[PATCH] D49317: Move __construct_forward (etc.) out of std::allocator_traits.

Louis Dionne via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Mon Jul 30 08:43:57 PDT 2018

ldionne added a comment.

In https://reviews.llvm.org/D49317#1180200, @mclow.lists wrote:

> I am not in favor of this patch.
> I'm in favor of fixing the problem that Arthur has described, but not like this, for the following reasons:
> - Conceptually, these are (similar to) "Allocator-based versions of the algorithms proposed in P0040 <https://wg21.link/P0040>", and should (probably? possibly?) look more like them.
> - Mainly, though, I think that the goal of this patch (which is see as 'getting to memcpy') is not the direction that libc++ should take.  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. (See https://libcxx.llvm.org/cxx2a_status.html (comments on `std::copy` and https://bugs.llvm.org/show_bug.cgi?id=25165).

Marshall makes a great point about `memcpy` and `constexpr`... We're trying to make the default allocator constexpr-friendly for C++20, and this doesn't play very nicely with that.

  rCXX libc++


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