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Make svector moves and swap non-allocating #2949

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120 changes: 85 additions & 35 deletions include/xtensor/containers/xstorage.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -680,7 +680,7 @@ namespace xt
requires(N != N2);

svector(const svector& other);
svector(svector&& other) noexcept(std::is_nothrow_move_constructible<value_type>::value);
svector(svector&& other) noexcept(std::is_nothrow_move_assignable<value_type>::value);

void assign(size_type n, const value_type& v);

Expand Down Expand Up @@ -761,6 +761,7 @@ namespace xt

void grow(size_type min_capacity = 0);
void destroy_range(T* begin, T* end);
void move_from(svector& rhs) noexcept(std::is_nothrow_move_assignable<value_type>::value);
};

template <class T, std::size_t N, class A, bool Init>
Expand Down Expand Up @@ -846,7 +847,10 @@ namespace xt
inline svector<T, N, A, Init>& svector<T, N, A, Init>::operator=(svector&& rhs
) noexcept(std::is_nothrow_move_assignable<value_type>::value)
{
assign(rhs.begin(), rhs.end());
if (this != &rhs)
{
move_from(rhs);
}
return *this;
}

Expand Down Expand Up @@ -889,9 +893,10 @@ namespace xt

template <class T, std::size_t N, class A, bool Init>
inline svector<T, N, A, Init>::svector(svector&& rhs
) noexcept(std::is_nothrow_move_constructible<value_type>::value)
) noexcept(std::is_nothrow_move_assignable<value_type>::value)
: m_allocator(rhs.m_allocator)
{
this->swap(rhs);
move_from(rhs);
}

template <class T, std::size_t N, class A, bool Init>
Expand Down Expand Up @@ -1275,46 +1280,89 @@ namespace xt
return;
}

// We can only avoid copying elements if neither vector is small.
if (!this->on_stack() && !rhs.on_stack())
if constexpr (std::is_same<svector<T, ON, OA, InitA>, self_type>::value)
{
swap(this->m_begin, rhs.m_begin);
swap(this->m_end, rhs.m_end);
swap(this->m_capacity, rhs.m_capacity);
return;
// Moves never allocate, so swapping two vectors of the same type cannot fail.
self_type tmp(std::move(*this));
*this = std::move(rhs);
rhs = std::move(tmp);
}
else
{
// We can only avoid copying elements if neither vector is small.
if (!this->on_stack() && !rhs.on_stack())
{
swap(this->m_begin, rhs.m_begin);
swap(this->m_end, rhs.m_end);
swap(this->m_capacity, rhs.m_capacity);
return;
}

size_type rhs_old_size = rhs.size();
size_type old_size = this->size();
size_type rhs_old_size = rhs.size();
size_type old_size = this->size();

if (rhs_old_size > old_size)
{
this->resize(rhs_old_size);
}
else if (old_size > rhs_old_size)
{
rhs.resize(old_size);
}
if (rhs_old_size > old_size)
{
this->resize(rhs_old_size);
}
else if (old_size > rhs_old_size)
{
rhs.resize(old_size);
}

// Swap the shared elements.
size_type min_size = (std::min)(old_size, rhs_old_size);
for (size_type i = 0; i < min_size; ++i)
{
swap((*this)[i], rhs[i]);
// Swap the shared elements.
size_type min_size = (std::min)(old_size, rhs_old_size);
for (size_type i = 0; i < min_size; ++i)
{
swap((*this)[i], rhs[i]);
}

// Copy over the extra elts.
if (old_size > rhs_old_size)
{
std::copy(this->begin() + min_size, this->end(), rhs.begin() + min_size);
this->destroy_range(this->begin() + min_size, this->end());
this->m_end = this->begin() + min_size;
}
else if (rhs_old_size > old_size)
{
std::copy(rhs.begin() + min_size, rhs.end(), this->begin() + min_size);
this->destroy_range(rhs.begin() + min_size, rhs.end());
rhs.m_end = rhs.begin() + min_size;
}
}
}

// Copy over the extra elts.
if (old_size > rhs_old_size)
template <class T, std::size_t N, class A, bool Init>
inline void svector<T, N, A, Init>::move_from(svector& rhs
) noexcept(std::is_nothrow_move_assignable<value_type>::value)
{
if (rhs.on_stack())
{
std::copy(this->begin() + min_size, this->end(), rhs.begin() + min_size);
this->destroy_range(this->begin() + min_size, this->end());
this->m_end = this->begin() + min_size;
// rhs holds at most N elements, and the current storage (inline buffer or a heap
// buffer, whose capacity is always greater than N) can hold them.
m_end = std::move(rhs.m_begin, rhs.m_end, m_begin);
rhs.m_end = rhs.m_begin;
}
else if (rhs_old_size > old_size)
else
{
std::copy(rhs.begin() + min_size, rhs.end(), this->begin() + min_size);
this->destroy_range(rhs.begin() + min_size, rhs.end());
rhs.m_end = rhs.begin() + min_size;
// Take ownership of the heap buffer of rhs together with the allocator that
// allocated it.
if (!on_stack())
{
detail::safe_destroy_deallocate(
m_allocator,
m_begin,
static_cast<std::size_t>(m_capacity - m_begin)
);
}
m_allocator = rhs.m_allocator;
m_begin = rhs.m_begin;
m_end = rhs.m_end;
m_capacity = rhs.m_capacity;
rhs.m_begin = std::begin(rhs.m_data);
rhs.m_end = std::begin(rhs.m_data);
rhs.m_capacity = std::end(rhs.m_data);
}
}

Expand Down Expand Up @@ -1398,7 +1446,9 @@ namespace xt
}

template <class T, std::size_t N, class A, bool Init>
inline void swap(svector<T, N, A, Init>& lhs, svector<T, N, A, Init>& rhs) noexcept
inline void
swap(svector<T, N, A, Init>& lhs, svector<T, N, A, Init>& rhs) noexcept(std::is_nothrow_move_assignable<T>::value
)
{
lhs.swap(rhs);
}
Expand Down
39 changes: 39 additions & 0 deletions test/test_xstorage.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -8,6 +8,8 @@
****************************************************************************/

#include <numeric>
#include <type_traits>
#include <utility>

#include "xtensor/containers/xstorage.hpp"
#include "xtensor/core/xtensor_config.hpp"
Expand Down Expand Up @@ -345,4 +347,41 @@ namespace xt
EXPECT_EQ(a.front(), size_t(3));
EXPECT_EQ(a.size(), size_t(3));
}

TEST(svector, move)
{
static_assert(std::is_nothrow_move_constructible<svector_type>::value);
static_assert(std::is_nothrow_move_assignable<svector_type>::value);
static_assert(std::is_nothrow_swappable<svector_type>::value);

// A heap buffer is transferred, not copied.
svector_type a = {1, 3, 4, 6, 7};
const std::size_t* heap = a.data();
svector_type b(std::move(a));
EXPECT_EQ(b.data(), heap);
EXPECT_EQ(b, svector_type({1, 3, 4, 6, 7}));
EXPECT_TRUE(a.empty());

svector_type c = {1, 2};
c = std::move(b);
EXPECT_EQ(c.data(), heap);
EXPECT_EQ(c, svector_type({1, 3, 4, 6, 7}));
EXPECT_TRUE(b.empty());

// Elements stored inline are moved into the existing storage.
svector_type d = {5, 6};
svector_type e(std::move(d));
EXPECT_EQ(e, svector_type({5, 6}));
EXPECT_TRUE(d.empty());

c = std::move(e);
EXPECT_EQ(c.data(), heap);
EXPECT_EQ(c, svector_type({5, 6}));
EXPECT_TRUE(e.empty());

// Moved-from vectors can be reused.
a = {8, 9};
EXPECT_EQ(a, svector_type({8, 9}));
}

}
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