[gcc r12-9344] libstdc++: Optimize std::bitset<N>::to_string

[gcc r12-9344] libstdc++: Optimize std::bitset<N>::to_string

[Jonathan Wakely]

https://gcc.gnu.org/g:33ff7cb3386fb9477f145fa4ab74bdf70d0cb2f3

commit r12-9344-g33ff7cb3386fb9477f145fa4ab74bdf70d0cb2f3
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Sep 22 18:36:04 2022 +0100

    libstdc++: Optimize std::bitset<N>::to_string
    
    This makes to_string approximately twice as fast at any optimization
    level. Instead of iterating through every bit, jump straight to the next
    bit that is set, by using _Find_first and _Find_next.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/bitset (bitset::_M_copy_to_string): Find set bits
            instead of iterating over individual bits.
    
    (cherry picked from commit ffb03fa12850df3a4f53435d5f20ff122c83732a)

Diff:
---
 libstdc++-v3/include/std/bitset | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/libstdc++-v3/include/std/bitset b/libstdc++-v3/include/std/bitset
index 438c2f7efe9..8ff9d7fdefd 100644
--- a/libstdc++-v3/include/std/bitset
+++ b/libstdc++-v3/include/std/bitset
@@ -1415,9 +1415,12 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 			_CharT __zero, _CharT __one) const
       {
 	__s.assign(_Nb, __zero);
-	for (size_t __i = _Nb; __i > 0; --__i)
-	  if (_Unchecked_test(__i - 1))
-	    _Traits::assign(__s[_Nb - __i], __one);
+	size_t __n = this->_Find_first();
+	while (__n < _Nb)
+	  {
+	    __s[_Nb - __n - 1] = __one;
+	    __n = _Find_next(__n);
+	  }
       }
 
   // 23.3.5.3 bitset operations: