NFC: pointer_offset* functions always return void*

This requires that the caller perform the cast on the output rather than the
input, which is a little closer to the truth.  Shuffle some casts into the right
position.

src/ds/address.h

@@ -18,19 +18,19 @@ namespace snmalloc
   /**
    * Perform pointer arithmetic and return the adjusted pointer.
    */
-  template<typename T>
-  inline T* pointer_offset(T* base, size_t diff)
+  template<typename U = void, typename T>
+  inline U* pointer_offset(T* base, size_t diff)
   {
-    return reinterpret_cast<T*>(reinterpret_cast<char*>(base) + diff);
+    return reinterpret_cast<U*>(reinterpret_cast<char*>(base) + diff);
   }
 
   /**
    * Perform pointer arithmetic and return the adjusted pointer.
    */
-  template<typename T>
-  inline T* pointer_offset_signed(T* base, ptrdiff_t diff)
+  template<typename U = void, typename T>
+  inline U* pointer_offset_signed(T* base, ptrdiff_t diff)
   {
-    return reinterpret_cast<T*>(reinterpret_cast<char*>(base) + diff);
+    return reinterpret_cast<U*>(reinterpret_cast<char*>(base) + diff);
   }
 
   /**

src/ds/cdllist.h

@@ -31,7 +31,7 @@ namespace snmalloc
 
     SNMALLOC_FAST_PATH T* get_next()
     {
-      return pointer_offset_signed(static_cast<T*>(this), to_next);
+      return pointer_offset_signed<T>(this, to_next);
     }
   };
 

src/mem/address_space.h

@@ -101,8 +101,7 @@ namespace snmalloc
         void* bigger = remove_block(align_bits + 1);
         if (bigger != nullptr)
         {
-          void* left_over =
-            pointer_offset(bigger, bits::one_at_bit(align_bits));
+          auto left_over = pointer_offset(bigger, bits::one_at_bit(align_bits));
           ranges[align_bits][0] = left_over;
           check_block(left_over, align_bits);
         }

src/mem/alloc.h

@@ -391,7 +391,7 @@ namespace snmalloc
       while (chunkmap_slab_kind >= CMLargeRangeMin)
       {
         // This is a large alloc redirect.
-        ss = pointer_offset_signed(
+        ss = pointer_offset_signed<Superslab>(
           ss,
           -(static_cast<ptrdiff_t>(1)
             << (chunkmap_slab_kind - CMLargeRangeMin + SUPERSLAB_BITS)));
@@ -402,7 +402,7 @@ namespace snmalloc
       {
         if constexpr ((location == End) || (location == OnePastEnd))
           // We don't know the End, so return MAX_PTR
-          return pointer_offset<void>(nullptr, UINTPTR_MAX);
+          return pointer_offset<void, void>(nullptr, UINTPTR_MAX);
         else
           // We don't know the Start, so return MIN_PTR
           return nullptr;
@@ -1181,8 +1181,8 @@ namespace snmalloc
       Slab* slab = alloc_slab<allow_reserve>(sizeclass);
       if (slab == nullptr)
         return nullptr;
-      bp = reinterpret_cast<SlabNext*>(pointer_offset(
-        slab, get_initial_offset(sizeclass, Metaslab::is_short(slab))));
+      bp = pointer_offset<SlabNext>(
+        slab, get_initial_offset(sizeclass, Metaslab::is_short(slab)));
 
       return small_alloc_build_free_list<zero_mem, allow_reserve>(sizeclass);
     }

src/mem/superslab.h

@@ -183,8 +183,8 @@ namespace snmalloc
     Slab* alloc_slab(sizeclass_t sizeclass)
     {
       uint8_t h = head;
-      Slab* slab = pointer_offset(
-        reinterpret_cast<Slab*>(this), (static_cast<size_t>(h) << SLAB_BITS));
+      Slab* slab = reinterpret_cast<Slab*>(
+        pointer_offset(this, (static_cast<size_t>(h) << SLAB_BITS)));
 
       uint8_t n = meta[h].next;