macro: implement ABS via __builtin_imaxabs()

src/basic/errno-util.h

@@ -95,6 +95,7 @@ static inline int errno_or_else(int fallback) {
         return -ABS(fallback);
 }
 
+/* abs(3) says: Trying to take the absolute value of the most negative integer is not defined. */
 #define _DEFINE_ABS_WRAPPER(name)                         \
         static inline bool ERRNO_IS_##name(intmax_t r) {  \
                 if (r == INTMAX_MIN)                      \

src/fundamental/macro-fundamental.h

@@ -176,12 +176,12 @@
                 UNIQ_T(A, aq) > UNIQ_T(B, bq) ? UNIQ_T(A, aq) : UNIQ_T(B, bq); \
         })
 
-#define ABS(a) __ABS(UNIQ, (a))
-#define __ABS(aq, a)                                    \
-({                                                      \
-        const typeof(a) UNIQ_T(A, aq) = (a);            \
-        UNIQ_T(A, aq) < 0 ? -UNIQ_T(A, aq) : UNIQ_T(A, aq); \
-})
+#ifdef __clang__
+#  define ABS(a) __builtin_llabs(a)
+#else
+#  define ABS(a) __builtin_imaxabs(a)
+#endif
+assert_cc(sizeof(intmax_t) <= sizeof(long long));
 
 #define IS_UNSIGNED_INTEGER_TYPE(type) \
         (__builtin_types_compatible_p(typeof(type), unsigned char) ||   \