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Recommit r370502: Make `vector` unconditionally move elements when exceptions…

Authored by EricWF on Sep 13 2019, 9:09 AM.

Description

Recommit r370502: Make vector unconditionally move elements when exceptions are disabled.

The patch was reverted due to some confusion about non-movable types. ie
types
that explicitly delete their move constructors. However, such types do
not meet
the requirement for MoveConstructible, which is required by
std::vector:

Summary:

std::vector<T> is free choose between using copy or move operations
when it
needs to resize. The standard only candidates that the correct exception
safety
guarantees are provided. When exceptions are disabled these guarantees
are
trivially satisfied. Meaning vector is free to optimize it's
implementation by
moving instead of copying.

This patch makes std::vector unconditionally move elements when
exceptions are
disabled. This optimization is conforming according to the current
standard wording.

There are concerns that moving in -fno-noexceptionsmode will be a
surprise to
users. For example, a user may be surprised to find their code is slower
with
exceptions enabled than it is disabled. I'm sympathetic to this
surprised, but
I don't think it should block this optimization.

Reviewers: mclow.lists, ldionne, rsmith
Reviewed By: ldionne
Subscribers: zoecarver, christof, dexonsmith, libcxx-commits
Tags: #libc
Differential Revision: https://reviews.llvm.org/D62228

llvm-svn: 371867

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