libstdc++: Split std::basic_string::_M_use_local_data into two functions

This splits out the activate-the-union-member-for-constexpr logic from
_M_use_local_data, so that it can be used separately in cases that don't
need to use std::pointer_traits<pointer>::pointer_to to obtain the
return value.

This leaves only three uses of _M_use_local_data() which are all of the
same form:

  __s._M_data(_M_use_local_data());
  __s._M_set_length(0);

We could remove _M_use_local_data() and change those three places to use
a new _M_reset() function that does:

  _M_init_local_buf();
  _M_data(_M_local_data());
  _M_set_length(0);

This is left for a future change.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (_M_init_local_buf()): New
	function.
	(_M_use_local_data()): Use _M_init_local_buf.
	(basic_string(), basic_string(const Alloc&))
	(basic_string(basic_string&&))
	(basic_string(basic_string&&, const Alloc&)): Use
	_M_init_local_buf instead of _M_use_local_data().
	* include/bits/basic_string.tcc (swap(basic_string&))
	(_M_construct(InIter, InIter, input_iterator_tag))
	(_M_construct(InIter, InIter, forward_iterator_tag))
	(_M_construct(size_type, CharT), reserve()): Likewise.

2 files changed, 26 insertions, 15 deletions

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 4f94cd9..0fa32af 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -353,14 +353,24 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       // Ensure that _M_local_buf is the active member of the union.
       __attribute__((__always_inline__))
       _GLIBCXX14_CONSTEXPR
-      pointer
-      _M_use_local_data() _GLIBCXX_NOEXCEPT
+      void
+      _M_init_local_buf() _GLIBCXX_NOEXCEPT
       {
 #if __cpp_lib_is_constant_evaluated
 	if (std::is_constant_evaluated())
 	  for (size_type __i = 0; __i <= _S_local_capacity; ++__i)
 	    _M_local_buf[__i] = _CharT();
 #endif
+      }
+
+      __attribute__((__always_inline__))
+      _GLIBCXX14_CONSTEXPR
+      pointer
+      _M_use_local_data() _GLIBCXX_NOEXCEPT
+      {
+#if __cpp_lib_is_constant_evaluated
+	_M_init_local_buf();
+#endif
 	return _M_local_data();
       }
 
@@ -522,7 +532,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       _GLIBCXX_NOEXCEPT_IF(is_nothrow_default_constructible<_Alloc>::value)
       : _M_dataplus(_M_local_data())
       {
-	_M_use_local_data();
+	_M_init_local_buf();
 	_M_set_length(0);
       }
 
@@ -534,7 +544,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       basic_string(const _Alloc& __a) _GLIBCXX_NOEXCEPT
       : _M_dataplus(_M_local_data(), __a)
       {
-	_M_use_local_data();
+	_M_init_local_buf();
 	_M_set_length(0);
       }
 
@@ -678,7 +688,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       {
 	if (__str._M_is_local())
 	  {
-	    (void)_M_use_local_data();
+	    _M_init_local_buf();
 	    traits_type::copy(_M_local_buf, __str._M_local_buf,
 			      __str.length() + 1);
 	  }
@@ -718,7 +728,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       {
 	if (__str._M_is_local())
 	  {
-	    (void)_M_use_local_data();
+	    _M_init_local_buf();
 	    traits_type::copy(_M_local_buf, __str._M_local_buf,
 			      __str.length() + 1);
 	    _M_length(__str.length());
diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
index 4bc98f2..f0a44e5 100644
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -79,7 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	      }
 	    else if (__s.length())
 	      {
-		(void)_M_use_local_data();
+		_M_init_local_buf();
 		traits_type::copy(_M_local_buf, __s._M_local_buf,
 				  __s.length() + 1);
 		_M_length(__s.length());
@@ -88,7 +88,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	      }
 	    else if (length())
 	      {
-		(void)__s._M_use_local_data();
+		__s._M_init_local_buf();
 		traits_type::copy(__s._M_local_buf, _M_local_buf,
 				  length() + 1);
 		__s._M_length(length());
@@ -99,7 +99,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	else
 	  {
 	    const size_type __tmp_capacity = __s._M_allocated_capacity;
-	    (void)__s._M_use_local_data();
+	    __s._M_init_local_buf();
 	    traits_type::copy(__s._M_local_buf, _M_local_buf,
 			      length() + 1);
 	    _M_data(__s._M_data());
@@ -111,7 +111,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  const size_type __tmp_capacity = _M_allocated_capacity;
 	  if (__s._M_is_local())
 	    {
-	      (void)_M_use_local_data();
+	      _M_init_local_buf();
 	      traits_type::copy(_M_local_buf, __s._M_local_buf,
 				__s.length() + 1);
 	      __s._M_data(_M_data());
@@ -174,11 +174,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	size_type __len = 0;
 	size_type __capacity = size_type(_S_local_capacity);
 
-	pointer __p = _M_use_local_data();
+	_M_init_local_buf();
 
 	while (__beg != __end && __len < __capacity)
 	  {
-	    __p[__len++] = *__beg;
+	    _M_local_buf[__len++] = *__beg;
 	    ++__beg;
 	  }
 
@@ -230,7 +230,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    _M_capacity(__dnew);
 	  }
 	else
-	  _M_use_local_data();
+	  _M_init_local_buf();
 
 	// Check for out_of_range and length_error exceptions.
 	struct _Guard
@@ -263,7 +263,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  _M_capacity(__n);
 	}
       else
-	_M_use_local_data();
+	_M_init_local_buf();
 
       if (__n)
 	this->_S_assign(_M_data(), __n, __c);
@@ -372,7 +372,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       if (__length <= size_type(_S_local_capacity))
 	{
-	  this->_S_copy(_M_use_local_data(), _M_data(), __length + 1);
+	  _M_init_local_buf();
+	  this->_S_copy(_M_local_buf, _M_data(), __length + 1);
 	  _M_destroy(__capacity);
 	  _M_data(_M_local_data());
 	}