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libstdc++: Make move ctor noexcept for fully-dynamic string

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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.
This commit is contained in:
Jonathan Wakely 2021-04-30 15:04:34 +01:00
parent ce709ad3dc
commit 10b6d89bad
1 changed files with 14 additions and 7 deletions
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21
libstdc++-v3/include/bits/cow_string.h
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@ -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.