Re: Bitshifting negatives in Pike is a little sketchy

Hi Douglas,

this is indeed a bug in 7.8 (which has already been fixed in pike 8).
This is caused by an overflow check in the operator. The reason is that
in C a right shift by a negative number is implementation defined. I
suppose there are no relevant implementations which don't do an
arithmetic right shift, so the check can be made less strict.
Thanks for the report, will fix.

Arne

On 10/21/16 10:46, Douglas Gardner wrote:
> Hi all,
> 
> In Pike 7.8, the following operations happen:
> 
> 25 >> 1 → 12
> -25 >> 1 → -1
> 25 >> -1 → Bad argument 2 to >>(). Expected int(0..)|object.
> -25 >> -1 → Bad argument 2 to >>(). Expected int(0..)|object.
> 
> Only the first one of these is handled correctly.
> Bitshifting negative integers is a generally sketchy business in many
> languages, but most seem to agree that -25 >> 1 is -13. However, in
> Pike, right bitshifting any negative integer results in -1 (which is
> rather wrong!).
> 
> 25 >> -1 and -25 >> -1 are a little undecided. In most languages, such
> as Python, Java and Perl, they return 0. In C and Ruby, a negative
> right shift turns into a left shift, so they return 50 and -50.
> In Python and Go, they raise errors. All three approaches are valid,
> but in Pike we hit a fairly opaque unknown program error -- this
> should change to match the behaviour of <<(), which is to tell you off
> about a negative shift count.
> 
> 
> All in all, we should:
> * make a negative shifted by a positive work correctly
> * make a negative shifted by a negative either work sensibly, or tell
> you off properly
> 
> Thanks,
> Douglas
> 
>