ds/address: add pointer diff function

And use it rather than open-coding subtraction of two address_cast-s.

src/ds/address.h

@@ -87,4 +87,16 @@ namespace snmalloc
     return pointer_cast<T>(bits::align_up(address_cast(p), alignment));
 #endif
   }
+
+  /**
+   * Compute the difference in pointers in units of char.  base is
+   * expected to point to the base of some (sub)allocation into which cursor
+   * points; would-be negative answers trip an assertion in debug builds.
+   */
+  inline size_t pointer_diff(void* base, void* cursor)
+  {
+    assert(cursor >= base);
+    return static_cast<size_t>(
+      static_cast<char*>(cursor) - static_cast<char*>(base));
+  }
 } // namespace snmalloc

src/mem/alloc.h

@@ -1193,7 +1193,7 @@ namespace snmalloc
 #ifdef CHECK_CLIENT
       if (!is_multiple_of_sizeclass(
             sizeclass_to_size(sizeclass),
-            address_cast(slab) + SUPERSLAB_SIZE - address_cast(p)))
+            pointer_diff(p, pointer_offset(slab, SUPERSLAB_SIZE))))
       {
         error("Not deallocating start of an object");
       }

src/mem/mediumslab.h

@@ -125,8 +125,7 @@ namespace snmalloc
     uint16_t pointer_to_index(void* p)
     {
       // Get the offset from the slab for a memory location.
-      return static_cast<uint16_t>(
-        ((address_cast(p) - address_cast(this))) >> 8);
+      return static_cast<uint16_t>(pointer_diff(this, p) >> 8);
     }
   };
 } // namespace snmalloc

src/mem/slab.h

@@ -16,7 +16,7 @@ namespace snmalloc
     uint16_t pointer_to_index(void* p)
     {
       // Get the offset from the slab for a memory location.
-      return static_cast<uint16_t>(address_cast(p) - address_cast(this));
+      return static_cast<uint16_t>(pointer_diff(this, p));
     }
 
   public:
@@ -144,7 +144,7 @@ namespace snmalloc
       Metaslab& meta = super->get_meta(this);
       return is_multiple_of_sizeclass(
         sizeclass_to_size(meta.sizeclass),
-        address_cast(this) + SLAB_SIZE - address_cast(p));
+        pointer_diff(p, pointer_offset(this, SLAB_SIZE)));
     }
 
     // Returns true, if it deallocation can proceed without changing any status

src/mem/superslab.h

@@ -44,9 +44,9 @@ namespace snmalloc
     // Used size_t as results in better code in MSVC
     size_t slab_to_index(Slab* slab)
     {
-      auto res = ((address_cast(slab) - address_cast(this)) >> SLAB_BITS);
-      assert(res == (uint8_t)res);
-      return res;
+      auto res = (pointer_diff(this, slab) >> SLAB_BITS);
+      assert(res == static_cast<uint8_t>(res));
+      return static_cast<uint8_t>(res);
     }
 
   public: