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Strange multiplication behavior in Guile Scheme interpreter

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.)

Solution

  • 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.