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 <jwak...@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.
Tested powerpc64le-linux. Committed to trunk.
commit 10b6d89baddd86139480ba902f491903fcb464a6
Author: Jonathan Wakely <jwak...@redhat.com>
Date: Fri Apr 30 15:04:34 2021
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 <jwak...@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 --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
}
+#else
+ : _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.
