Improved fast path for pagemap.

src/mem/pagemap.h

@@ -112,9 +112,30 @@ namespace snmalloc
       // to see that correctly.
       PagemapEntry* value = e->load(std::memory_order_relaxed);
 
-      if ((value == nullptr) || (value == LOCKED_ENTRY))
+      if (likely((value != nullptr) && (value != LOCKED_ENTRY)))
       {
+        result = true;
+        return value;
+      }
       if constexpr (create_addr)
+      {
+        return get_node_slow(e, result);
+      }
+      else
+      {
+        result = false;
+        return nullptr;
+      }
+    }
+    
+    NOINLINE PagemapEntry* get_node_slow(std::atomic<PagemapEntry*>* e, bool& result)
+    {
+      // The page map nodes are all allocated directly from the OS zero
+      // initialised with a system call.  We don't need any ordered to guarantee
+      // to see that correctly.
+      PagemapEntry* value = e->load(std::memory_order_relaxed);
+
+      if ((value == nullptr) || (value == LOCKED_ENTRY))
       {
         value = nullptr;
 
@@ -135,12 +156,6 @@ namespace snmalloc
           value = e->load(std::memory_order_acquire);
         }
       }
-        else
-        {
-          result = false;
-          return nullptr;
-        }
-      }
       result = true;
       return value;
     }