https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120987
--- Comment #10 from Tom de Vries <vries at gcc dot gnu.org> ---
(In reply to Tom de Vries from comment #4)
> and then rethrown:
> ...
> if (file_exception.reason < 0)
> throw_exception (std::move (file_exception));
> ...
>
> The gdb_exception class contains a message member:
> ...
> std::shared_ptr<std::string> message;
> ...
> and the std::move should leave file_exception.message in a "valid but
> unspecified state" but gcc seems optimize that away.
Let's look at this in a bit more detail, at "-O0 -g".
We set a breakpoint at the line containing throw_exception, and run to it.
At that point, message is:
...
(gdb) p file_exception.message
$2 = std::shared_ptr<std::__cxx11::basic_string<char, std::char_traits<char>, s
td::allocator<char> >> (use count 1, weak count 0) = {get() = 0x1371640}
...
We step into throw_exception, until here:
...
188 else if (exception.reason == RETURN_ERROR)
> 189 throw gdb_exception_error (std::move (exception));
...
Then step into gdb_exception_error constructor:
...
280 explicit gdb_exception_error (gdb_exception &&ex) noexcept
> 281 : gdb_exception (std::move (ex))
...
Then into gdb_exception constructor:
...
> 144 explicit gdb_exception (gdb_exception &&other) noexcept = default
...
Then into shared_ptr constructor:
...
358 shared_ptr(shared_ptr&& __r) noexcept
> 359 : __shared_ptr<_Tp>(std::move(__r)) { }
...
Then into __shared_ptr constructor:
...
│ 1532 __shared_ptr(__shared_ptr&& __r) noexcept │
│ > 1533 : _M_ptr(__r._M_ptr), _M_refcount() │
│ 1534 { │
│ 1535 _M_refcount._M_swap(__r._M_refcount); │
│ 1536 __r._M_ptr = nullptr; │
│ 1537 } │
...
When entering this function, file_exception.message still has the same value.
After the swap, we have:
...
(gdb) p file_exception.message
$10 = std::shared_ptr<std::__cxx11::basic_string<char, std::char_traits<char>,
std::allocator<char> >> (empty) = {get() = 0x1371640}
(gdb)
...
and after the nullptr assignment we have:
...
gdb) p file_exception.message
$12 = std::shared_ptr<std::__cxx11::basic_string<char, std::char_traits<char>,
std::allocator<char> >> (empty) = {get() = 0x0}
...
Now with "-O2 -flto=auto".
We start out with:
...
(gdb) p file_exception.message
$2 = std::shared_ptr<std::__cxx11::basic_string<char, std::char_traits<char>,
std::allocator<char> >> (use count 1, weak count 0) = {get() = <optimized out>}
...
We step into throw_exception, then gdb_exception_error constructor, then
gdb_exception constructor, then shared_ptr constructor, then __shared_ptr
constructor. The code is scattered by lto, so I stepi through it.
Finally, we reach __cxa_throw.
At that point, file_exception.message is still the same (though the optimized
out part may have changed):
...
(gdb) p file_exception.message
$21 = std::shared_ptr<std::__cxx11::basic_string<char, std::char_traits<char>,
std::allocator<char> >> (use count 1, weak count 0) = {
get() = <optimized out>}
...
but the use count should have been 0 at that point.
Looking at the first argument of __cxa_throw, the construction of the exception
object went ok:
...
(gdb) p /x $rdi
$23 = 0xe145b0
(gdb) p ((gdb_exception *)0xe145b0)->message
$24 = std::shared_ptr<std::__cxx11::basic_string<char, std::char_traits<char>,
std::allocator<char> >> (use count 1, weak count 0) = {get() = 0xe14620}
(gdb) p *((gdb_exception *)0xe145b0)->message
$25 = "No symbol table is loaded. Use the \"file\" command."
...
