[9] RFR (XS): 8036588 : VerifyFieldClosure fails instanceKlass:3133

[Vladimir Kozlov]

Yes, the return_value_is_used() method is incorrect, it does not take 
into account that the returned value could be used after deoptimization 
for reallocated objects.

return_oop is checked by deoptimization only when EliminateAllocations 
is true. Only in such case and when objects are reallocated GC can 
happen during deoptimization.

The fix will also preserve a return value when it is really not used:

A a = new A();
(void)foo();
a.f = 1;

or simple

(void)foo();

Which is fine since deoptimization code is not critical for performance.

return_value_is_used() is used only by this code. I would suggest to 
rename it to returns_pointer() and copy-paste code from 
CallNode::returns_pointer() (MachCallNode is not based on CallNode). And 
use it in the check.

Thanks,
Vladimir

On 7/8/14 12:13 PM, David Chase wrote:
>
> bug: https://bugs.openjdk.java.net/browse/JDK-8036588 (closed because only seen in SQE)
>
> webrev: http://cr.openjdk.java.net/~drchase/8036588/webrev.00/
>
> cause+fix:
> The root cause is use of the wrong liveness information at deoptimization point.
> The old code uses the optimizer's notion of "live" -- but deoptimization transfers to the
> interpreter and which can (will) manipulate values that are dead to the optimizer.
> The trigger is very tricky -- the following things need to happen:
>
> 1) an object D that will be dead is allocated
> 2) a method M is invoked that returns an object F, to only be stored in a field f of D
> 3) the optimizer eliminates the allocation of D and the storefield into D.f
> 4) deoptimization hits an execution of M; deoptimization reallocates D for the
>      interpreter; BUT the reallocation triggers a GC, which would forward F if
>      it had been correctly noted as live out of the call to M (but the bug is that it
>      was not).
> 5) the interpreter evaluates D.f = F (this succeeds)
> 6) before the frame with D in it exits, ANOTHER garbage collection occurs (or perhaps
>       GC was running concurrently in some way) and attempts to trace/copy through
>       D and D.f.
> 7) Hilarity ensues.
> 8) For extra giggles, this has only ever been observed with -Xmx=32G (or the corresponding
>       -XX:MaxRAMFraction= option) plus of course -XX:+DeoptimizeALot.  Also setting
>      -XX:DeoptimizeALotInterval=1 increases the failure rate to about 10% of test runs.
>      There's some additional missing context, because following this recipe to write a simpler
>      test for public consumption did not result in a crashing program.
>
> fix: Use a simpler test for "pointer is live from M" -- if the return type is an object,
>        then it is "live", at least for the interpreter.
>
> testing:
>    jtreg of runtime, gc, compiler
>
>    got to the point where I could see fails often enough for the two tests known to trigger this,
>    and after the fix neither test was observed to fail even once, even with hundreds of repetitions.
>