address.h: add pointer alignment functions

More explicit than address_cast and bitwise and

src/ds/address.h

@@ -56,4 +56,35 @@ namespace snmalloc
       0;
   }
 
+  /**
+   * Align a pointer down to a statically specified granularity, which must be a
+   * power of two.
+   */
+  template<size_t alignment, typename T = void>
+  inline T* pointer_align_down(void* p)
+  {
+    static_assert(alignment > 0);
+    static_assert(bits::next_pow2_const(alignment) == alignment);
+#if __has_builtin(__builtin_align_down)
+    return static_cast<T*>(__builtin_align_down(p, alignment));
+#else
+    return pointer_cast<T>(bits::align_down(address_cast(p), alignment));
+#endif
+  }
+
+  /**
+   * Align a pointer up to a statically specified granularity, which must be a
+   * power of two.
+   */
+  template<size_t alignment, typename T = void>
+  inline T* pointer_align_up(void* p)
+  {
+    static_assert(alignment > 0);
+    static_assert(bits::next_pow2_const(alignment) == alignment);
+#if __has_builtin(__builtin_align_up)
+    return static_cast<T*>(__builtin_align_up(p, alignment));
+#else
+    return pointer_cast<T>(bits::align_up(address_cast(p), alignment));
+#endif
+  }
 } // namespace snmalloc

src/mem/largealloc.h

@@ -166,11 +166,12 @@ namespace snmalloc
         remaining -= size;
       }
 
-      auto page_start = bits::align_down(address_cast(p), OS_PAGE_SIZE);
-      auto page_end = bits::align_up(address_cast(p) + size, OS_PAGE_SIZE);
+      auto page_start = pointer_align_down<OS_PAGE_SIZE, char>(p);
+      auto page_end =
+        pointer_align_up<OS_PAGE_SIZE, char>(pointer_offset(p, size));
 
       PAL::template notify_using<NoZero>(
-        pointer_cast<void>(page_start), page_end - page_start);
+        page_start, static_cast<size_t>(page_end - page_start));
 
       return new (p) T(std::forward<Args...>(args)...);
     }
@@ -295,8 +296,7 @@ namespace snmalloc
           reserved_start =
             memory_provider.template reserve<false>(&add, SUPERSLAB_SIZE);
           reserved_end = pointer_offset(reserved_start, add);
-          reserved_start = pointer_cast<void>(
-            bits::align_up(address_cast(reserved_start), SUPERSLAB_SIZE));
+          reserved_start = pointer_align_up<SUPERSLAB_SIZE>(reserved_start);
 
           if (add < need)
             return false;

src/mem/mediumslab.h

@@ -41,7 +41,7 @@ namespace snmalloc
 
     static Mediumslab* get(void* p)
     {
-      return pointer_cast<Mediumslab>(address_cast(p) & SUPERSLAB_MASK);
+      return pointer_align_down<SUPERSLAB_SIZE, Mediumslab>(p);
     }
 
     void init(RemoteAllocator* alloc, sizeclass_t sc, size_t rsize)

src/mem/metaslab.h

@@ -15,7 +15,7 @@ namespace snmalloc
 
     Slab* get_slab()
     {
-      return pointer_cast<Slab>(address_cast(this) & SLAB_MASK);
+      return pointer_align_down<SLAB_SIZE, Slab>(this);
     }
   };
 

src/mem/pagemap.h

@@ -273,9 +273,7 @@ namespace snmalloc
     void* page_for_address(uintptr_t p)
     {
       bool success;
-      return reinterpret_cast<void*>(
-        ~(OS_PAGE_SIZE - 1) &
-        reinterpret_cast<uintptr_t>(get_addr<true>(p, success)));
+      return pointer_align_down<OS_PAGE_SIZE>(get_addr<true>(p, success));
     }
 
     T get(uintptr_t p)

src/mem/superslab.h

@@ -67,7 +67,7 @@ namespace snmalloc
 
     static Superslab* get(void* p)
     {
-      return pointer_cast<Superslab>(address_cast(p) & SUPERSLAB_MASK);
+      return pointer_align_down<SUPERSLAB_SIZE, Superslab>(p);
     }
 
     static bool is_short_sizeclass(sizeclass_t sizeclass)