AK: Remove excessive hashing caused by FlyString table

Before this change, the global FlyString table looked like this:

    HashMap<StringView, Detail::StringBase>

After this change, we have:

    HashTable<Detail::StringData const*, FlyStringTableHashTraits>

The custom hash traits are used to extract the stored hash from
StringData which avoids having to rehash the StringView repeatedly like
we did before.

This necessitated a handful of smaller changes to make it work.

AK/FlyString.cpp

@@ -8,14 +8,21 @@
 #include <AK/FlyString.h>
 #include <AK/HashMap.h>
 #include <AK/Singleton.h>
+#include <AK/String.h>
+#include <AK/StringData.h>
 #include <AK/StringView.h>
 #include <AK/Utf8View.h>
 
 namespace AK {
 
+struct FlyStringTableHashTraits : public Traits<Detail::StringData const*> {
+    static u32 hash(Detail::StringData const* string) { return string->hash(); }
+    static bool equals(Detail::StringData const* a, Detail::StringData const* b) { return *a == *b; }
+};
+
 static auto& all_fly_strings()
 {
-    static Singleton<HashMap<StringView, Detail::StringBase>> table;
+    static Singleton<HashTable<Detail::StringData const*, FlyStringTableHashTraits>> table;
     return *table;
 }
 
@@ -31,14 +38,19 @@ FlyString::FlyString(String const& string)
         return;
     }
 
-    auto it = all_fly_strings().find(string.bytes_as_string_view());
+    if (string.m_data->is_fly_string()) {
+        m_data = string;
+        return;
+    }
+
+    auto it = all_fly_strings().find(string.m_data);
     if (it == all_fly_strings().end()) {
         m_data = string;
-
-        all_fly_strings().set(string.bytes_as_string_view(), m_data);
-        string.did_create_fly_string({});
+        all_fly_strings().set(string.m_data);
+        string.m_data->set_fly_string(true);
     } else {
-        m_data = it->value;
+        m_data.m_data = *it;
+        m_data.m_data->ref();
     }
 }
 
@@ -104,9 +116,9 @@ bool FlyString::operator==(char const* string) const
     return bytes_as_string_view() == string;
 }
 
-void FlyString::did_destroy_fly_string_data(Badge<Detail::StringData>, StringView string_data)
+void FlyString::did_destroy_fly_string_data(Badge<Detail::StringData>, Detail::StringData const& string_data)
 {
-    all_fly_strings().remove(string_data);
+    all_fly_strings().remove(&string_data);
 }
 
 Detail::StringBase FlyString::data(Badge<String>) const

AK/FlyString.h

@@ -48,7 +48,7 @@ public:
 
     [[nodiscard]] int operator<=>(FlyString const& other) const;
 
-    static void did_destroy_fly_string_data(Badge<Detail::StringData>, StringView);
+    static void did_destroy_fly_string_data(Badge<Detail::StringData>, Detail::StringData const&);
     [[nodiscard]] Detail::StringBase data(Badge<String>) const;
 
     // This is primarily interesting to unit tests.

AK/String.h

@@ -186,7 +186,7 @@ public:
     static ErrorOr<String> from_byte_string(T&&) = delete;
 
 private:
-    friend FlyString;
+    friend class ::AK::FlyString;
 
     using ShortString = Detail::ShortString;
 

AK/StringBase.cpp

@@ -90,12 +90,6 @@ bool StringBase::operator==(StringBase const& other) const
     return bytes() == other.bytes();
 }
 
-void StringBase::did_create_fly_string(Badge<FlyString>) const
-{
-    VERIFY(!is_short_string());
-    m_data->set_fly_string(true);
-}
-
 ErrorOr<Bytes> StringBase::replace_with_uninitialized_buffer(size_t byte_count)
 {
     if (byte_count <= MAX_SHORT_STRING_BYTE_COUNT)

AK/StringBase.h

@@ -69,8 +69,6 @@ public:
 
     [[nodiscard]] bool operator==(StringBase const&) const;
 
-    void did_create_fly_string(Badge<FlyString>) const;
-
     [[nodiscard]] ALWAYS_INLINE FlatPtr raw(Badge<FlyString>) const { return bit_cast<FlatPtr>(m_data); }
 
 protected:
@@ -96,6 +94,9 @@ protected:
     ErrorOr<StringBase> substring_from_byte_offset_with_shared_superstring(size_t start, size_t byte_count) const;
 
 private:
+    friend class ::AK::String;
+    friend class ::AK::FlyString;
+
     // NOTE: If the least significant bit of the pointer is set, this is a short string.
     static constexpr uintptr_t SHORT_STRING_FLAG = 1;
 

AK/StringData.h

@@ -44,22 +44,15 @@ public:
 
     void operator delete(void* ptr)
     {
-        free(ptr);
-    }
-
-    void unref() const
-    {
-        if (m_is_fly_string && m_ref_count == 2) {
-            m_is_fly_string = false; // Otherwise unref from did_destroy_fly_string_data will cause infinite recursion.
-            FlyString::did_destroy_fly_string_data({}, *this);
-        }
-        RefCounted::unref();
+        kfree_sized(ptr, allocation_size_for_string_data(static_cast<StringData const*>(ptr)->m_byte_count));
     }
 
     ~StringData()
     {
         if (m_substring)
             substring_data().superstring->unref();
+        if (m_is_fly_string)
+            FlyString::did_destroy_fly_string_data({}, *this);
     }
 
     SubstringData const& substring_data() const