std:vec for classes with constructor?

Richard Biener richard.guenther@gmail.com
Thu Aug 6 10:31:06 GMT 2020


On Thu, Aug 6, 2020 at 12:19 PM Jonathan Wakely <jwakely@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 <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.

It's probably also historic because we couldn't even implement
the case of re-allocation correctly without std::move, could we?

> >> 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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