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