On Wed, Jul 17, 2019 at 10:46:37AM +0800, Wei Yang wrote: > On Wed, Jul 17, 2019 at 09:59:10AM +0800, Peter Xu wrote: > >On Wed, Jul 17, 2019 at 09:29:02AM +0800, Wei Yang wrote: > >> The value left in nr is the number of bits for the last word, which > >> could be calculate the last word mask directly. > > > >Is it true even if start does not align to BITS_PER_LONG? > > > > Yes. Let me see how to explain this. > > When you look into the definition of BITMAP_LAST_WORD_MASK, it takes the > number of total bits and give the number of bits in last word. While the value > matters for the input is the number of last word. This means the following > equation stands > > BITMAP_LAST_WORD_MASK(size) == BITMAP_FIRST_WORD_MASK(size % BITS_PER_LONG) > > Now let look at the calculation for nr. In each iteration, nr will be > truncated to be aligned to BITS_PER_LONG. So when we exit the loop, nr keeps > the number of bits in last word. > > So we can leverage the result, no matter the start is aligned or not.
Yes, you are right. Do you have plan to write some unit tests for these functions? :) It'll be tests/test-bitmap.c. IMHO the test cases could be even more helpful to the QEMU project as a whole comparing to this patch to guarantee changes like your patch won't break. At the meantime I think you can also do that to bitmap_set_atomic. Thanks, -- Peter Xu
