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AK: Make empty FixedArray smaller

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Move the FixedArray's size field into the heap-allocated storage. This
makes zero-sized FixedArrays take up 8 bytes instead of 16.
This commit is contained in:
Andreas Kling 2022-08-26 15:03:46 +02:00
parent 98672e09dd
commit f03f70a84a
1 changed files with 48 additions and 58 deletions
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106
AK/FixedArray.h
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@ -37,12 +37,13 @@ public:
{
if (size == 0)
return FixedArray<T>();
T* elements = static_cast<T*>(kmalloc_array(size, sizeof(T)));
if (!elements)
auto* new_storage = static_cast<Storage*>(kmalloc(storage_allocation_size(size)));
if (!new_storage)
return Error::from_errno(ENOMEM);
new_storage->size = size;
for (size_t i = 0; i < size; ++i)
new (&elements[i]) T();
return FixedArray<T>(size, elements);
new (&new_storage->elements[i]) T();
return FixedArray<T>(new_storage);
}
static FixedArray<T> must_create_but_fixme_should_propagate_errors(size_t size)
@ -50,21 +51,10 @@ public:
return MUST(try_create(size));
}
// NOTE:
// Even though it may look like there will be a template instantiation of this function for every size,
// the compiler will inline this anyway and therefore not generate any duplicate code.
template<size_t N>
static ErrorOr<FixedArray<T>> try_create(T (&&array)[N])
{
if (N == 0)
return FixedArray<T>();
T* elements = static_cast<T*>(kmalloc_array(N, sizeof(T)));
if (!elements)
return Error::from_errno(ENOMEM);
for (size_t i = 0; i < N; ++i)
new (&elements[i]) T(move(array[i]));
return FixedArray<T>(N, elements);
return try_create(Span(array, N));
}
template<typename U>
@ -72,24 +62,23 @@ public:
{
if (span.size() == 0)
return FixedArray<T>();
T* elements = static_cast<T*>(kmalloc_array(span.size(), sizeof(T)));
if (!elements)
auto* new_storage = static_cast<Storage*>(kmalloc(storage_allocation_size(span.size())));
if (!new_storage)
return Error::from_errno(ENOMEM);
new_storage->size = span.size();
for (size_t i = 0; i < span.size(); ++i)
new (&elements[i]) T(span[i]);
return FixedArray<T>(span.size(), elements);
new (&new_storage->elements[i]) T(span[i]);
return FixedArray<T>(new_storage);
}
ErrorOr<FixedArray<T>> try_clone() const
{
if (m_size == 0)
return FixedArray<T>();
T* elements = static_cast<T*>(kmalloc_array(m_size, sizeof(T)));
if (!elements)
return Error::from_errno(ENOMEM);
for (size_t i = 0; i < m_size; ++i)
new (&elements[i]) T(m_elements[i]);
return FixedArray<T>(m_size, elements);
return try_create(span());
}
static size_t storage_allocation_size(size_t size)
{
return sizeof(Storage) + size * sizeof(T);
}
// Nobody can ever use these functions, since it would be impossible to make them OOM-safe due to their signatures. We just explicitly delete them.
@ -97,42 +86,37 @@ public:
FixedArray<T>& operator=(FixedArray<T> const&) = delete;
FixedArray(FixedArray<T>&& other)
: m_size(other.m_size)
, m_elements(other.m_elements)
: m_storage(exchange(other.m_storage, nullptr))
{
other.m_size = 0;
other.m_elements = nullptr;
}
// This function would violate the contract, as it would need to deallocate this FixedArray. As it also has no use case, we delete it.
FixedArray<T>& operator=(FixedArray<T>&&) = delete;
~FixedArray()
{
if (!m_elements)
if (!m_storage)
return;
for (size_t i = 0; i < m_size; ++i)
m_elements[i].~T();
kfree_sized(m_elements, sizeof(T) * m_size);
// NOTE: should prevent use-after-free early
m_size = 0;
m_elements = nullptr;
for (size_t i = 0; i < m_storage->size; ++i)
m_storage->elements[i].~T();
kfree_sized(m_storage, storage_allocation_size(m_storage->size));
m_storage = nullptr;
}
size_t size() const { return m_size; }
bool is_empty() const { return m_size == 0; }
T* data() { return m_elements; }
T const* data() const { return m_elements; }
size_t size() const { return m_storage ? m_storage->size : 0; }
bool is_empty() const { return size() == 0; }
T* data() { return m_storage ? m_storage->elements : nullptr; }
T const* data() const { return m_storage ? m_storage->elements : nullptr; }
T& at(size_t index)
{
VERIFY(index < m_size);
return m_elements[index];
VERIFY(index < m_storage->size);
return m_storage->elements[index];
}
T const& at(size_t index) const
{
VERIFY(index < m_size);
return m_elements[index];
VERIFY(index < m_storage->size);
return m_storage->elements[index];
}
T& operator[](size_t index)
@ -147,8 +131,10 @@ public:
bool contains_slow(T const& value) const
{
for (size_t i = 0; i < m_size; ++i) {
if (m_elements[i] == value)
if (!m_storage)
return false;
for (size_t i = 0; i < m_storage->size; ++i) {
if (m_storage->elements[i] == value)
return true;
}
return false;
@ -156,14 +142,15 @@ public:
void swap(FixedArray<T>& other)
{
::swap(m_size, other.m_size);
::swap(m_elements, other.m_elements);
::swap(m_storage, other.m_storage);
}
void fill_with(T const& value)
{
for (size_t i = 0; i < m_size; ++i)
m_elements[i] = value;
if (!m_storage)
return;
for (size_t i = 0; i < m_storage->size; ++i)
m_storage->elements[i] = value;
}
using Iterator = SimpleIterator<FixedArray, T>;
@ -179,14 +166,17 @@ public:
Span<T const> span() const { return { data(), size() }; }
private:
FixedArray(size_t size, T* elements)
: m_size(size)
, m_elements(elements)
struct Storage {
size_t size { 0 };
T elements[0];
};
FixedArray(Storage* storage)
: m_storage(storage)
{
}
size_t m_size { 0 };
T* m_elements { nullptr };
Storage* m_storage { nullptr };
};
}