Summary

arithmetic-shift fails in extreme cases

Metadata

Description

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.

 
 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. :)

Changes and comments

[2013-10-15 20:52:36 UTC] sjamaan changed summary

[2013-10-15 20:52:36 UTC] sjamaan wrote:

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?

[2014-02-17 08:47:13 UTC] sjamaan changed milestone from 4.9.0 to 4.10.0

[2014-02-17 08:47:13 UTC] sjamaan wrote:

Not important enough to delay 4.9.0 any longer

[2015-08-04 10:38:45 UTC] sjamaan changed status from new to closed

[2015-08-04 10:38:45 UTC] sjamaan set resolution to fixed

[2015-08-04 10:38:45 UTC] sjamaan wrote:

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