Make `vector` unconditionally move elements when exceptions are disabled.
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
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@370502 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/memory b/include/memory
index 321d5fe..ec1f3d9 100644
--- a/include/memory
+++ b/include/memory
@@ -1609,10 +1609,16 @@
_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)
{
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 +1631,7 @@
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 +1678,18 @@
_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)
{
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 +1703,7 @@
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;
diff --git a/include/vector b/include/vector
index 2d83484..a07243a 100644
--- a/include/vector
+++ b/include/vector
@@ -948,7 +948,8 @@
vector<_Tp, _Allocator>::__swap_out_circular_buffer(__split_buffer<value_type, allocator_type&>& __v)
{
__annotate_delete();
- __alloc_traits::__construct_backward(this->__alloc(), this->__begin_, this->__end_, __v.__begin_);
+ __alloc_traits::__construct_backward_with_exception_guarantees(
+ this->__alloc(), this->__begin_, this->__end_, __v.__begin_);
_VSTD::swap(this->__begin_, __v.__begin_);
_VSTD::swap(this->__end_, __v.__end_);
_VSTD::swap(this->__end_cap(), __v.__end_cap());
@@ -963,8 +964,10 @@
{
__annotate_delete();
pointer __r = __v.__begin_;
- __alloc_traits::__construct_backward(this->__alloc(), this->__begin_, __p, __v.__begin_);
- __alloc_traits::__construct_forward(this->__alloc(), __p, this->__end_, __v.__end_);
+ __alloc_traits::__construct_backward_with_exception_guarantees(
+ this->__alloc(), this->__begin_, __p, __v.__begin_);
+ __alloc_traits::__construct_forward_with_exception_guarantees(
+ this->__alloc(), __p, this->__end_, __v.__end_);
_VSTD::swap(this->__begin_, __v.__begin_);
_VSTD::swap(this->__end_, __v.__end_);
_VSTD::swap(this->__end_cap(), __v.__end_cap());
diff --git a/test/libcxx/containers/sequences/vector/exception_safety_exceptions_disabled.sh.cpp b/test/libcxx/containers/sequences/vector/exception_safety_exceptions_disabled.sh.cpp
new file mode 100644
index 0000000..1e9cbde
--- /dev/null
+++ b/test/libcxx/containers/sequences/vector/exception_safety_exceptions_disabled.sh.cpp
@@ -0,0 +1,57 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// RUN: %build -fno-exceptions
+// RUN: %run
+
+// UNSUPPORTED: c++98, c++03
+
+// <vector>
+
+// Test that vector always moves elements when exceptions are disabled.
+// vector is allowed to move or copy elements while resizing, so long as
+// it still provides the strong exception safety guarantee.
+
+#include <vector>
+#include <cassert>
+
+#include "test_macros.h"
+
+#ifndef TEST_HAS_NO_EXCEPTIONS
+#error exceptions should be disabled.
+#endif
+
+bool allow_moves = false;
+
+class A {
+public:
+ A() {}
+ A(A&&) { assert(allow_moves); }
+ explicit A(int) {}
+ A(A const&) { assert(false); }
+};
+
+int main(int, char**) {
+ std::vector<A> v;
+
+ // Create a vector containing some number of elements that will
+ // have to be moved when it is resized.
+ v.reserve(10);
+ size_t old_cap = v.capacity();
+ for (int i = 0; i < v.capacity(); ++i) {
+ v.emplace_back(42);
+ }
+ assert(v.capacity() == old_cap);
+ assert(v.size() == v.capacity());
+
+ // The next emplace back should resize.
+ allow_moves = true;
+ v.emplace_back(42);
+
+ return 0;
+}
