From: "François Dumont" <frs.dumont@gmail.com>
To: Jonathan Wakely <jwakely@redhat.com>
Cc: "libstdc++@gcc.gnu.org" <libstdc++@gcc.gnu.org>,
gcc-patches <gcc-patches@gcc.gnu.org>
Subject: Re: Fix move_if_noexcept usages in _Hashtable
Date: Tue, 04 Dec 2018 21:43:00 -0000 [thread overview]
Message-ID: <81e3fa4e-e4cc-67b5-0329-e6800cc1d62a@gmail.com> (raw)
In-Reply-To: <20181204143836.GC27131@redhat.com>
On 12/4/18 3:38 PM, Jonathan Wakely wrote:
> On 04/12/18 07:45 +0100, François Dumont wrote:
>> Hi
>>
>> Â This patch fix a minor problem with usage of std::move_if_noexcept.
>> We use it to move node content if move construtor is noexcept but we
>> eventually use the allocator_type::construct method which might be
>> slightly different. I think it is better to check for this method
>> noexcept qualification.
>
> This is likely to pessimize some code, since most allocators do not
> have an exception-specification on their construct members.
>
Perhaps but the Standard mandates to call allocator construct so we
don't have choice. It is surprising to consider value_type move
constructor when we don't know what allocator construct does.
Most users do not change from std::allocator or, even if they do, do not
implement construct so impact should be limited.
>> Â Moreover I have added a special overload for nodes containing a
>> std::pair. It is supposed to allow move semantic in associative
>> containers where Key is stored as const deleting std::pair move
>> constructor. In this case we should still move the Value part.
>>
>> Â It doesn't work for the moment because the std::pair piecewise
>> constructor has no noexcept qualification. Is there any plan to add
>> it ? I think adding it will force including <tuple> in stl_pair.h, is
>> it fine ?
No feedback on this point ? Is using std::pair piecewise constructor ok ?
>>
>> Â If this evolution is accepted I'll adapt it for _Rb_tree that has
>> the same problem.
>>
>> Â Working on this I also notice that content of initialization_list
>> is not moved. Is there a plan to make initialization_list iterator
>> type like move_iterator ? Should containers use
>> __make_move_iterator_if_noexcept ?
>
> No.
>
> Whether to allow moving from std::initializer_list is an active topic,
> see
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1249r0.html
>
Ok, nice, allowing emplace build of values would be even better, I'll
have a closer look.
>> Â Tested under Linux x86_64 normal mode.
>>
>> Â Ok to commit this first step ?
>
> No, this is not suitable for stage 3. It seems too risky.
>
> We can reconsider it during stage 1, but I'd like to see (at least) a
> new test showing a bug with the current code. For example, a type with
> a move constructor that is noexcept, but when used with a
> scoped_allocator_adaptor (which calls something other than the move
> constructor) we incorrectly move elements, and lose data when an
> exception happens.
>
>
Ok, I'll try.
next prev parent reply other threads:[~2018-12-04 21:43 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-12-04 6:45 François Dumont
2018-12-04 14:38 ` Jonathan Wakely
2018-12-04 21:43 ` François Dumont [this message]
2018-12-16 18:10 ` François Dumont
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