AK: Specialize Atomic<Integral> for clang compatibility

While Clang claims to implement GCC's atomics libcall API, a small
incompatibility caused our builds to fail on Clang.

Clang requires requires the operands to its fixed-size functions to be
integer types, while GCC will take any type with the same size and
alignment as the various integer primitives. This was problematic, as
atomic `enum class`es would not compile.

Furthermore, Clang does not like if only one operand pointer is marked
volatile. Because it only affects the standalone atomic functions, that
will be fixed in a later commit.

As an added benefit, the code is more type-safe, as it won't let us
perform arithmetic on non-integer types. Types with overloaded
arithmetic types won't cause unexpected behavior anymore.

The constructors for the various atomic types can now be used in
constant expressions.

AK/Atomic.h

@@ -6,6 +6,7 @@
 
 #pragma once
 
+#include <AK/Concepts.h>
 #include <AK/Platform.h>
 #include <AK/Types.h>
 
@@ -143,7 +144,76 @@ public:
     Atomic(const Atomic&) = delete;
     Atomic(Atomic&&) = delete;
 
-    Atomic(T val) noexcept
+    constexpr Atomic(T val) noexcept
+        : m_value(val)
+    {
+    }
+
+    volatile T* ptr() noexcept
+    {
+        return &m_value;
+    }
+
+    T exchange(T desired, MemoryOrder order = DefaultMemoryOrder) volatile noexcept
+    {
+        // We use this hack to prevent unnecessary initialization, even if T has a default constructor.
+        // NOTE: Will need to investigate if it pessimizes the generated assembly.
+        alignas(T) u8 buffer[sizeof(T)];
+        T* ret = reinterpret_cast<T*>(buffer);
+        __atomic_exchange(&m_value, &desired, ret, order);
+        return *ret;
+    }
+
+    [[nodiscard]] bool compare_exchange_strong(T& expected, T desired, MemoryOrder order = DefaultMemoryOrder) volatile noexcept
+    {
+        if (order == memory_order_acq_rel || order == memory_order_release)
+            return __atomic_compare_exchange(&m_value, &expected, &desired, false, memory_order_release, memory_order_acquire);
+        else
+            return __atomic_compare_exchange(&m_value, &expected, &desired, false, order, order);
+    }
+
+    ALWAYS_INLINE operator T() const volatile noexcept
+    {
+        return load();
+    }
+
+    ALWAYS_INLINE T load(MemoryOrder order = DefaultMemoryOrder) const volatile noexcept
+    {
+        alignas(T) u8 buffer[sizeof(T)];
+        T* ret = reinterpret_cast<T*>(buffer);
+        __atomic_load(&m_value, ret, order);
+        return *ret;
+    }
+
+    ALWAYS_INLINE T operator=(T desired) volatile noexcept
+    {
+        store(desired);
+        return desired;
+    }
+
+    ALWAYS_INLINE void store(T desired, MemoryOrder order = DefaultMemoryOrder) volatile noexcept
+    {
+        __atomic_store(&m_value, &desired, order);
+    }
+
+    ALWAYS_INLINE bool is_lock_free() const volatile noexcept
+    {
+        return __atomic_is_lock_free(sizeof(m_value), &m_value);
+    }
+};
+
+template<Integral T, MemoryOrder DefaultMemoryOrder>
+class Atomic<T, DefaultMemoryOrder> {
+    T m_value { 0 };
+
+public:
+    Atomic() noexcept = default;
+    Atomic& operator=(const Atomic&) volatile = delete;
+    Atomic& operator=(Atomic&&) volatile = delete;
+    Atomic(const Atomic&) = delete;
+    Atomic(Atomic&&) = delete;
+
+    constexpr Atomic(T val) noexcept
         : m_value(val)
     {
     }
@@ -274,7 +344,7 @@ public:
     Atomic(const Atomic&) = delete;
     Atomic(Atomic&&) = delete;
 
-    Atomic(T* val) noexcept
+    constexpr Atomic(T* val) noexcept
         : m_value(val)
     {
     }
@@ -363,7 +433,6 @@ public:
         return __atomic_is_lock_free(sizeof(m_value), &m_value);
     }
 };
-
 }
 
 using AK::Atomic;