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LibJS: Add ASTNodeWithTailArray template to pack AST node + array

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This template allows us to allocate an AST node and an array of some
arbitrary type T with one allocation instead of two. This can save
a lot of memory in some cases.

Thanks to Jonathan Müller for suggesting this technique! :^)
This commit is contained in:
Andreas Kling 2022-11-27 12:33:02 +01:00 committed by Linus Groh
parent c767535ca2
commit 8a8d8ecb35
Notes: sideshowbarker 2024-07-17 06:51:40 +09:00 
Author: https://github.com/awesomekling
Commit: https://github.com/SerenityOS/serenity/commit/8a8d8ecb35
Pull-request: https://github.com/SerenityOS/serenity/pull/16215
Reviewed-by: https://github.com/ADKaster
Reviewed-by: https://github.com/trflynn89
1 changed files with 44 additions and 0 deletions
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44
Userland/Libraries/LibJS/AST.h
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@ -49,6 +49,10 @@ create_ast_node(SourceRange range, Args&&... args)
class ASTNode : public RefCounted<ASTNode> {
public:
virtual ~ASTNode() = default;
// NOTE: This is here to stop ASAN complaining about mismatch between new/delete sizes in ASTNodeWithTailArray.
void operator delete(void* ptr) { ::operator delete(ptr); }
virtual Completion execute(Interpreter&) const = 0;
virtual Bytecode::CodeGenerationErrorOr<void> generate_bytecode(Bytecode::Generator&) const;
virtual void dump(int indent) const;
@ -97,6 +101,46 @@ private:
u32 m_end_offset { 0 };
};
// This is a helper class that packs an array of T after the AST node, all in the same allocation.
template<typename Derived, typename Base, typename T>
class ASTNodeWithTailArray : public Base {
public:
virtual ~ASTNodeWithTailArray() override
{
for (auto& value : tail_span())
value.~T();
}
Span<T const> tail_span() const { return { tail_data(), tail_size() }; }
T const* tail_data() const { return reinterpret_cast<T const*>(reinterpret_cast<uintptr_t>(this) + sizeof(Derived)); }
size_t tail_size() const { return m_tail_size; }
protected:
template<typename ActualDerived = Derived, typename... Args>
static NonnullRefPtr<ActualDerived> create(size_t tail_size, SourceRange source_range, Args&&... args)
{
static_assert(sizeof(ActualDerived) == sizeof(Derived), "This leaf class cannot add more members");
static_assert(alignof(ActualDerived) % alignof(T) == 0, "Need padding for tail array");
auto memory = ::operator new(sizeof(ActualDerived) + tail_size * sizeof(T));
return adopt_ref(*::new (memory) ActualDerived(move(source_range), forward<Args>(args)...));
}
ASTNodeWithTailArray(SourceRange source_range, Span<T const> values)
: Base(move(source_range))
, m_tail_size(values.size())
{
VERIFY(values.size() <= NumericLimits<u32>::max());
for (size_t i = 0; i < values.size(); ++i)
new (&tail_data()[i]) T(values[i]);
}
private:
T* tail_data() { return reinterpret_cast<T*>(reinterpret_cast<uintptr_t>(this) + sizeof(Derived)); }
u32 m_tail_size { 0 };
};
class Statement : public ASTNode {
public:
explicit Statement(SourceRange source_range)