Re: [racket-dev] Inconsistent behavior when doing bitwise arithmetic

[John Boyle]

I can confirm weird behavior (and I get different weird results):

Welcome to Racket v6.5.
> (define num #x)
(for ([i 5])
  (printf "~a~n" (bitwise-ior (bitwise-and num -2) 0)))
2192618128
2192618180
2192618200
2192618668
2192618688
> (for ([i 5])
  (printf "~a~n" (bitwise-ior (bitwise-and num -2) 0)))
2192791964
2192792016
2192792036
2192792504
2192792524
> (bitwise-and num -2)
18446744073709551614
> (bitwise-and num -2)
18446744073709551614
> (bitwise-and num -2)
18446744073709551614
> (bitwise-and num -2)
18446744073709551614
> (define h (bitwise-and num -2))
> (for ([i 5])
  (printf "~a~n" (bitwise-ior h 0)))
18446744073709551614
18446744073709551614
18446744073709551614
18446744073709551614
18446744073709551614
> (for ([i 5])
  (printf "~a~n" (bitwise-ior h 0)))
18446744073709551614
18446744073709551614
18446744073709551614
18446744073709551614
18446744073709551614

--John Boyle
*Science is what we understand well enough to explain to a computer. Art is
everything else we do.* --Knuth

On Thu, Oct 13, 2016 at 9:17 AM,  wrote:

> Hi all,
>
> I get a weird behavior when using bitwise-ior and bitwise-and with large
> numbers. Tested on 2 machines (racket 6.6, Ubuntu 16.04 and 14.04):
>
> Here is the test example:
>
> #lang racket
> (define num #x) ;; remove one f, and the results are fine
> in both cases
> (for ([i 5])
>   (printf "~a~n" (bitwise-ior (bitwise-and num -2) 0)))
>
>
> When run from DrRacket using ctr+r, it works properly:
> 18446744073709551614
> 18446744073709551614
> 18446744073709551614
> 18446744073709551614
> 18446744073709551614
>
> When pasted into DrRacket's REPL (or run from command line via "racket
> random-bug.rkt"), the code does not only produce wrong result, but also
> seems to be counting something :)
> 70112357026588
> 70112357038692
> 70112357043588
> 70112357048576
> 70112357053344
>
> Removing one "f" from the test number results in correct behavior in both
> cases.
> Very large numbers are are reduced to the same range (around 54 bits).
> Replacing the expression (bitwise-and num -2) by the corresponding number
> it computes results in correct behavior.
>
> Cheers,
> Peter
>
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[racket-dev] Inconsistent behavior when doing bitwise arithmetic

[peter . samarin]

Hi all,

I get a weird behavior when using bitwise-ior and bitwise-and with large 
numbers. Tested on 2 machines (racket 6.6, Ubuntu 16.04 and 14.04):

Here is the test example:

#lang racket
(define num #x) ;; remove one f, and the results are fine 
in both cases
(for ([i 5])
  (printf "~a~n" (bitwise-ior (bitwise-and num -2) 0)))


When run from DrRacket using ctr+r, it works properly:
18446744073709551614
18446744073709551614
18446744073709551614
18446744073709551614
18446744073709551614

When pasted into DrRacket's REPL (or run from command line via "racket 
random-bug.rkt"), the code does not only produce wrong result, but also 
seems to be counting something :)
70112357026588
70112357038692
70112357043588
70112357048576
70112357053344

Removing one "f" from the test number results in correct behavior in both 
cases.
Very large numbers are are reduced to the same range (around 54 bits).
Replacing the expression (bitwise-and num -2) by the corresponding number 
it computes results in correct behavior.

Cheers,
Peter

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