Compact Delta 80B -> 56B (#1747)

Make special structure for old_disk_key. std::optional<std::string> was
40B, which is the largest member of out action union. Replaced with 8B,
structure.

This makes largest member now vertex_edge at 24B, this means Delta is
now only 56B.

🥳🎉 Now less than a cacheline 🎊✨

src/memory/global_memory_control.cpp

@@ -122,11 +122,11 @@ static bool my_commit(extent_hooks_t *extent_hooks, void *addr, size_t size, siz
 
   [[maybe_unused]] auto blocker = memgraph::utils::MemoryTracker::OutOfMemoryExceptionBlocker{};
   if (GetQueriesMemoryControl().IsThreadTracked()) [[unlikely]] {
-    bool ok = GetQueriesMemoryControl().TrackAllocOnCurrentThread(length);
+    [[maybe_unused]] bool ok = GetQueriesMemoryControl().TrackAllocOnCurrentThread(length);
     DMG_ASSERT(ok);
   }
 
-  auto ok = memgraph::utils::total_memory_tracker.Alloc(static_cast<int64_t>(length));
+  [[maybe_unused]] auto ok = memgraph::utils::total_memory_tracker.Alloc(static_cast<int64_t>(length));
   DMG_ASSERT(ok);
 
   return false;

src/mg_import_csv.cpp

@@ -416,7 +416,7 @@ memgraph::storage::PropertyValue StringToValue(const std::string &str, const std
 std::string GetIdSpace(const std::string &type) {
   // The format of this field is as follows:
   // [START_|END_]ID[(<id_space>)]
-  std::regex format(R"(^(START_|END_)?ID(\(([^\(\)]+)\))?$)", std::regex::extended);
+  static std::regex format(R"(^(START_|END_)?ID(\(([^\(\)]+)\))?$)", std::regex::extended);
   std::smatch res;
   if (!std::regex_match(type, res, format))
     throw LoadException(

src/query/procedure/mg_procedure_impl.cpp

@@ -3798,7 +3798,7 @@ void PrintFuncSignature(const mgp_func &func, std::ostream &stream) {
 
 bool IsValidIdentifierName(const char *name) {
   if (!name) return false;
-  std::regex regex("[_[:alpha:]][_[:alnum:]]*");
+  static std::regex regex("[_[:alpha:]][_[:alnum:]]*");
   return std::regex_match(name, regex);
 }
 

src/storage/v2/delta.hpp

@@ -123,6 +123,26 @@ inline bool operator==(const PreviousPtr::Pointer &a, const PreviousPtr::Pointer
 
 inline bool operator!=(const PreviousPtr::Pointer &a, const PreviousPtr::Pointer &b) { return !(a == b); }
 
+struct opt_str {
+  opt_str(std::optional<std::string> const &other) : str_{other ? new_cstr(*other) : nullptr} {}
+
+  ~opt_str() { delete[] str_; }
+
+  auto as_opt_str() const -> std::optional<std::string> {
+    if (!str_) return std::nullopt;
+    return std::optional<std::string>{std::in_place, str_};
+  }
+
+ private:
+  static auto new_cstr(std::string const &str) -> char const * {
+    auto *mem = new char[str.length() + 1];
+    strcpy(mem, str.c_str());
+    return mem;
+  }
+
+  char const *str_ = nullptr;
+};
+
 struct Delta {
   enum class Action : std::uint8_t {
     /// Use for Vertex and Edge
@@ -160,7 +180,7 @@ struct Delta {
   // Because of this object was created in past txs, we create timestamp by ourselves inside instead of having it from
   // current tx. This timestamp we got from RocksDB timestamp stored in key.
   Delta(DeleteDeserializedObjectTag /*tag*/, uint64_t ts, std::optional<std::string> old_disk_key)
-      : timestamp(new std::atomic<uint64_t>(ts)), command_id(0), old_disk_key{.value = std::move(old_disk_key)} {}
+      : timestamp(new std::atomic<uint64_t>(ts)), command_id(0), old_disk_key{.value = old_disk_key} {}
 
   Delta(DeleteObjectTag /*tag*/, std::atomic<uint64_t> *timestamp, uint64_t command_id)
       : timestamp(timestamp), command_id(command_id), action(Action::DELETE_OBJECT) {}
@@ -222,7 +242,7 @@ struct Delta {
       case Action::REMOVE_OUT_EDGE:
         break;
       case Action::DELETE_DESERIALIZED_OBJECT:
-        old_disk_key.value.reset();
+        std::destroy_at(&old_disk_key.value);
         delete timestamp;
         timestamp = nullptr;
         break;
@@ -242,7 +262,7 @@ struct Delta {
     Action action;
     struct {
       Action action = Action::DELETE_DESERIALIZED_OBJECT;
-      std::optional<std::string> value;
+      opt_str value;
     } old_disk_key;
     struct {
       Action action;

src/utils/disk_utils.hpp

@@ -21,7 +21,7 @@ inline std::optional<std::string> GetOldDiskKeyOrNull(storage::Delta *head) {
     head = head->next;
   }
   if (head->action == storage::Delta::Action::DELETE_DESERIALIZED_OBJECT) {
-    return head->old_disk_key.value;
+    return head->old_disk_key.value.as_opt_str();
   }
   return std::nullopt;
 }

src/utils/string.hpp

@@ -242,7 +242,7 @@ std::vector<TString, TAllocator> *Split(std::vector<TString, TAllocator> *out, c
   if (src.empty()) return out;
   // TODO: Investigate how much regex allocate and perhaps replace with custom
   // solution doing no allocations.
-  std::regex not_whitespace("[^\\s]+");
+  static std::regex not_whitespace("[^\\s]+");
   auto matches_begin = std::cregex_iterator(src.data(), src.data() + src.size(), not_whitespace);
   auto matches_end = std::cregex_iterator();
   out->reserve(std::distance(matches_begin, matches_end));