diff --git a/libcxx/src/memory.cpp b/libcxx/src/memory.cpp
--- a/libcxx/src/memory.cpp
+++ b/libcxx/src/memory.cpp
@@ -194,7 +194,11 @@
     if (size <= space)
     {
         char* p1 = static_cast<char*>(ptr);
-        char* p2 = reinterpret_cast<char*>(reinterpret_cast<size_t>(p1 + (alignment - 1)) & -alignment);
+#if __has_builtin(__builtin_align_up)
+        char* p2 = __builtin_align_up(p1, alignment);
+#else
+        char* p2 = reinterpret_cast<char*>(reinterpret_cast<uintptr_t>(p1 + (alignment - 1)) & -alignment);
+#endif
         size_t d = static_cast<size_t>(p2 - p1);
         if (d <= space - size)
         {