max() preprocessor macro with single-evaluation, constant expression for constants

max_macro.c

/* https://lwn.net/Articles/748870/ */

#define __single_eval_max(t1, t2, max1, max2, x, y) ({	\
    t1 max1 = (x);					\
    t2 max2 = (y);					\
    (void) (&max1 == &max2);			\
    max1 > max2 ? max1 : max2; })

#define __max(t1, t2, x, y)						\
    __builtin_choose_expr(__builtin_constant_p(x) &&		\
		                      __builtin_constant_p(y),			\
		                      (t1)(x) > (t2)(y) ? (t1)(x) : (t2)(y),	\
		                      __single_eval_max(t1, t2,			\
					                __UNIQUE_ID(max1_),	\
					                __UNIQUE_ID(max2_),	\
					                x, y))

#define max(x, y)	__max(typeof(x), typeof(y), x, y)