Messages in this thread | | | From | Andy Shevchenko <> | Date | Wed, 24 Aug 2022 20:56:02 +0300 | Subject | Re: [PATCH v2 3/3] lib/find_bit: optimize find_next_bit() functions |
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On Wed, Aug 24, 2022 at 8:54 PM Andy Shevchenko <andy.shevchenko@gmail.com> wrote: > On Wed, Aug 24, 2022 at 4:53 PM Yury Norov <yury.norov@gmail.com> wrote: > > On Wed, Aug 24, 2022 at 12:19:05PM +0300, Andy Shevchenko wrote: > > > On Wed, Aug 24, 2022 at 4:56 AM Yury Norov <yury.norov@gmail.com> wrote:
...
> > > > +#define FIND_NEXT_BIT(EXPRESSION, size, start) \ > > > > +({ \ > > > > + unsigned long mask, idx, tmp, sz = (size), __start = (start); \ > > > > + \ > > > > + if (unlikely(__start >= sz)) \ > > > > + goto out; \ > > > > + \ > > > > + mask = word_op(BITMAP_FIRST_WORD_MASK(__start)); \ > > > > + idx = __start / BITS_PER_LONG; \ > > > > + \ > > > > + for (tmp = (EXPRESSION) & mask; !tmp; tmp = (EXPRESSION)) { \ > > > > > > for (unsigned long tmp ...; > > > But hey, why not loop over idx (which probably should be named as > > > offset) > > > > Offset in structure, index in array, isn't? > > > > > as I proposed in the first patch? You will drop a lot of > > > divisions / multiplications, no? > > > > Those divisions and multiplications are optimized away, and > > what you suggested blows up the EXPRESSION. > > > > I tried like this: > > mask = word_op(BITMAP_FIRST_WORD_MASK(__start)); > > idx = __start / BITS_PER_LONG; > > tmp = (EXPRESSION); > > > > while (1) { > > if (tmp) { > > sz = min(idx * BITS_PER_LONG + __ffs(word_op(tmp)), sz); > > break; > > } > > > > if (++idx > sz) > > break; > > > > tmp = (EXPRESSION); > > } > > > > And it generated the same code, but looks less expressive to me. > > If you have some elegant approach in mind - can you please share > > it, and how the generated code looks? > > for (unsigned long idx = 0; idx < sz; idx++) {
Of source 0 should be changed to whatever start you have there.
> unsigned long tmp; > > tmp = (EXPRESSION); > if (tmp) { > ... > } > } > > No?
-- With Best Regards, Andy Shevchenko
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