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 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 > > >
