[committed] libstdc++: Optimize std::bitset<N>::to_string

[committed] libstdc++: Optimize std::bitset<N>::to_string

[Jonathan Wakely]

Tested x86_64-linux. Pushed to trunk.

-- >8 --

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.
---
 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 0c84f15fda0..83c6416b770 100644
--- a/libstdc++-v3/include/std/bitset
+++ b/libstdc++-v3/include/std/bitset
@@ -1495,9 +1495,12 @@ _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);
+	  }
       }
 #endif // HOSTED
 
-- 
2.37.3