From f03f70a84abcec720140ed39d5cba209c8a69475 Mon Sep 17 00:00:00 2001 From: Andreas Kling Date: Fri, 26 Aug 2022 15:03:46 +0200 Subject: [PATCH] AK: Make empty FixedArray smaller Move the FixedArray's size field into the heap-allocated storage. This makes zero-sized FixedArrays take up 8 bytes instead of 16. --- AK/FixedArray.h | 106 ++++++++++++++++++++++-------------------------- 1 file changed, 48 insertions(+), 58 deletions(-) diff --git a/AK/FixedArray.h b/AK/FixedArray.h index 90cc2d6bb8..312d94f830 100644 --- a/AK/FixedArray.h +++ b/AK/FixedArray.h @@ -37,12 +37,13 @@ public: { if (size == 0) return FixedArray(); - T* elements = static_cast(kmalloc_array(size, sizeof(T))); - if (!elements) + auto* new_storage = static_cast(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(size, elements); + new (&new_storage->elements[i]) T(); + return FixedArray(new_storage); } static FixedArray 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 static ErrorOr> try_create(T (&&array)[N]) { - if (N == 0) - return FixedArray(); - T* elements = static_cast(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(N, elements); + return try_create(Span(array, N)); } template @@ -72,24 +62,23 @@ public: { if (span.size() == 0) return FixedArray(); - T* elements = static_cast(kmalloc_array(span.size(), sizeof(T))); - if (!elements) + auto* new_storage = static_cast(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(span.size(), elements); + new (&new_storage->elements[i]) T(span[i]); + return FixedArray(new_storage); } ErrorOr> try_clone() const { - if (m_size == 0) - return FixedArray(); - T* elements = static_cast(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(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& operator=(FixedArray const&) = delete; FixedArray(FixedArray&& 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& operator=(FixedArray&&) = 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& 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; @@ -179,14 +166,17 @@ public: Span 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 }; }; }