Re: std:vec for classes with constructor?

[Richard Sandiford <richard.sandiford@arm.com>]

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 :-)

> 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.

(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