, Andy Shevchenko wrote:
> > > On Tue, Aug 15, 2023 at 04:59:34PM -0700, Yury Norov wrote:

...

> > > > int n = bitmap_pos_to_ord(old, oldbit, nbits);
> > > >
> > > > + bit = (n < 0) ? oldbit : /* identity map */
> > >
> > > Can't you also optimize this case?
> > >
> > > Something like
> > >
> > > bitmap_xor(tmp, old, new) // maybe even better approach, dunno
> >
> > > bitmap_empty(tmp) // can be replaced by find first bit
> >
> > Or reuse bitmap_weight()...
>
> That way it wouldn't work,

Why not? AFAIU there are two cases when we may copy:
1) the new mapping is empty;
2) the old == new.

The other cases we need to remap.

The case 2) is easy with xor and weight.

diff --git a/lib/bitmap.c b/lib/bitmap.c
index 24284caadbcc..917eea5219ac 100644
--- a/lib/bitmap.c
+++ b/lib/bitmap.c
@@ -958,7 +958,7 @@ EXPORT_SYMBOL(bitmap_parse);
* gets mapped to (returns) @ord value 3 in this example, that means
* that bit 7 is the 3rd (starting with 0th) set bit in @buf.
*
- * The bit positions 0 through @bits are valid positions in @buf.
+ * The bit positions 0 through @nbits are valid positions in @buf.
*/
static int bitmap_pos_to_ord(const unsigned long *buf, unsigned int pos, unsigned int nbits)
{
@@ -1008,17 +1008,30 @@ void bitmap_remap(unsigned long *dst, const unsigned long *src,

if (dst == src) /* following doesn't handle inplace remaps */
return;
- bitmap_zero(dst, nbits);
+
+ bitmap_xor(dst, old, new, nbits);
+ if (bitmap_empty(dst, nbits))
+ goto identity_map;

w = bitmap_weight(new, nbits);
+ if (w == 0)
+ goto identity_map;
+
+ bitmap_zero(dst, nbits);
+
for_each_set_bit(oldbit, src, nbits) {
int n = bitmap_pos_to_ord(old, oldbit, nbits);

- if (n < 0 || w == 0)
+ if (n < 0)
set_bit(oldbit, dst); /* identity map */
else
set_bit(find_nth_bit(new, nbits, n % w), dst);
}
+
+ return;
+
+identity_map:
+ bitmap_copy(dst, src, nbits);
}
EXPORT_SYMBOL(bitmap_remap);

But this gives +89 bytes on x86_64... :-(
Inside the loop we can also break when n gets equal to w, but it seems
a special case (we don't need bitmap_weight_from() for that, do we?).

--
With Best Regards,
Andy Shevchenko


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