C2: cyclic IR for JVMS

[Vladimir Ivanov]

No, I have hard time gathering more data about the problem.
I'm able to reproduce it only with product binaries and it happens 
during huge method compilation. Will try to add additional 
instrumentation, but I wouldn't expect much.

Best regards,
Vladimir Ivanov

On 12/19/14 11:35 PM, Vladimir Kozlov wrote:
> Did you find why "the call node is live, though the loop was dead"?
> I think I assumed that calls are safe - can't be in dead loop.
> Your fix is reasonable but, first, fins why call is alive.
>
> Thanks,
> Vladimir
>
> On 12/19/14 7:46 AM, Vladimir Ivanov wrote:
>> So, if return value projection is not considered dead loop safe [1], the
>> problem goes away.
>>
>> I'm curious whether it was an overlook in 6625997 [1] [2].
>>
>> Best regards,
>> Vladimir Ivanov
>>
>> [1]
>> diff --git a/src/share/vm/opto/multnode.hpp
>> b/src/share/vm/opto/multnode.hpp
>> --- a/src/share/vm/opto/multnode.hpp
>> +++ b/src/share/vm/opto/multnode.hpp
>> @@ -68,7 +68,7 @@
>>     {
>>       init_class_id(Class_Proj);
>>       // Optimistic setting. Need additional checks in
>> Node::is_dead_loop_safe().
>> -    if (con != TypeFunc::Memory || src->is_Start())
>> +    if ((con != TypeFunc::Memory && con != TypeFunc::Parms) ||
>> src->is_Start())
>>         init_flags(Flag_is_dead_loop_safe);
>>       debug_only(check_con());
>>     }
>>
>> [2] https://jbs.oracle.com/bugs/browse/JDK-6625997
>> [3]
>> http://hg.openjdk.java.net/jdk9/hs-comp/hotspot/diff/72f4a668df19/src/share/vm/opto/multnode.hpp
>>
>>
>>
>> On 12/18/14 10:11 PM, Vladimir Ivanov wrote:
>>> Roland,
>>>
>>> The code is reachable by the time inlining occurs, but the call node is
>>> broken.
>>>
>>> The transformation which breaks the code is performed in
>>> RegionNode::Ideal()
>>> Phi (Region) (Proj #5 (CallStaticJava ... Phi(backref)...))
>>> ==>
>>> (Proj #5 (CallStaticJava ... Proj(backref)...))
>>>
>>> The RegionNode and Phi are removed, but CallStaticJava is broken.
>>>
>>> It seems that if RegionNode::is_unreachable_region() detected that
>>> RegionNode is dead, it would break the vicious circle (PhiNode is
>>> replaced by top()).
>>>
>>> Not sure why the call node is live, though the loop was dead.
>>>
>>> Best regards,
>>> Vladimir Ivanov
>>>
>>> On 12/18/14, 8:25 PM, Roland Westrelin wrote:
>>>>> As my investigations show, it's a leftover from a loop.
>>>>> CallStaticJava represents loop counter increment.
>>>>>
>>>>> The graph is as follows:
>>>>> Phi (Region) (Proj #5 (CallStaticJava ... Phi(backref)...))
>>>>>
>>>>> The Region is dead, but RegionNode::is_unreachable_region doesn't
>>>>> detect that. Node::simple_data_loop_check(Phi, Proj) doesn't find the
>>>>> cycle, because most of Proj nodes have Flag_is_dead_loop_safe set [1].
>>>>> And I don't understand why it is so. Is the check in
>>>>> Node::is_dead_loop_safe() too weak or Flag_is_dead_loop_safe too
>>>>> "optimistic"? :-)
>>>>
>>>> Isn’t the problem that you try to late inline a call that is dead and
>>>> that the checks in LateInlineCallGenerator::do_late_inline():
>>>>
>>>>    if (call == NULL || call->outcnt() == 0 ||
>>>>        call->in(0) == NULL || call->in(0)->is_top()) {
>>>>      return;
>>>>    }
>>>>
>>>>    const TypeTuple *r = call->tf()->domain();
>>>>    for (int i1 = 0; i1 < method()->arg_size(); i1++) {
>>>>      if (call->in(TypeFunc::Parms + i1)->is_top() &&
>>>> r->field_at(TypeFunc::Parms + i1) != Type::HALF) {
>>>>        assert(Compile::current()->inlining_incrementally(), "shouldn't
>>>> happen during parsing");
>>>>        return;
>>>>      }
>>>>    }
>>>>
>>>>    if (call->in(TypeFunc::Memory)->is_top()) {
>>>>      assert(Compile::current()->inlining_incrementally(), "shouldn't
>>>> happen during parsing");
>>>>      return;
>>>>    }
>>>>
>>>> don’t detect it?
>>>> FWIW, I know there are rare cases where those tests are not
>>>> sufficient. Ideally, you would need  Compile::inline_incrementally()
>>>> to do:
>>>> PhaseIdealLoop ideal_loop( igvn, false, true );
>>>> after every inline but that’s very expensive.
>>>>
>>>> Roland.
>>>>