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-noexceptions`mode 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
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 2a573784f3679a7abab921018eb1c585dce8e669
diff --git a/include/memory b/include/memory
index 321d5fe..12573ca 100644
--- a/include/memory
+++ b/include/memory
@@ -1507,6 +1507,31 @@
template <class _Tp>
struct __is_default_allocator<_VSTD::allocator<_Tp> > : true_type {};
+
+
+template <class _Alloc,
+ bool = __has_construct<_Alloc, typename _Alloc::value_type*, typename _Alloc::value_type&&>::value && !__is_default_allocator<_Alloc>::value
+ >
+struct __is_cpp17_move_insertable;
+template <class _Alloc>
+struct __is_cpp17_move_insertable<_Alloc, true> : std::true_type {};
+template <class _Alloc>
+struct __is_cpp17_move_insertable<_Alloc, false> : std::is_move_constructible<typename _Alloc::value_type> {};
+
+template <class _Alloc,
+ bool = __has_construct<_Alloc, typename _Alloc::value_type*, const typename _Alloc::value_type&>::value && !__is_default_allocator<_Alloc>::value
+ >
+struct __is_cpp17_copy_insertable;
+template <class _Alloc>
+struct __is_cpp17_copy_insertable<_Alloc, true> : __is_cpp17_move_insertable<_Alloc> {};
+template <class _Alloc>
+struct __is_cpp17_copy_insertable<_Alloc, false> : integral_constant<bool,
+ std::is_copy_constructible<typename _Alloc::value_type>::value &&
+ __is_cpp17_move_insertable<_Alloc>::value>
+ {};
+
+
+
template <class _Alloc>
struct _LIBCPP_TEMPLATE_VIS allocator_traits
{
@@ -1609,10 +1634,18 @@
_LIBCPP_INLINE_VISIBILITY
static
void
- __construct_forward(allocator_type& __a, _Ptr __begin1, _Ptr __end1, _Ptr& __begin2)
+ __construct_forward_with_exception_guarantees(allocator_type& __a, _Ptr __begin1, _Ptr __end1, _Ptr& __begin2)
{
+ static_assert(__is_cpp17_move_insertable<allocator_type>::value,
+ "The specified type does not meet the requirements of Cpp17MoveInsertible");
for (; __begin1 != __end1; ++__begin1, (void) ++__begin2)
- construct(__a, _VSTD::__to_raw_pointer(__begin2), _VSTD::move_if_noexcept(*__begin1));
+ construct(__a, _VSTD::__to_raw_pointer(__begin2),
+#ifdef _LIBCPP_NO_EXCEPTIONS
+ _VSTD::move(*__begin1)
+#else
+ _VSTD::move_if_noexcept(*__begin1)
+#endif
+ );
}
template <class _Tp>
@@ -1625,7 +1658,7 @@
is_trivially_move_constructible<_Tp>::value,
void
>::type
- __construct_forward(allocator_type&, _Tp* __begin1, _Tp* __end1, _Tp*& __begin2)
+ __construct_forward_with_exception_guarantees(allocator_type&, _Tp* __begin1, _Tp* __end1, _Tp*& __begin2)
{
ptrdiff_t _Np = __end1 - __begin1;
if (_Np > 0)
@@ -1672,12 +1705,20 @@
_LIBCPP_INLINE_VISIBILITY
static
void
- __construct_backward(allocator_type& __a, _Ptr __begin1, _Ptr __end1, _Ptr& __end2)
+ __construct_backward_with_exception_guarantees(allocator_type& __a, _Ptr __begin1, _Ptr __end1, _Ptr& __end2)
{
+ static_assert(__is_cpp17_move_insertable<allocator_type>::value,
+ "The specified type does not meet the requirements of Cpp17MoveInsertable");
while (__end1 != __begin1)
{
- construct(__a, _VSTD::__to_raw_pointer(__end2-1), _VSTD::move_if_noexcept(*--__end1));
- --__end2;
+ construct(__a, _VSTD::__to_raw_pointer(__end2 - 1),
+#ifdef _LIBCPP_NO_EXCEPTIONS
+ _VSTD::move(*--__end1)
+#else
+ _VSTD::move_if_noexcept(*--__end1)
+#endif
+ );
+ --__end2;
}
}
@@ -1691,7 +1732,7 @@
is_trivially_move_constructible<_Tp>::value,
void
>::type
- __construct_backward(allocator_type&, _Tp* __begin1, _Tp* __end1, _Tp*& __end2)
+ __construct_backward_with_exception_guarantees(allocator_type&, _Tp* __begin1, _Tp* __end1, _Tp*& __end2)
{
ptrdiff_t _Np = __end1 - __begin1;
__end2 -= _Np;