[racket-dev] Output value of program changes depending on whether I pretty-print an opaque struct value

From: Matthew Flatt (mflatt at cs.utah.edu)
Date: Tue Aug 27 18:33:02 EDT 2013

I haven't been able to get a different result by changing printing.

One thing that printing might do, however, is assign `eq?`-based hash
codes to objects that did not already have them. That assignment, in
turn, could affect the order in which objects appear later in a hash

I hacked Racket to make the internal hash-code generator count down
instead of up, and I was able to get a different result that way.

Are you using `eq?`-based hashing at all? Could there be some
dependency on the order of values within a hash table (or hash set)?

At Tue, 27 Aug 2013 14:02:50 -0400 (EDT), "J. Ian Johnson" wrote:
> This is mostly an mflatt-only problem.
> My analysis framework is now using a generic dictionary for its abstract heap, 
> and depending on whether or not I pretty-print this heap before continuing on 
> analyzing, the result of the analysis changes (sound versus unsound). I found 
> the problem doing some printf debugging, and when fixed, I tried removing the 
> print statements. The answer changed. There must be some problem in the C code 
> somewhere, because this is just bonkers.
> (Note: I've had previous bugs where rewriting a (begin (void) e) => e caused a 
> similar value-changing bug. Updating the compiler to head fixed it at that 
> point. I'm running yesterday's HEAD now, though.)
> https://github.com/dvanhorn/oaam/tree/thocon
> See the ;; mflatt: comment in code/kcfa.rkt
> To see the problem,
> racket code/run-benchmark.rkt --lcg benchmarks/temp-c/sort2.sch
> Thanks,
> -Ian
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