Strange multiplication behavior in Guile Scheme interpreter

Question

I was practicing Scheme in Guile 1.8.8 interpreter on OS X. I noticed something interesting.

Here's expt function which is basically does exponentiation expt(b,n) = b^n :

 (define (square x) (* x x))  (define (even? x) (= (remainder x 2) 0))  (define (expt b n)        (cond ((= n 0) 1)         ((even? n) (square (expt b (/ n 2))))         (else (* b (expt b (- n 1))))       )) 

If I try it with some inputs

 > (expt 2 10)  1024  > (expt 2 63)  9223372036854775808 

Here comes the strange part:

 > (expt 2 64)  0 

More strangely, until n=488 it stays at 0:

 > (expt 2 487)  0  > (expt 2 488)  79916762888089401123.....  > (expt 2 1000)  1071508607186267320948425049060....  > (expt 2 10000)  0 

When I try this code with repl.it online interpreter, it works as expected. So what the hell is wrong with Guile?

(Note: On some dialects, remainder function is called as mod.)

Answer 1

I recently fixed this bug in Guile 2.0. The bug came into existence when C compilers started optimizing out overflow checks, on the theory that if a signed integer overflow occurs then the behavior is unspecified and thus the compiler can do whatever it likes.