Is it? I always internalized int as “the most natural sized number for the platform”. For a lot of uses, that’s a pretty reasonable choice, despite there more specific/constrained types available now.
Basically, to get decent perf out of C loops, arithmetic overflow has to be left as UB for int. If the standard index variable type had historically been unsigned and defined to terminate on overflow or something, you could make wrap-on-overflow standard for int without killing the performance of hot inner loops.
GCC says:
> -faggressive-loop-optimizations This option tells the loop optimizer to use language constraints to derive bounds for the number of iterations of a loop. This assumes that loop code does not invoke undefined behavior by for example causing signed integer overflows or out-of-bound array accesses. The bounds for the number of iterations of a loop are used to guide loop unrolling and peeling and loop exit test optimizations.