Endless loops in computation code (1.6.0_22).

[Tom Rodriguez]

On Mar 3, 2011, at 4:39 AM, Dawid Weiss wrote:

> Hi. We see an infrequent, but very annoying VM hangups in purely
> computational code, as in here:
> 
> http://builds.carrot2.org/build/result/viewBuildResultsFailedTests.action?buildKey=JSA-JSAHEAD-JOB1&buildNumber=8
> 
> The loop in question is like this:
> 
>            for (c0 = ALPHABET_SIZE - 2, j = m; 0 < j; --c0)
>            {
>                for (c1 = ALPHABET_SIZE - 1; c0 < c1; j = i, --c1)
>                {
>                    i = bucket_B[(c0) * ALPHABET_SIZE + (c1)];
>                    if (1 < (j - i))
>                    {
>                        ssSort(PAb, i, j, buf, bufsize, 2, n, SA[i] == (m - 1));
>                    }
>                }
>            }
> 
> Unfortunately I cannot provide an always-halting example, but the bug
> seems to be JVM-related because:
> 
> 1) once the VM hangs in debugging mode, breaking out to the debugger
> and stepping through the code terminates normally (so a deopt. I
> assume),

Is it hanging, as in making no progress, or not terminating?  Does it terminate if given a lot more time?  It's not GC bound?  By debugger do you mean gdb or a Java debugger?

tom

> 
> 2) the freeze is non-deterministic, while the test is (regardless of
> the 'randomness', it always starts from the same seed),
> 
> 3) we could reproduce this occasionally under different VMs and
> different OSs (64-bit linux, 64-bit Windows).
> 
> Has there been any bug in the JIT that might be causing this? Thanks
> for pointers if you recognize an evil code pattern above.
> 
> Dawid
> 
> P.S. The above code is a translation from another person's C snippet,
> so I can't easily explain why the loop is built this way or why c0 is
> passed between the inner and outer loop :).