On Thu, Jun 08, 2017 at 04:48:04AM +0300, Andy Shevchenko wrote:
> > Commit 7dd968163f ("bitmap: bitmap_equal memcmp optimization") was
> > rather more restrictive than necessary; we can use memcmp() to implement
> > bitmap_equal() as long as the number of bits can be proved to be a
> > multiple of 8. And architectures other than s390 may be able to make
> > good use of this optimisation.
>
> > - if (__builtin_constant_p(nbits) && (nbits % BITS_PER_LONG) == 0)
> > + if (__builtin_constant_p(nbits & 7) && IS_ALIGNED(nbits, 8))
> > return !memcmp(src1, src2, nbits / 8);
>
> I'm not sure this is a fully correct change.
> What exactly ' & 7' part does?
> For me looks like you may just drop it.
We only need to know if the bottom 3 bits are 0 to apply this optimisation.
For example, if we have a user which does this:
nbits = 8;
if (argle)
nbits += 8;
if (bitmap_equal(ptr1, ptr2, nbits))
blah();
then we can use memcmp() because gcc can deduce that the bottom 3 bits
are never set (try it! it works!). We don't need nbits as a whole to
be const.
