std:vec for classes with constructor?

Jonathan Wakely jwakely@redhat.com
Thu Aug 6 10:19:17 GMT 2020


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 <mjambor@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.

>> I quizzed some libstdc++ folks, and there has been a lot of
>> optimizations done on std::vec over the last few years,.. They think its
>> pretty good now, and we were encouraged to use it.
>>
>> We can visit the question tho...  What is the rationale for not using
>> std::vec in the compiler?  We currently use std::swap, std:pair,
>> std::map, std::sort, and a few others.
>> is there some aspect of using std::vec I am not aware of that makes it
>> something we need to avoid?
>
>One reason to prefer vec<> for general interfaces is that it
>works with auto_vec<…, N>, making it possible to pre-allocate a
>reasonably-sized buffer on the stack without needing a round-trip
>through the allocators.
>
>FWIW, that isn't simply a GCC thing.  LLVM (which is obviously much
>more C++-intensive than GCC) still makes heavy use of SmallVector for
>automatic variables.  And the reason we have things like memory_block.h
>is that malloc did used to show up high in profiles.

Yes, LLVM's SmallVector is very useful. You can achieve a similar
thing with a custom allocator in std::vector, but it's more cumbersome
and it alters the type from std::vector<X> to std::vector<X, Y>.

The beauty of the LLVM design is the common base class for
SmallVector<T, N> is the same for all N, so you can pass it to APIs
that don't care about the size and just work with the base interface.

>(FWIW, I'm not saying that's an argument in favour of avoiding
>std::vector completely.  It's just a reason why it might not always
>be the right choice.)
>
>Thanks,
>Richard
>



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