No, this optimization also happens when you `#include <stdio.h>`. More accurately speaking, GCC (among others) evidently assumes that printf is the standard C l

No, this optimization also happens when you `#include <stdio.h>`. More accurately speaking, GCC (among others) evidently assumes that printf is the standard C library function no matter prototype is defined or not and replaces it with puts under the same assumption plus the following additional conditions: - No other arguments are present. - The format argument has no format specifiers and ends with `\n`. - The return value is unused. (So `return printf("...\n");` would have prevented this optimization.)