[gcc r12-4065] libstdc++: Make move ctor noexcept for fully-dynamic string

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:10b6d89baddd86139480ba902f491903fcb464a6

commit r12-4065-g10b6d89baddd86139480ba902f491903fcb464a6
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Apr 30 15:04:34 2021 +0100

    libstdc++: Make move ctor noexcept for fully-dynamic string
    
    The move constructor for the "fully-dynamic" COW string is not noexcept,
    because it allocates a new empty string rep for the moved-from string.
    However, there is no need to do that, because the moved-from string does
    not have to be left empty. Instead, implement move construction for the
    fully-dynamic case as a reference count increment, so the string is
    shared.
    
    Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/cow_string.h [_GLIBCXX_FULLY_DYNAMIC_STRING]
            (basic_string(basic_string&&)): Add noexcept and avoid
            allocation, by sharing rep with the rvalue string.

Diff:
---
 libstdc++-v3/include/bits/cow_string.h | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/libstdc++-v3/include/bits/cow_string.h b/libstdc++-v3/include/bits/cow_string.h
index 61edaa85484..ba4a8cc2e98 100644
--- a/libstdc++-v3/include/bits/cow_string.h
+++ b/libstdc++-v3/include/bits/cow_string.h
@@ -620,18 +620,25 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *  The newly-created string contains the exact contents of @a __str.
        *  @a __str is a valid, but unspecified string.
        */
-      basic_string(basic_string&& __str)
+      basic_string(basic_string&& __str) noexcept
 #if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
-      noexcept // FIXME C++11: should always be noexcept.
-#endif
       : _M_dataplus(std::move(__str._M_dataplus))
       {
-#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
 	__str._M_data(_S_empty_rep()._M_refdata());
+      }
 #else
-	__str._M_data(_S_construct(size_type(), _CharT(), get_allocator()));
-#endif
+      : _M_dataplus(__str._M_rep())
+      {
+	// Rather than allocate an empty string for the rvalue string,
+	// just share ownership with it by incrementing the reference count.
+	// If the rvalue string was "leaked" then it was the unique owner,
+	// so need an extra increment to indicate shared ownership.
+	if (_M_rep()->_M_is_leaked())
+	  __gnu_cxx::__atomic_add_dispatch(&_M_rep()->_M_refcount, 2);
+	else
+	  __gnu_cxx::__atomic_add_dispatch(&_M_rep()->_M_refcount, 1);
       }
+#endif
 
       /**
        *  @brief  Construct string from an initializer %list.