arithmetic-shift fails in extreme cases

[sjamaan]

I don't know whether it's worth the trouble to fix this, but I'll put it on here so at least it's documented.

(arithmetic-shift -123 most-negative-fixnum) => -123

This should be -1. It fails because in chicken.h the C_i_fixnum_arithmetic_shift function calls C_fixnum_shift_right with the negated value of the shift amount. This overflows and wraps around to zero.

Fixing it would be simplest by just inlining the shift and negating _after_ unboxing. This is easy, but there are some other places that call C_fixnum_shift_right in this way.

This code is obviously wrong but I'm not sure it's worth fixing, since it's such an extreme edge case (if you subtract 1 from the shift amount you get an error that the argument is not a fixnum). On the other hand, not fixing it just seems, I dunno... lazy and possibly dangerous. :)

[sjamaan]

Turns out there's also a bug when shifting more than C_WORD_SIZE positions.

According to the ​C spec, shifting more than the number of bits in the type is undefined (#3 in section 6.5.7, "bitwise shift operators").
Shifting negative values is also pretty ill-defined (#4 and #5, final sentence in both).

C: Don't you just love it?

[sjamaan]

Not important enough to delay 4.9.0 any longer

[sjamaan]

This has been fixed by the numbers integration in CHICKEN 5. It's not important enough to fix in CHICKEN 4.