[boost] [dynamic_bitset] Using of GCC built-in functions may increase performance of some methods

Subject: [boost] [dynamic_bitset] Using of GCC built-in functions may increase performance of some methods
From: Evgeny Shulgin (izaronplatz_at_[hidden])
Date: 2018-08-22 19:03:49

Hi all, when I was playing around the Boost.DynamicBitset library, I
noticed the last lines of the count() function (which returns the number of
bits in the bitset that are set):

return do_count(m_bits.begin(), num_blocks(), Block(0),
    static_cast<value_to_type<(bool)mode> *>(0));

Well, this line was modified 10 years ago last time.

Let's go deeper. GNU C provides several language features not found in ISO
standard C. These include a bunch of bit operations - you can look at them
here: https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html.
For example, *__builtin_popcountll *counts the number of bits in an
'unsigned long long', so we can use this kind of code to calculate bits
(don't judge, I wrote this fast and didn't think of coding standards):

size_type res = 0;
for (size_type i = 0; i < m_bits.size(); i++) {
    res += __builtin_popcountll(m_bits[i]);
}

I checked that m_bits[] and popcount() both work with 64-bit values on my
machine. It halved the running time of a program that has a bitset and
calls count() over and over again - here's the code
<https://gist.github.com/Izaron/5005fed4184ee6483699191dc5ef3677> I used.
These __builtin_XXX functions can be used everywhere where it's possible,
with preprocessor directives. Or is it actually used and longer time means
something else?
I used the latest version of Boost.