mclow.lists added inline comments.
================ Comment at: include/vector:318 + } +} + ---------------- Quuxplusone wrote: > Marshall writes: > > Instead, we should be writing simple loops that the compiler can optimize > > into a call to memcpy if it chooses. Having calls to memcpy in the code > > paths makes it impossible to "constexp-ify" this code. > > Well, I have three thoughts on that. > > (A), "removing the calls to memcpy" sounds like you want to just call the > *actual* move-constructor in a loop, and then later call the actual > destructor in a loop. Which is to say, you don't want libc++ to have a > codepath for this speed optimization at all. That's just leaving a ton of > performance on the table, and I strongly disagree with that idea. > > (B), regardless, couldn't you achieve that goal simply by taking this patch > almost exactly as it is except removing the overloads that take `true_type`? > If you want constexpr-friendliness badly enough that you're willing to call > the move-constructor and destructor even of trivially copyable types, then > you can still use this framework; you just have to remove the overloads that > call memcpy. That wouldn't be a major refactoring. > > (C), surely if you want the best of both worlds, you should be pushing > someone to invent a constexpr memcpy and/or a way to [detect > constexpr-context at compile > time](https://quuxplusone.github.io/blog/2018/06/12/perennial-impossibilities/#detect-the-constexprness-of-the-current-context)? > I don't think it makes sense to pessimize existing (non-constexpr) users in > C++03-through-C++17 just because someone hypothetically might in > C++2a-or-later want to mutate a std::vector in a constexpr context. > Which is to say, you don't want libc++ to have a codepath for this speed > optimization at all. You're completely correct. I don't want libc++ to have such a code path. I want clang to generate a `memcpy` from the code w/o ever mentioning `memcpy` in the source. Repository: rCXX libc++ https://reviews.llvm.org/D49317 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
