Adds unsafe_store operation

internals/testing/smoke_test.cpp

@@ -9,12 +9,14 @@ using namespace trade_v1;
 
 auto smoke_test = test([]() {
   atom<int> xA = 1;
-  atom<float> yA = 2.f;
+  atom<float> yA = 1.f;
   atom<int> zA = 3;
   atom<std::shared_ptr<int>> p(std::make_shared<int>(32));
 
   verify(atomically(assume_readonly, []() { return true; }));
 
+  yA.unsafe_store(2);
+
   {
     verify(1 == xA.unsafe_load());
     verify(2 == yA.unsafe_load());

provides/include/trade_v1/private/private-methods.hpp

@@ -109,6 +109,27 @@ Value &trade_v1::Private::store(atom_t<Value> &atom, Forwardable &&value) {
   return access->m_current;
 }
 
+template <class Value>
+const Value &trade_v1::Private::unsafe_store(atom_t<Value> &atom,
+                                             const Value &value) {
+  auto &lock = s_locks[lock_ix_of(&atom)];
+  molecular::backoff backoff;
+  while (true) {
+    auto u = lock.m_clock.load();
+    if (0 <= static_cast<signed_clock_t>(u)) {
+      if (lock.m_clock.compare_exchange_weak(
+              u, ~u, std::memory_order_acquire)) {
+        atom.m_value.store(value, std::memory_order_relaxed);
+        if (auto first = lock.m_first)
+          signal(first);
+        lock.m_clock.store(s_clock++, std::memory_order_release);
+        return value;
+      }
+    }
+    backoff();
+  }
+}
+
 template <class Value> Value &trade_v1::Private::ref(atom_t<Value> &atom) {
   auto transaction = s_transaction;
   auto access = insert(transaction, &atom);

provides/include/trade_v1/private/private.hpp

@@ -106,6 +106,9 @@ class Private {
   template <class Value, class Forwardable>
   static Value &store(atom_t<Value> &atom, Forwardable &&value);
 
+  template <class Value>
+  static const Value &unsafe_store(atom_t<Value> &atom, const Value &value);
+
   template <class Value> static Value &ref(atom_t<Value> &atom);
 
   template <class Config, class Action>

provides/include/trade_v1/synopsis.hpp

@@ -41,6 +41,11 @@ template <class Value> struct atom : Private::atom_t<Value> {
   /// value` is equivalent to `atom.store(value)`.
   template <class Forwardable> Value &store(Forwardable &&value);
 
+  /// Store the given value to the given atom as a single transaction.  Note
+  /// that this cannot be used safely inside an `atomically` block, because it
+  /// can prevent the `atomically` block from ever completing.
+  const Value &unsafe_store(const Value &value);
+
   /// Returns a mutable reference to the current value of the atom within a
   /// transaction.  `atom.ref()` is roughly equivalent to
   /// `atom.store(atom.load())`, but accesses the transaction log only once.

provides/include/trade_v1/trade.hpp

@@ -39,6 +39,11 @@ Value &trade_v1::atom<Value>::store(Forwardable &&value) {
   return Private::store(*this, std::forward<Forwardable>(value));
 }
 
+template <class Value>
+const Value &trade_v1::atom<Value>::unsafe_store(const Value &value) {
+  return Private::unsafe_store(*this, value);
+}
+
 template <class Config, class Action>
 std::invoke_result_t<Action> trade_v1::atomically(Config config,
                                                   Action &&action) {