Implement rotate. Not tested

VersionedMap.cpp

@@ -102,8 +102,8 @@ struct Node {
     uint32_t nextFree;
   };
   Entry *entry;
-  uint32_t pointers[3];
-  bool replacePointer;
+  uint32_t pointer[3];
+  bool replacedPointer;
   std::atomic<bool> updated;
 };
 
@@ -224,21 +224,21 @@ struct MemManager {
         uint32_t p = stack[--stackIndex];
         auto &node = base[p];
         if (node.updated.load(std::memory_order_relaxed)) {
-          if (node.pointers[!node.replacePointer] != 0) {
-            tryPush(node.pointers[!node.replacePointer]);
+          if (node.pointer[!node.replacedPointer] != 0) {
+            tryPush(node.pointer[!node.replacedPointer]);
           }
           if (oldestVersion < node.updateVersion) {
-            if (node.pointers[node.replacePointer] != 0) {
-              tryPush(node.pointers[node.replacePointer]);
+            if (node.pointer[node.replacedPointer] != 0) {
+              tryPush(node.pointer[node.replacedPointer]);
             }
           }
-          tryPush(node.pointers[2]);
+          tryPush(node.pointer[2]);
         } else {
-          if (node.pointers[0] != 0) {
-            tryPush(node.pointers[0]);
+          if (node.pointer[0] != 0) {
+            tryPush(node.pointer[0]);
           }
-          if (node.pointers[1] != 0) {
-            tryPush(node.pointers[1]);
+          if (node.pointer[1] != 0) {
+            tryPush(node.pointer[1]);
           }
         }
       }
@@ -375,6 +375,94 @@ private:
 
 struct VersionedMap::Impl {
 
+  template <std::memory_order kOrder>
+  uint32_t child(uint32_t node, bool which, int64_t at) {
+    auto &n = mm.base[node];
+    if (n.updated.load(kOrder) && n.updateVersion <= at &&
+        which == n.replacedPointer) {
+      return n.pointer[2];
+    } else {
+      return n.pointer[which];
+    }
+  }
+
+  template <std::memory_order kOrder>
+  uint32_t left(uint32_t node, bool which, int64_t at) {
+    return child<kOrder>(node, false, at);
+  }
+
+  template <std::memory_order kOrder>
+  uint32_t right(uint32_t node, bool which, int64_t at) {
+    return child<kOrder>(node, true, at);
+  }
+
+  // Returns the node that results from setting `which` to `child` on `node`
+  uint32_t update(uint32_t node, int64_t version, bool which, uint32_t child) {
+    if (this->child<std::memory_order_relaxed>(node, which, version) == child) {
+      return node;
+    }
+    auto &n = mm.base[node];
+    const bool updated = n.updated.load(std::memory_order_relaxed);
+
+    auto doCopy = [&]() {
+      uint32_t copy = mm.allocate();
+      auto &c = mm.base[copy];
+      n.entry->addref();
+      c.entry = n.entry;
+      c.pointer[which] = child;
+      c.pointer[!which] = n.pointer[!which];
+      c.updated.store(false, std::memory_order_relaxed);
+      c.updateVersion = version;
+      return copy;
+    };
+
+    if (n.updateVersion == version) {
+      if (updated && n.replacedPointer != which) {
+        // We can't update n.replacedPointer without introducing a data race
+        // (unless we packed it into the atomic?) so we copy. pointer[2] becomes
+        // unreachable, but need to tell the garbage collector.
+        n.pointer[2] = 0;
+        return doCopy();
+      } else if (updated) {
+        n.pointer[2] = child;
+      } else {
+        n.pointer[which] = child;
+      }
+      return node;
+    }
+
+    if (updated) {
+      // We already used this node's in-place update
+      return doCopy();
+    } else {
+      n.updateVersion = version;
+      n.pointer[2] = child;
+      n.replacedPointer = which;
+      n.updated.store(true, std::memory_order_release); // Must be last
+      return node;
+    }
+  }
+
+  void rotate(uint32_t &n, int64_t at, bool right) {
+    auto l = child<std::memory_order_relaxed>(n, !right, at);
+    n = update(
+        l, right,
+        update(n, !right, child<std::memory_order_relaxed>(l, right, at), at),
+        at);
+  }
+
+  uint32_t newNode(int64_t version, const uint8_t *key, int keyLen,
+                   const uint8_t *val, int valLen, bool clearTo) {
+    auto result = mm.allocate();
+    auto &node = mm.base[result];
+    node.updateVersion = version;
+    node.pointer[0] = 0;
+    node.pointer[1] = 0;
+    node.updated.store(false, std::memory_order_relaxed);
+    node.entry = Entry::make(version, key, keyLen, val, valLen, clearTo);
+    return result;
+  }
+
   MemManager mm;
   RootSet roots;
 };
@@ -389,22 +477,22 @@ int main() {
   weaselab::MemManager mm;
   bench.run("allocate", [&]() {
     auto x = mm.allocate();
-    mm.base[x].pointers[0] = 0;
-    mm.base[x].pointers[1] = 0;
+    mm.base[x].pointer[0] = 0;
+    mm.base[x].pointer[1] = 0;
     mm.base[x].updated.store(false, std::memory_order_relaxed);
   });
   mm.gc(nullptr, 0, 0);
   for (int i = 0; i < 10000; ++i) {
     auto x = mm.allocate();
-    mm.base[x].pointers[0] = 0;
-    mm.base[x].pointers[1] = 0;
+    mm.base[x].pointer[0] = 0;
+    mm.base[x].pointer[1] = 0;
     mm.base[x].updated.store(false, std::memory_order_relaxed);
   }
   auto root = mm.allocate();
   mm.base[root].entry = weaselab::Entry::make(0, nullptr, 0, nullptr, 0,
                                               weaselab::VersionedMap::Set);
-  mm.base[root].pointers[0] = 0;
-  mm.base[root].pointers[1] = 0;
+  mm.base[root].pointer[0] = 0;
+  mm.base[root].pointer[1] = 0;
   mm.base[root].updated.store(false, std::memory_order_relaxed);
   bench.run("gc", [&]() { mm.gc(&root, 1, 0); });