Added reverse lookup to Pagemap

src/backend/pagemap.h

@@ -185,6 +185,21 @@ namespace snmalloc
       body = new_body;
     }
 
+    /**
+     * Get the number of entries.
+     */
+    constexpr size_t num_entries() const
+    {
+      if constexpr (has_bounds)
+      {
+        return size >> GRANULARITY_BITS;
+      }
+      else
+      {
+        return bits::one_at_bit(bits::ADDRESS_BITS - GRANULARITY_BITS);
+      }
+    }
+
     /**
      * If the location has not been used before, then
      * `potentially_out_of_range` should be set to true.
@@ -223,6 +238,21 @@ namespace snmalloc
       return body[p >> SHIFT];
     }
 
+    /**
+     * Return the starting address corresponding to a given entry within the
+     * Pagemap. Also checks that the reference actually points to a valid entry.
+     */
+    address_t get_address(const T& t) const
+    {
+      address_t entry_offset = address_cast(&t) - address_cast(body);
+      address_t entry_index = entry_offset / sizeof(T);
+      if (entry_offset % sizeof(T) != 0 || entry_index >= num_entries())
+      {
+        PAL::error("Internal error: Invalid Pagemap entry for reverse lookup.");
+      }
+      return base + (entry_index << GRANULARITY_BITS);
+    }
+
     void set(address_t p, T t)
     {
 #ifdef SNMALLOC_TRACING