On 10 January 2017 at 07:40, Robert Haas <robertmhaas@gmail.com> wrote:
> On Sat, Jan 7, 2017 at 7:45 PM, Jim Nasby <Jim.Nasby@bluetreble.com> wrote:
>> Well, now that there's 3 places that need to do almost the same thing, I
>> think it'd be best to just centralize this somewhere. I realize that's not
>> going to save any significant amount of code, but it would make it crystal
>> clear what's going on (assuming the excellent comment above RIGHTMOST_ONE
>> was kept).
>
> Hmm. This sounds a lot like what fls() and my_log2() also do. I've
> been quietly advocating for fls() because we only provide an
> implementation in src/port if the operating system doesn't have it,
> and the operating system may have an implementation that optimizes to
> a single machine-language instruction (bsrl on x86, I think, see
> 4f658dc851a73fc309a61be2503c29ed78a1592e). But the fact that our
> src/port implementation uses a loop instead of the RIGHTMOST_ONE()
> trick seems non-optimal.
It does really sound like we need a bitutils.c as mentioned in [1].
It would be good to make use of GCC's __builtin_popcount [2] instead
of the number_of_ones[] array in bitmapset.c. It should be a bit
faster and less cache polluting.
[1] https://www.postgresql.org/message-id/14578.1462595165@sss.pgh.pa.us
[2] https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
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