Make pagemap check for init on some gets.

When we are accessing potentially out of range, then we might be
accessing before the pagemap has been initialised.  Move the check
into the pagemap for better codegen.

src/backend/pagemap.h

@@ -40,6 +40,12 @@ namespace snmalloc
      */
     T* body{const_cast<T*>(&default_value)};
 
+    /**
+     * The representation of the pagemap, but nullptr if it has not been
+     * initialised.  Used to combine init checking and lookup.
+     */
+    T* body_opt{nullptr};
+
     /**
      * If `has_bounds` is set, then these should contain the
      * bounds of the heap that is being managed by this pagemap.
@@ -94,6 +100,7 @@ namespace snmalloc
       static_assert(
         has_bounds_ == has_bounds, "Don't set SFINAE template parameter!");
       body = address;
+      body_opt = address;
     }
 
     /**
@@ -124,7 +131,7 @@ namespace snmalloc
       // Put pagemap at start of range.
       // TODO CHERI capability bound here!
       body = reinterpret_cast<T*>(b);
-
+      body_opt = body;
       // Advance by size of pagemap.
       // Note that base needs to be aligned to GRANULARITY for the rest of the
       // code to work
@@ -183,6 +190,7 @@ namespace snmalloc
       new_body[0] = body[0];
 
       body = new_body;
+      body_opt = new_body;
     }
 
     /**
@@ -209,6 +217,12 @@ namespace snmalloc
     template<bool potentially_out_of_range>
     const T& get(address_t p)
     {
+      if constexpr (potentially_out_of_range)
+      {
+        if (unlikely(body_opt == nullptr))
+          return default_value;
+      }
+
       if constexpr (has_bounds)
       {
         if (p - base > size)
@@ -235,7 +249,10 @@ namespace snmalloc
         register_range(p, 1);
       }
 
-      return body[p >> SHIFT];
+      if constexpr (potentially_out_of_range)
+        return body_opt[p >> SHIFT];
+      else
+        return body[p >> SHIFT];
     }
 
     /**