Optimize truncating a basic_string

	* include/bits/basic_string.h [_GLIBCXX_USE_CXX11_ABI]
	(basic_string::erase(size_type, size_type)): Add fast path for
	truncating the string, by calling _M_set_length directly.
	(basic_string::erase(__const_iterator, __const_iterator)): Likewise.
	* include/bits/basic_string.tcc [_GLIBCXX_USE_CXX11_ABI]
	(basic_string::resize(size_type, _CharT)): Likewise.

From-SVN: r240446

libstdc++-v3/ChangeLog

+2016-09-23  Jonathan Wakely  <jwakely@redhat.com>
+
+	* include/bits/basic_string.h [_GLIBCXX_USE_CXX11_ABI]
+	(basic_string::erase(size_type, size_type)): Add fast path for
+	truncating the string, by calling _M_set_length directly.
+	(basic_string::erase(__const_iterator, __const_iterator)): Likewise.
+	* include/bits/basic_string.tcc [_GLIBCXX_USE_CXX11_ABI]
+	(basic_string::resize(size_type, _CharT)): Likewise.
+
 2016-09-22  Jason Merrill  <jason@redhat.com>
 
 	* configure.ac: Define HAVE_MEMALIGN for newlib.

libstdc++-v3/include/bits/basic_string.h

@@ -1709,8 +1709,11 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       basic_string&
       erase(size_type __pos = 0, size_type __n = npos)
       {
-	this->_M_erase(_M_check(__pos, "basic_string::erase"),
-		       _M_limit(__pos, __n));
+	_M_check(__pos, "basic_string::erase");
+	if (__n == npos)
+	  this->_M_set_length(__pos);
+	else if (__n != 0)
+	  this->_M_erase(__pos, _M_limit(__pos, __n));
 	return *this;
       }
 
@@ -1747,7 +1750,10 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	_GLIBCXX_DEBUG_PEDASSERT(__first >= begin() && __first <= __last
 				 && __last <= end());
         const size_type __pos = __first - begin();
-	this->_M_erase(__pos, __last - __first);
+	if (__last == end())
+	  this->_M_set_length(__pos);
+	else
+	  this->_M_erase(__pos, __last - __first);
 	return iterator(this->_M_data() + __pos);
       }
 

libstdc++-v3/include/bits/basic_string.tcc

@@ -351,7 +351,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       if (__size < __n)
 	this->append(__n - __size, __c);
       else if (__n < __size)
-	this->_M_erase(__n, __size - __n);
+	this->_M_set_length(__n);
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>