libutils: util.h: Relax ROUNDDOWN() and add ROUNDDOWN2()

Remove constraint on ROUNDDOWN() to have its size argument being a power
of 2 and add new ROUNDDOWN2() macro with that constraint.

The previous implementation of ROUNDDOWN(), optimized for when size
argument is a power of 2, is now used for ROUNDDOWN2() but this latter
also asserts (in debug build mode) that the size argument conforms to
this condition.

The new implementation of ROUNDDOWN() is less optimal but modern
compilers produce the same optimized assembly code with this macro when
the size argument is a constant value known from the compiler so all
use of ROUNDDOWN() with a known constant value do not need move to
ROUNDDOWN2().

Performance sensitive routines should now on use ROUNDDOWN2() to
leverage the power-of-2 rounding optimization.

Signed-off-by: Etienne Carriere <[email protected]>
Reviewed-by: Jens Wiklander <[email protected]>

lib/libutils/ext/include/util.h

@@ -135,8 +135,15 @@
  */
 #define ROUNDUP_DIV(x, y) (ROUNDUP((x), (y)) / (__typeof__(x))(y))
 
+/* Round down the even multiple of size */
+#define ROUNDDOWN(x, y)		(((x) / (__typeof__(x))(y)) * (__typeof__(x))(y))
+
 /* Round down the even multiple of size, size has to be a power of 2 */
-#define ROUNDDOWN(v, size) ((v) & ~((__typeof__(v))(size) - 1))
+#define ROUNDDOWN2(v, size) \
+	(__extension__({ \
+		assert(IS_POWER_OF_TWO(size)); \
+		((v) & ~((__typeof__(v))(size) - 1)); \
+	}))
 
 /*
  * Round up the result of x / y to the nearest upper integer if result is not