On 06/08/20 12:31 +0200, Richard Biener wrote:
On Thu, Aug 6, 2020 at 12:19 PM Jonathan Wakely <jwak...@redhat.com> wrote:
On 06/08/20 06:16 +0100, Richard Sandiford wrote:
>Andrew MacLeod via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
>> On 8/5/20 12:54 PM, Richard Biener via Gcc-patches wrote:
>>> On August 5, 2020 5:09:19 PM GMT+02:00, Martin Jambor <mjam...@suse.cz>
wrote:
>>>> On Fri, Jul 31 2020, Aldy Hernandez via Gcc-patches wrote:
>>>> [...]
>>>>
>>>>> * ipa-cp changes from vec<value_range> to std::vec<value_range>.
>>>>>
>>>>> We are using std::vec to ensure constructors are run, which they
>>>> aren't
>>>>> in our internal vec<> implementation. Although we usually steer away
>>>>> from using std::vec because of interactions with our GC system,
>>>>> ipcp_param_lattices is only live within the pass and allocated with
>>>> calloc.
>>>> Ummm... I did not object but I will save the URL of this message in the
>>>> archive so that I can waive it in front of anyone complaining why I
>>>> don't use our internal vec's in IPA data structures.
>>>>
>>>> But it actually raises a broader question: was this supposed to be an
>>>> exception, allowed only not to complicate the irange patch further, or
>>>> will this be generally accepted thing to do when someone wants to have
>>>> a
>>>> vector of constructed items?
>>> It's definitely not what we want. You have to find another solution to this
problem.
>>>
>>> Richard.
>>>
>>
>> Why isn't it what we want?
>>
>> This is a small vector local to the pass so it doesn't interfere with
>> our PITA GTY.
>> The class is pretty straightforward, but we do need a constructor to
>> initialize the pointer and the max-size field. There is no allocation
>> done per element, so a small number of elements have a couple of fields
>> initialized per element. We'd have to loop to do that anyway.
>>
>> GCC's vec<> does not provide he ability to run a constructor, std::vec
>> does.
>
>I realise you weren't claiming otherwise, but: that could be fixed :-)
It really should be.
Artificial limitations like that are just a booby trap for the unwary.
It's probably also historic because we couldn't even implement
the case of re-allocation correctly without std::move, could we?
I don't see why not. std::vector worked fine without std::move, it's
just more efficient with std::move, and can be used with a wider set
of element types.
When reallocating you can just copy each element to the new storage
and destroy the old element. If your type is non-copyable then you
need std::move, but I don't think the types I see used with vec<> are
non-copyable. Most of them are trivially-copyable.
I think the benefit of std::move to GCC is likely to be permitting
cheap copies to be made where previously they were banned for
performance reasons, but not because those copies were impossible.
