On 01/02/21 19:30 +0100, François Dumont via Libstdc++ wrote:
On 01/02/21 6:43 pm, Jonathan Wakely wrote:
On 31/01/21 16:59 +0100, François Dumont via Libstdc++ wrote:
After the debug issue has been fixed in PR 98466 the problem was
not in the debug iterator implementation itself but in the deque
iterator operator- implementation.
libstdc++: Make deque iterator operator- usable with
value-init iterators
N3644 implies that operator- can be used on value-init
iterators. We now return
0 if both iterators are value initialized. If only one is
value initialized we
keep the UB by returning the result of a normal computation
which is an unexpected
value.
libstdc++/ChangeLog:
PR libstdc++/70303
* include/bits/stl_deque.h
(std::deque<>::operator-(iterator, iterator)):
Return 0 if both iterators are value-initialized.
* testsuite/23_containers/deque/70303.cc: New test.
* testsuite/23_containers/vector/70303.cc: New test.
Tested under Linux x86_64, ok to commit ?
OK.
I don't like adding the branch there though. Even with the
__builtin_expect it causes worse code to be generated than the
original.
This would be branchless, but a bit harder to understand:
return difference_type(__x._S_buffer_size())
* (__x._M_node - __y._M_node - int(__x._M_node == __y._M_node))
+ (__x._M_cur - __x._M_first) + (__y._M_last - __y._M_cur);
Please commit the fix and we can think about it later.
I do not think this work because for value-init iterators we have
__x._M_node == __y._M_node == nullptr so the diff would give
-_S_buffer_size().
But I got the idear, I'll prepare the patch.
Yeah, I just came back to the computer to say that it's wrong. But
maybe this:
return difference_type(_S_buffer_size())
* (__x._M_node - __y._M_node - int(__x._M_node && __y._M_node))
+ (__x._M_cur - __x._M_first) + (__y._M_last - __y._M_cur);
For value-init'd iterators we'd get _S_buffer_size() * 0 + 0 - 0
We could even just use int(__x._M_node != 0) because if one is null
and the other isn't, it's UB anyway.
