On Mon, Apr 29, 2024 at 09:34:06AM -0700, Yury Norov wrote:
> On Mon, Apr 29, 2024 at 11:40:34PM +0800, Kuan-Wei Chiu wrote:
> > On Mon, Apr 29, 2024 at 10:05:12AM +0200, Rasmus Villemoes wrote:
> > > On 28/04/2024 18.08, Yury Norov wrote:
> > > > + Rasmus
> > > >
> > > > On Sat, Apr 27, 2024 at 01:33:55PM +0800, Kuan-Wei Chiu wrote:
> > > >> Before:
> > > >> Start testing find_bit() with random-filled bitmap
> > > >> [ 0.299085] fbcon: Taking over console
> > > >> [ 0.299820] find_next_bit: 606286 ns, 164169 iterations
> > > >> [ 0.300463] find_next_zero_bit: 641072 ns, 163512 iterations
> > > >> [ 0.300996] find_last_bit: 531027 ns, 164169 iterations
> > > >> [ 0.305233] find_nth_bit: 4235859 ns, 16454 iterations
> > > >> [ 0.306434] find_first_bit: 1199357 ns, 16455 iterations
> > > >> [ 0.321616] find_first_and_bit: 15179667 ns, 32869 iterations
> > > >> [ 0.321917] find_next_and_bit: 298836 ns, 73875 iterations
> > > >> [ 0.321918]
> > > >> Start testing find_bit() with sparse bitmap
> > > >> [ 0.321953] find_next_bit: 7931 ns, 656 iterations
> > > >> [ 0.323201] find_next_zero_bit: 1246980 ns, 327025 iterations
> > > >> [ 0.323210] find_last_bit: 8000 ns, 656 iterations
> > > >> [ 0.324427] find_nth_bit: 1213161 ns, 655 iterations
> > > >> [ 0.324813] find_first_bit: 384747 ns, 656 iterations
> > > >> [ 0.324817] find_first_and_bit: 2220 ns, 1 iterations
> > > >> [ 0.324820] find_next_and_bit: 1831 ns, 1 iterations
> > > >>
> > > >> After:
> > > >> Start testing find_bit() with random-filled bitmap
> > > >> [ 0.305081] fbcon: Taking over console
> > > >> [ 0.306126] find_next_bit: 854517 ns, 163960 iterations
> > > >> [ 0.307041] find_next_zero_bit: 911725 ns, 163721 iterations
> > > >> [ 0.307711] find_last_bit: 668261 ns, 163960 iterations
> > > >> [ 0.311160] find_nth_bit: 3447530 ns, 16372 iterations
> > > >> [ 0.312358] find_first_bit: 1196633 ns, 16373 iterations
> > > >> [ 0.327191] find_first_and_bit: 14830129 ns, 32951 iterations
> > > >> [ 0.327503] find_next_and_bit: 310560 ns, 73719 iterations
> > > >> [ 0.327504]
> > > >> Start testing find_bit() with sparse bitmap
> > > >> [ 0.327539] find_next_bit: 7633 ns, 656 iterations
> > > >> [ 0.328787] find_next_zero_bit: 1247398 ns, 327025 iterations
> > > >> [ 0.328797] find_last_bit: 8425 ns, 656 iterations
> > > >> [ 0.330034] find_nth_bit: 1234044 ns, 655 iterations
> > > >> [ 0.330428] find_first_bit: 392086 ns, 656 iterations
> > > >> [ 0.330431] find_first_and_bit: 1980 ns, 1 iterations
> > > >> [ 0.330434] find_next_and_bit: 1831 ns, 1 iterations
> > > >>
> > > >> Some benchmarks seem to have worsened after applying this patch.
> > > >> However, unless I'm mistaken, the fns() changes should only affect the
> > > >> results of find_nth_bit, while the others are just random fluctuations.
> > > >
> > > > So...
> > > >
> > > > The patch itself looks good,
> > >
> > > Well, I think it could be even better. While I agree that bit=ffs();
> > > clear_bit(bit, ) is probably a bad way of doing it, I think the basic
> > > structure of the function is good. Introducing a "count from 0 up to n"
> > > loop is rarely a good thing, keeping the n counting down to 0 is likely
> > > better.
> > >
> > > So I'd instead just change the function to
> > >
> > > static inline unsigned long fns(unsigned long word, unsigned int n)
> > > {
> > > while (word) {
> > > if (n-- == 0)
> > > return __ffs(word);
> > > word &= word - 1;
> > > }
> > >
> > > return BITS_PER_LONG;
> > > }
> > >
> > How about rewriting it as follows:
> >
> > static inline unsigned long fns(unsigned long word, unsigned int n)
> > {
> > while (word && n--)
> > word &= word - 1;
> >
> > return word ? __ffs(word) : BITS_PER_LONG;
> > }
> >
> > IMHO, this way the code can be shorter and more elegant.
>
> Rasmus' code looks shorter because it tests the 'word != 0' only once
> when n == 0, for example. But we never sure how the compiler would
> optimize it. Can you show a disassembly of both and pick the best?
>
> Thanks,
> Yury
It appears that gcc with the O2 flag is smart enough to generate the
same code. The only difference in my disassembly results is the
generated label names. Tested with gcc 11.4.0.
Regards,
Kuan-Wei
