Expose mutable reference to pagemap

src/backend/backend.h

@@ -272,6 +272,20 @@ namespace snmalloc
         return concretePagemap.template get<potentially_out_of_range>(p);
       }
 
+      /**
+       * Get the metadata associated with a chunk.
+       *
+       * Set template parameter to true if it not an error
+       * to access a location that is not backed by a chunk.
+       */
+      template<bool potentially_out_of_range = false>
+      SNMALLOC_FAST_PATH static MetaEntry&
+      get_metaentry_mut(LocalState* ls, address_t p)
+      {
+        UNUSED(ls);
+        return concretePagemap.template get_mut<potentially_out_of_range>(p);
+      }
+
       /**
        * Set the metadata associated with a chunk.
        */

src/backend/pagemap.h

@@ -236,12 +236,12 @@ namespace snmalloc
      * read/write.
      */
     template<bool potentially_out_of_range>
-    const T& get(address_t p)
+    T& get_mut(address_t p)
     {
       if constexpr (potentially_out_of_range)
       {
         if (SNMALLOC_UNLIKELY(body_opt == nullptr))
-          return default_value;
+          return const_cast<T&>(default_value);
       }
 
       if constexpr (has_bounds)
@@ -250,14 +250,14 @@ namespace snmalloc
         {
           if constexpr (potentially_out_of_range)
           {
-            return default_value;
+            return const_cast<T&>(default_value);
           }
           else
           {
             // Out of range null should
             // still return the default value.
             if (p == 0)
-              return default_value;
+              return const_cast<T&>(default_value);
             PAL::error("Internal error: Pagemap read access out of range.");
           }
         }
@@ -278,6 +278,18 @@ namespace snmalloc
         return body[p >> SHIFT];
     }
 
+    /**
+     * If the location has not been used before, then
+     * `potentially_out_of_range` should be set to true.
+     * This will ensure there is a location for the
+     * read/write.
+     */
+    template<bool potentially_out_of_range>
+    const T& get(address_t p)
+    {
+      return get_mut<potentially_out_of_range>(p);
+    }
+
     /**
      * Check if the pagemap has been initialised.
      */