> Those numbers look like pointers interpreted as fixnums

Yes.

The optimizer thought that the result of (bitwise-and num -2) was a
fixnum, so it changed
  (bitwise-ior (bitwise-and num -2) 0)
to
  (unsafe-fxior (bitwise-and num -2) 0)


All the pointers are even (actually a multiple of 4 in 32 bits and a
multiple of 8 in 64 bits). Fixnum are implemented as "pointers" to an
odd address, the fixnum n is encoded as 2*n+1.

I.E. If the "pointer" is even then it's a pointer to a real object. If
the "pointer" is odd then it's a fixnum.

In this case, the result of (bitwise-and num -2) was a pointer to a
bignum, but unsafe-fxior interpreted it as fixnum without checking the
parity, so what you see is the address of the bignum divided by 2.

Each iteration produces a new bignum, so you will usually get results
in an increasing order, as the allocator creates new objects.

Gustavo


On Fri, Oct 14, 2016 at 9:39 AM, Vincent St-Amour
<stamo...@eecs.northwestern.edu> wrote:
> Those numbers look like pointers interpreted as fixnums.
>
> So my guess as to why they differ so much is that between each
> operation, you call `printf`, which allocates enough to claim the
> addresses between, e.g., 70112357026588 and 70112357038692. So that next
> time around the loop, the result of the `bitwise-and` lives at
> 70112357038692.
>
> Just a guess, though.
>
> Vincent
>
>
>
> On Fri, 14 Oct 2016 01:11:56 -0500,
> peter.sama...@gmail.com wrote:
>>
>> Do you have any guess why the resulting numbers vary so much?
>>
>> On Thursday, October 13, 2016 at 7:20:47 PM UTC+2, mflatt wrote:
>>
>>     Thanks for the report!
>>
>>     This is a bug in the optimizer's handling of `bitwise-and`, where I
>>     made it assume a fixnum result if either argument is a fixnum.
>>     That's
>>     obviously not correct if the fixnum is negative. I'll push a repair.
>>
>>     The difference you see when running in a module is because the
>>     optimizer constant-folds the calculation, so there's no `bitwise-and
>>     `
>>     call left to make incorrect assumptions about.
>>
>>     At Thu, 13 Oct 2016 09:17:52 -0700 (PDT), peter....@gmail.com 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 #xffffffffffffffff) ;; 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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