AK: Add inline storage support for the Function class

This allows us to allocate most Function objects on the stack or as
part of other objects and gets rid of quite a few allocations.

AK/Function.h

@@ -1,5 +1,6 @@
 /*
  * Copyright (C) 2016 Apple Inc. All rights reserved.
+ * Copyright (c) 2021, Gunnar Beutner <gbeutner@serenityos.org>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -26,8 +27,12 @@
 #pragma once
 
 #include <AK/Assertions.h>
-#include <AK/OwnPtr.h>
+#include <AK/Atomic.h>
+#include <AK/BitCast.h>
+#include <AK/Noncopyable.h>
+#include <AK/ScopeGuard.h>
 #include <AK/StdLibExtras.h>
+#include <AK/Types.h>
 
 namespace AK {
 
@@ -36,46 +41,75 @@ class Function;
 
 template<typename Out, typename... In>
 class Function<Out(In...)> {
+    AK_MAKE_NONCOPYABLE(Function);
+
 public:
     Function() = default;
 
+    ~Function()
+    {
+        clear();
+    }
+
     template<typename CallableType, class = typename EnableIf<!(IsPointer<CallableType> && IsFunction<RemovePointer<CallableType>>)&&IsRvalueReference<CallableType&&>>::Type>
     Function(CallableType&& callable)
-        : m_callable_wrapper(make<CallableWrapper<CallableType>>(move(callable)))
     {
+        init_with_callable(move(callable));
     }
 
     template<typename FunctionType, class = typename EnableIf<IsPointer<FunctionType> && IsFunction<RemovePointer<FunctionType>>>::Type>
     Function(FunctionType f)
-        : m_callable_wrapper(make<CallableWrapper<FunctionType>>(move(f)))
     {
+        init_with_callable(move(f));
+    }
+
+    Function(Function&& other)
+    {
+        move_from(move(other));
     }
 
     Out operator()(In... in) const
     {
-        VERIFY(m_callable_wrapper);
-        return m_callable_wrapper->call(forward<In>(in)...);
+        auto* wrapper = callable_wrapper();
+        VERIFY(wrapper);
+        ++m_call_nesting_level;
+        ScopeGuard guard([this] {
+            --m_call_nesting_level;
+        });
+        return wrapper->call(forward<In>(in)...);
     }
 
-    explicit operator bool() const { return !!m_callable_wrapper; }
+    explicit operator bool() const { return !!callable_wrapper(); }
 
     template<typename CallableType, class = typename EnableIf<!(IsPointer<CallableType> && IsFunction<RemovePointer<CallableType>>)&&IsRvalueReference<CallableType&&>>::Type>
     Function& operator=(CallableType&& callable)
     {
-        m_callable_wrapper = make<CallableWrapper<CallableType>>(move(callable));
+        clear();
+        init_with_callable(move(callable));
         return *this;
     }
 
     template<typename FunctionType, class = typename EnableIf<IsPointer<FunctionType> && IsFunction<RemovePointer<FunctionType>>>::Type>
     Function& operator=(FunctionType f)
     {
-        m_callable_wrapper = make<CallableWrapper<FunctionType>>(move(f));
+        clear();
+        if (f)
+            init_with_callable(move(f));
         return *this;
     }
 
     Function& operator=(std::nullptr_t)
     {
-        m_callable_wrapper = nullptr;
+        clear();
+        return *this;
+    }
+
+    Function& operator=(Function&& other)
+    {
+        if (this != &other) {
+            clear();
+            move_from(move(other));
+        }
         return *this;
     }
 
@@ -84,19 +118,21 @@ private:
     public:
         virtual ~CallableWrapperBase() = default;
         virtual Out call(In...) const = 0;
+        virtual void destroy() = 0;
+        virtual void init_and_swap(u8*, size_t) = 0;
     };
 
     template<typename CallableType>
     class CallableWrapper final : public CallableWrapperBase {
+        AK_MAKE_NONMOVABLE(CallableWrapper);
+        AK_MAKE_NONCOPYABLE(CallableWrapper);
+
     public:
         explicit CallableWrapper(CallableType&& callable)
             : m_callable(move(callable))
         {
         }
 
-        CallableWrapper(const CallableWrapper&) = delete;
-        CallableWrapper& operator=(const CallableWrapper&) = delete;
-
         Out call(In... in) const final override
         {
             if constexpr (requires { m_callable(forward<In>(in)...); }) {
@@ -110,11 +146,93 @@ private:
             }
         }
 
+        void destroy() final override
+        {
+            delete this;
+        }
+
+        void init_and_swap(u8* destination, size_t size) final override
+        {
+            VERIFY(size >= sizeof(CallableWrapper));
+            new (destination) CallableWrapper { move(m_callable) };
+        }
+
     private:
         CallableType m_callable;
     };
 
-    OwnPtr<CallableWrapperBase> m_callable_wrapper;
+    enum class FunctionKind {
+        NullPointer,
+        Inline,
+        Outline,
+    };
+
+    CallableWrapperBase* callable_wrapper() const
+    {
+        switch (m_kind) {
+        case FunctionKind::NullPointer:
+            return nullptr;
+        case FunctionKind::Inline:
+            return bit_cast<CallableWrapperBase*>(&m_storage);
+        case FunctionKind::Outline:
+            return *bit_cast<CallableWrapperBase**>(&m_storage);
+        default:
+            VERIFY_NOT_REACHED();
+        }
+    }
+
+    void clear()
+    {
+        auto* wrapper = callable_wrapper();
+        if (m_kind == FunctionKind::Inline) {
+            VERIFY(wrapper);
+            wrapper->~CallableWrapperBase();
+        } else if (m_kind == FunctionKind::Outline) {
+            VERIFY(wrapper);
+            wrapper->destroy();
+        }
+        m_kind = FunctionKind::NullPointer;
+    }
+
+    template<typename Callable>
+    void init_with_callable(Callable&& callable)
+    {
+        using WrapperType = CallableWrapper<Callable>;
+        if constexpr (sizeof(WrapperType) > inline_capacity) {
+            *bit_cast<CallableWrapperBase**>(&m_storage) = new WrapperType(move(callable));
+            m_kind = FunctionKind::Outline;
+        } else {
+            new (m_storage) WrapperType(move(callable));
+            m_kind = FunctionKind::Inline;
+        }
+    }
+
+    void move_from(Function&& other)
+    {
+        VERIFY(m_call_nesting_level == 0 && other.m_call_nesting_level == 0);
+        auto* other_wrapper = other.callable_wrapper();
+        switch (other.m_kind) {
+        case FunctionKind::NullPointer:
+            break;
+        case FunctionKind::Inline:
+            other_wrapper->init_and_swap(m_storage, inline_capacity);
+            m_kind = FunctionKind::Inline;
+            break;
+        case FunctionKind::Outline:
+            *bit_cast<CallableWrapperBase**>(&m_storage) = other_wrapper;
+            m_kind = FunctionKind::Outline;
+            break;
+        default:
+            VERIFY_NOT_REACHED();
+        }
+        other.m_kind = FunctionKind::NullPointer;
+    }
+
+    FunctionKind m_kind { FunctionKind::NullPointer };
+    mutable Atomic<u16> m_call_nesting_level { 0 };
+    // Empirically determined to fit most lambdas and functions.
+    static constexpr size_t inline_capacity = 4 * sizeof(void*);
+    alignas(max(alignof(CallableWrapperBase), alignof(CallableWrapperBase*))) u8 m_storage[inline_capacity];
 };
 
 }