commit 63b539ac150ca68fe4f6762fc341bbc4a7737152
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Sep 23 16:55:42 2016 +0100

    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.

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index e823f13..2708cbc 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/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);
       }
 
diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
index 0080d2b..df1e8dd 100644
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/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>