[gcc r12-5343] libstdc++: Set active member of union in std::string [PR103295]

[gcc r12-5343] libstdc++: Set active member of union in std::string [PR103295]

[Jonathan Wakely]

https://gcc.gnu.org/g:6afa1083c6ee7b31629fb0c16299b952cb17868c

commit r12-5343-g6afa1083c6ee7b31629fb0c16299b952cb17868c
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Nov 17 10:23:14 2021 +0000

    libstdc++: Set active member of union in std::string [PR103295]
    
    Clang diagnoses that the new constexpr std::string constructors are not
    usable in constant expressions, because they start to write to members
    of the union without setting an active member.
    
    This adds a new helper function which returns the address of the local
    buffer after making it the active member.
    
    This doesn't fix all problems with Clang, because it still refuses to
    write to memory returned by the allocator.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/103295
            * include/bits/basic_string.h (_M_use_local_data()): New
            member function to make local buffer the active member.
            (assign(const basic_string&)): Use it.
            * include/bits/basic_string.tcc (_M_construct, reserve()):
            Likewise.

Diff:
---
 libstdc++-v3/include/bits/basic_string.h   | 15 ++++++++++++++-
 libstdc++-v3/include/bits/basic_string.tcc | 10 ++++++++--
 2 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 0b7d6c0a981..9d281f5daf2 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -325,6 +325,19 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       _M_get_allocator() const
       { return _M_dataplus; }
 
+      // Ensure that _M_local_buf is the active member of the union.
+      __attribute__((__always_inline__))
+      _GLIBCXX14_CONSTEXPR
+      pointer
+      _M_use_local_data() _GLIBCXX_NOEXCEPT
+      {
+#if __cpp_lib_is_constant_evaluated
+	if (__builtin_is_constant_evaluated())
+	  _M_local_buf[0] = _CharT();
+#endif
+	return _M_local_data();
+      }
+
     private:
 
 #ifdef _GLIBCXX_DISAMBIGUATE_REPLACE_INST
@@ -1487,7 +1500,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 		if (__str.size() <= _S_local_capacity)
 		  {
 		    _M_destroy(_M_allocated_capacity);
-		    _M_data(_M_local_data());
+		    _M_data(_M_use_local_data());
 		    _M_set_length(0);
 		  }
 		else
diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
index 5743770b42a..5a51f7e21b5 100644
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -170,9 +170,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	size_type __len = 0;
 	size_type __capacity = size_type(_S_local_capacity);
 
+	pointer __p = _M_use_local_data();
+
 	while (__beg != __end && __len < __capacity)
 	  {
-	    _M_data()[__len++] = *__beg;
+	    __p[__len++] = *__beg;
 	    ++__beg;
 	  }
 
@@ -223,6 +225,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    _M_data(_M_create(__dnew, size_type(0)));
 	    _M_capacity(__dnew);
 	  }
+	else
+	  _M_use_local_data();
 
 	// Check for out_of_range and length_error exceptions.
 	__try
@@ -247,6 +251,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  _M_data(_M_create(__n, size_type(0)));
 	  _M_capacity(__n);
 	}
+      else
+	_M_use_local_data();
 
       if (__n)
 	this->_S_assign(_M_data(), __n, __c);
@@ -355,7 +361,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       if (__length <= size_type(_S_local_capacity))
 	{
-	  this->_S_copy(_M_local_data(), _M_data(), __length + 1);
+	  this->_S_copy(_M_use_local_data(), _M_data(), __length + 1);
 	  _M_destroy(__capacity);
 	  _M_data(_M_local_data());
 	}