Merge pull request #370 from ihaller/ihaller/reverse-lookup

Added the ability to perform a reverse lookup in the Pagemap

src/backend/pagemap.h

@@ -185,6 +185,21 @@ namespace snmalloc
       body = new_body;
     }
 
+    /**
+     * Get the number of entries.
+     */
+    [[nodiscard]] 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 +238,19 @@ 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.
+     */
+    [[nodiscard]] 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);
+      SNMALLOC_ASSERT(
+        entry_offset % sizeof(T) == 0 && entry_index < num_entries());
+      return base + (entry_index << GRANULARITY_BITS);
+    }
+
     void set(address_t p, T t)
     {
 #ifdef SNMALLOC_TRACING