diff --git a/sys/sys/cdefs.h b/sys/sys/cdefs.h
--- a/sys/sys/cdefs.h
+++ b/sys/sys/cdefs.h
@@ -884,4 +884,23 @@
 #define	__guarded_by(x)		__lock_annotate(guarded_by(x))
 #define	__pt_guarded_by(x)	__lock_annotate(pt_guarded_by(x))
 
+/* Alignment builtins for better type checking and improved code generation. */
+/* Provide fallback versions for other compilers (GCC/Clang < 10): */
+#if !__has_builtin(__builtin_is_aligned)
+#define __builtin_is_aligned(x, align)	\
+	(((__uintptr_t)x & ((align) - 1)) == 0)
+#endif
+#if !__has_builtin(__builtin_align_up)
+#define __builtin_align_up(x, align)	\
+	((__typeof__(x))(((__uintptr_t)(x)+((align)-1))&(~((align)-1))))
+#endif
+#if !__has_builtin(__builtin_align_down)
+#define __builtin_align_down(x, align)	\
+	((__typeof__(x))((x)&(~((align)-1))))
+#endif
+
+#define __align_up(x, y) __builtin_align_up(x, y)
+#define __align_down(x, y) __builtin_align_down(x, y)
+#define __is_aligned(x, y) __builtin_is_aligned(x, y)
+
 #endif /* !_SYS_CDEFS_H_ */
diff --git a/sys/sys/param.h b/sys/sys/param.h
--- a/sys/sys/param.h
+++ b/sys/sys/param.h
@@ -305,9 +305,9 @@
 #endif
 #define	nitems(x)	(sizeof((x)) / sizeof((x)[0]))
 #define	rounddown(x, y)	(((x)/(y))*(y))
-#define	rounddown2(x, y) ((x)&(~((y)-1)))          /* if y is power of two */
+#define	rounddown2(x, y) __align_down(x, y) /* if y is power of two */
 #define	roundup(x, y)	((((x)+((y)-1))/(y))*(y))  /* to any y */
-#define	roundup2(x, y)	(((x)+((y)-1))&(~((y)-1))) /* if y is powers of two */
+#define	roundup2(x, y)	__align_up(x, y) /* if y is powers of two */
 #define powerof2(x)	((((x)-1)&(x))==0)
 
 /* Macros for min/max. */