Simplify: use raw pointers instead of optional wrappers on deque.

lib/pls/include/pls/internal/scheduling/lock_free/external_trading_deque.h

@@ -9,7 +9,6 @@
 #include "pls/internal/base/error_handling.h"
 #include "pls/internal/base/system_details.h"
 
-#include "pls/internal/data_structures/optional.h"
 #include "pls/internal/data_structures/stamped_integer.h"
 
 #include "pls/internal/scheduling/lock_free/task.h"
@@ -38,8 +37,8 @@ class external_trading_deque {
  public:
   external_trading_deque(unsigned thread_id, size_t num_entries) : thread_id_(thread_id), entries_(num_entries) {}
 
-  static optional<task *> peek_traded_object(task *target_task);
-  static optional<task *> get_trade_object(task *target_task);
+  static task *peek_traded_object(task *target_task);
+  static task *get_trade_object(task *target_task);
 
   /**
    * Pushes a task on the bottom of the deque.
@@ -54,12 +53,12 @@ class external_trading_deque {
    *
    * @return optional<task*> holding the popped task if successful.
    */
-  optional<task *> pop_bot();
+  task *pop_bot();
 
   struct peek_result {
-    peek_result(optional<task *> top_task, stamped_integer top_pointer) : top_task_{std::move(top_task)},
+    peek_result(task *top_task, stamped_integer top_pointer) : top_task_{std::move(top_task)},
                                                                top_pointer_{top_pointer} {};
-    optional<task *> top_task_;
+    task *top_task_;
     stamped_integer top_pointer_;
   };
 
@@ -83,7 +82,7 @@ class external_trading_deque {
    *
    * @return optional<task*> holding the popped task if successful.
    */
-  optional<task *> pop_top(task *offered_task, peek_result peek_result);
+  task *pop_top(task *offered_task, peek_result peek_result);
 
  private:
   void reset_bot_and_top();

lib/pls/src/internal/scheduling/lock_free/external_trading_deque.cpp

@@ -3,26 +3,26 @@
 
 namespace pls::internal::scheduling::lock_free {
 
-optional<task *> external_trading_deque::peek_traded_object(task *target_task) {
+task * external_trading_deque::peek_traded_object(task *target_task) {
   traded_cas_field current_cas = target_task->external_trading_deque_cas_.load();
   if (current_cas.is_filled_with_object()) {
-    return optional<task *>{current_cas.get_trade_object()};
+    return current_cas.get_trade_object();
   } else {
-    return optional<task *>{};
+    return nullptr;
   }
 }
 
-optional<task *> external_trading_deque::get_trade_object(task *target_task) {
+task * external_trading_deque::get_trade_object(task *target_task) {
   traded_cas_field current_cas = target_task->external_trading_deque_cas_.load();
   if (current_cas.is_filled_with_object()) {
     task *result = current_cas.get_trade_object();
     traded_cas_field empty_cas;
     if (target_task->external_trading_deque_cas_.compare_exchange_strong(current_cas, empty_cas)) {
-      return optional<task *>{result};
+      return result;
     }
   }
 
-  return optional<task *>{};
+  return nullptr;
 }
 
 void external_trading_deque::push_bot(task *published_task) {
@@ -58,10 +58,10 @@ void external_trading_deque::decrease_bot() {
   bot_.store(bot_internal_.value, std::memory_order_relaxed);
 }
 
-optional<task *> external_trading_deque::pop_bot() {
+task * external_trading_deque::pop_bot() {
   if (bot_internal_.value == 0) {
     reset_bot_and_top();
-    return optional<task *>{};
+    return nullptr;
   }
   decrease_bot();
 
@@ -77,10 +77,10 @@ optional<task *> external_trading_deque::pop_bot() {
   if (popped_task->external_trading_deque_cas_.compare_exchange_strong(expected_sync_cas_field,
                                                                        empty_cas_field,
                                                                        std::memory_order_acq_rel)) {
-    return optional<task *>{popped_task};
+    return popped_task;
   } else {
     reset_bot_and_top();
-    return optional<task *>{};
+    return nullptr;
   }
 }
 
@@ -89,17 +89,17 @@ external_trading_deque::peek_result external_trading_deque::peek_top() {
   auto local_bot = bot_.load();
 
   if (local_top.value < local_bot) {
-    return peek_result{optional<task *>{entries_[local_top.value].traded_task_}, local_top};
+    return peek_result{entries_[local_top.value].traded_task_, local_top};
   } else {
-    return peek_result{optional<task *>{}, local_top};
+    return peek_result{nullptr, local_top};
   }
 }
 
-optional<task *> external_trading_deque::pop_top(task *offered_task, peek_result peek_result) {
+task * external_trading_deque::pop_top(task *offered_task, peek_result peek_result) {
   stamped_integer expected_top = peek_result.top_pointer_;
   auto local_bot = bot_.load();
   if (expected_top.value >= local_bot) {
-    return data_structures::optional<task *>{};
+    return nullptr;
   }
 
   auto &target_entry = entries_[expected_top.value];
@@ -119,7 +119,7 @@ optional<task *> external_trading_deque::pop_top(task *offered_task, peek_result
     // We got it, for sure move the top pointer forward.
     top_.compare_exchange_strong(expected_top, {expected_top.stamp + 1, expected_top.value + 1});
     // Return the stolen task
-    return data_structures::optional<task *>{result};
+    return result;
   } else {
     // We did not get it...help forwarding the top pointer anyway.
     if (expected_top.stamp == forwarding_stamp) {
@@ -130,7 +130,7 @@ optional<task *> external_trading_deque::pop_top(task *offered_task, peek_result
       // This means only updating the tag, as this location can still hold data we need.
       top_.compare_exchange_strong(expected_top, {forwarding_stamp, expected_top.value});
     }
-    return data_structures::optional<task *>{};
+    return nullptr;
   }
 }
 

lib/pls/src/internal/scheduling/lock_free/task_manager.cpp

@@ -36,12 +36,7 @@ void task_manager::push_local_task(base_task *pushed_task) {
 }
 
 base_task *task_manager::pop_local_task() {
-  auto result = deque_.pop_bot();
-  if (result) {
-    return *result;
-  } else {
-    return nullptr;
-  }
+  return deque_.pop_bot();
 }
 
 std::tuple<base_task *, base_task *> task_manager::steal_task(thread_state &stealing_state) {
@@ -50,27 +45,27 @@ std::tuple<base_task *, base_task *> task_manager::steal_task(thread_state &stea
 
   auto peek = deque_.peek_top();
   if (peek.top_task_) {
-    task *stolen_task = static_cast<task *>(*peek.top_task_);
+    task *stolen_task = peek.top_task_;
     // get a suitable task to trade in
     // TODO: opt. add debug marker to traded in tasks that we do not accidentally use them.
     task *traded_task = static_cast<task *>(&scheduler::task_chain_at(stolen_task->depth_, stealing_state));
     base_task *chain_after_stolen_task = traded_task->next_;
 
     // perform the actual pop operation
-    auto pop_result_task = deque_.pop_top(traded_task, peek);
+    task* pop_result_task = deque_.pop_top(traded_task, peek);
     if (pop_result_task) {
       PLS_ASSERT(stolen_task->thread_id_ != traded_task->thread_id_,
                  "It is impossible to steal an task we already own!");
-      PLS_ASSERT(*pop_result_task == stolen_task,
+      PLS_ASSERT(pop_result_task == stolen_task,
                  "We must only steal the task that we peeked at!");
 
       // update the resource stack associated with the stolen task
       stolen_task->push_task_chain(traded_task);
 
-      auto optional_exchanged_task = external_trading_deque::get_trade_object(stolen_task);
+      task* optional_exchanged_task = external_trading_deque::get_trade_object(stolen_task);
       if (optional_exchanged_task) {
         // All good, we pushed the task over to the stack, nothing more to do
-        PLS_ASSERT(*optional_exchanged_task == traded_task,
+        PLS_ASSERT(optional_exchanged_task == traded_task,
                    "We are currently executing this, no one else can put another task in this field!");
       } else {
         // The last other active thread took it as its spare resource...
@@ -93,9 +88,9 @@ base_task *task_manager::pop_clean_task_chain(base_task *base_task) {
   task *clean_chain = popped_task->pop_task_chain();
   if (clean_chain == nullptr) {
     // double-check if we are really last one or we only have unlucky timing
-    auto optional_cas_task = external_trading_deque::get_trade_object(popped_task);
+    task* optional_cas_task = external_trading_deque::get_trade_object(popped_task);
     if (optional_cas_task) {
-      clean_chain = *optional_cas_task;
+      clean_chain = optional_cas_task;
     } else {
       clean_chain = popped_task->pop_task_chain();
     }

test/scheduling_lock_free_tests.cpp

@@ -81,30 +81,30 @@ TEST_CASE("external trading deque", "[internal/scheduling/lock_free/external_tra
 
     // Local push/pop
     deque_1.push_bot(&tasks[0]);
-    REQUIRE(*deque_1.pop_bot() == &tasks[0]);
+    REQUIRE(deque_1.pop_bot() == &tasks[0]);
     REQUIRE(!deque_1.pop_bot());
 
     // Local push, external pop
     deque_1.push_bot(&tasks[0]);
     auto peek = deque_1.peek_top();
-    REQUIRE(*deque_1.pop_top(&tasks[1], peek) == &tasks[0]);
-    REQUIRE(*external_trading_deque::get_trade_object(&tasks[0]) == &tasks[1]);
+    REQUIRE(deque_1.pop_top(&tasks[1], peek) == &tasks[0]);
+    REQUIRE(external_trading_deque::get_trade_object(&tasks[0]) == &tasks[1]);
     REQUIRE(!deque_1.pop_top(&tasks[1], peek));
     REQUIRE(!deque_1.pop_bot());
 
     // Keeps push/pop order
     deque_1.push_bot(&tasks[0]);
     deque_1.push_bot(&tasks[1]);
-    REQUIRE(*deque_1.pop_bot() == &tasks[1]);
-    REQUIRE(*deque_1.pop_bot() == &tasks[0]);
+    REQUIRE(deque_1.pop_bot() == &tasks[1]);
+    REQUIRE(deque_1.pop_bot() == &tasks[0]);
     REQUIRE(!deque_1.pop_bot());
 
     deque_1.push_bot(&tasks[0]);
     deque_1.push_bot(&tasks[1]);
     auto peek1 = deque_1.peek_top();
-    REQUIRE(*deque_1.pop_top(&tasks[2], peek1) == &tasks[0]);
+    REQUIRE(deque_1.pop_top(&tasks[2], peek1) == &tasks[0]);
     auto peek2 = deque_1.peek_top();
-    REQUIRE(*deque_1.pop_top(&tasks[3], peek2) == &tasks[1]);
+    REQUIRE(deque_1.pop_top(&tasks[3], peek2) == &tasks[1]);
   }
 
   SECTION("Interwined execution #1") {
@@ -112,7 +112,7 @@ TEST_CASE("external trading deque", "[internal/scheduling/lock_free/external_tra
     deque_1.push_bot(&tasks[0]);
     auto peek1 = deque_1.peek_top();
     auto peek2 = deque_1.peek_top();
-    REQUIRE(*deque_1.pop_top(&tasks[1], peek1) == &tasks[0]);
+    REQUIRE(deque_1.pop_top(&tasks[1], peek1) == &tasks[0]);
     REQUIRE(!deque_1.pop_top(&tasks[2], peek2));
   }
 
@@ -120,7 +120,7 @@ TEST_CASE("external trading deque", "[internal/scheduling/lock_free/external_tra
     // Top and bottom access
     deque_1.push_bot(&tasks[0]);
     auto peek1 = deque_1.peek_top();
-    REQUIRE(*deque_1.pop_bot() == &tasks[0]);
+    REQUIRE(deque_1.pop_bot() == &tasks[0]);
     REQUIRE(!deque_1.pop_top(&tasks[2], peek1));
   }
 }