[9] RFR(S): 8150804: C2 Compilation fails with assert(_base >= OopPtr && _base <= AryPtr) failed: Not a Java pointer

[Vladimir Kozlov]

Looks good.

Thanks,
Vladimir

On 3/14/16 8:21 AM, Tobias Hartmann wrote:
> Hi,
>
> I triggered another problem with the latest webrev:
>
> #  Internal Error (/scratch/opt/jprt/T/P1/101046.tohartma/s/hotspot/src/share/vm/opto/phaseX.cpp:346), pid=11651, tid=11671
> #  assert(t == t_no_spec) failed: dead node in hash table or missed node during speculative cleanup
>
> We should add the input nodes to the IGVN worklist after cutting off the Phi because they may be dead now:
> http://cr.openjdk.java.net/~thartmann/8150804/webrev.02/
>
> Thanks,
> Tobias
>
> On 14.03.2016 11:09, Tobias Hartmann wrote:
>> Hi Vladimir,
>>
>> thanks for the review!
>>
>> On 14.03.2016 02:28, Vladimir Kozlov wrote:
>>> It is strange that next subgraph did not collapse - CheckCastPPNode::Identity() should remove second CheckCastPP:
>>>
>>>>          CheckCastPP
>>>>           A:NotNull
>>>>           /       \
>>>> CheckCastPP     |
>>>>    A:NotNull      |
>>>>            \     /
>>>>              Phi
>>>>               A
>>
>> When CheckCastPPNode::Identity() is first invoked, the graph looks like this
>>
>>   92	CheckCastPP	===  90  32  [[]]  #A:NotNull *  Oop:A:NotNull * !jvms: TestPhiElimination::test @ bci:8
>>   135	CheckCastPP	===  133  92  [[]]  #A:NotNull * (speculative=A:NotNull:exact * (inline_depth=2))  Oop:A:NotNull * (speculative=A:NotNull:exact * (inline_depth=2)) !jvms: A::get @ bci:9 TestPhiElimination::test @ bci:11
>>
>> Because 135 has a speculative part, the types are not equal and 135 is not replaced by 92.
>>
>>> I think next code in PhiNode::Ideal() should check can_reshape too since during parsing graph is incomplete:
>>>
>>>    if (uin == NULL) {
>>>      uncasted = true;
>>>      uin = unique_input(phase, true);
>>>    }
>>
>> Yes, that's a better solution. I verified that it works (Phi is folded after parsing and has correct A:NotNull type).
>>
>> Here is the new webrev:
>> http://cr.openjdk.java.net/~thartmann/8150804/webrev.01/
>>
>> Thanks,
>> Tobias
>>
>>>
>>> Thanks,
>>> Vladimir
>>>
>>> On 3/11/16 7:40 AM, Tobias Hartmann wrote:
>>>> Hi,
>>>>
>>>> please review the following patch.
>>>>
>>>> https://bugs.openjdk.java.net/browse/JDK-8150804
>>>> http://cr.openjdk.java.net/~thartmann/8150804/webrev.00/
>>>>
>>>> We fail in Compile::Process_OopMap_Node() while processing monitors of a safepoint node because the monitor object is TOP. The crash is rare but reproduces with my regression test. The problem is the elimination of Phi nodes with a unique input which was broken by the fixes for JDK-8139771 [1] and JDK-8146999 [2].
>>>>
>>>> Here are the details (for context, see 'TestPhiElimination.java'):
>>>> A::get() is inlined into test(obj) producing the following graph:
>>>>
>>>>           Parm (obj)
>>>>        TestPhiElimination
>>>>               |
>>>>             CastPP
>>>>    TestPhiElimination:NotNull
>>>>               |
>>>>          CheckCastPP
>>>>           A:NotNull
>>>>           /       \
>>>> CheckCastPP     |
>>>>    A:NotNull      |
>>>>            \     /
>>>>              Phi
>>>>               A
>>>>               |
>>>>           Safepoint
>>>>
>>>> https://bugs.openjdk.java.net/secure/attachment/57820/before_ideal.png
>>>>
>>>> PhiNode::ideal() then replaces the Phi by a CheckCastPP because it has a unique input (see PhiNode::unique_input()):
>>>>
>>>>           Parm (obj)
>>>>        TestPhiElimination
>>>>               |
>>>>          CheckCastPP
>>>>               A
>>>>               |
>>>>           Safepoint
>>>>
>>>> https://bugs.openjdk.java.net/secure/attachment/57821/after_ideal.png
>>>>
>>>> We completely lose the NotNull information provided by the CastPP. Therefore, we cannot prove that obj != null when accessing a field of obj and add an uncommon trap. Obj is also used as a monitor (A::get() is synchronized) and set to TOP in the uncommon trap branch. We are never able to prove that the null branch is not reachable and later fail when emitting code in Process_OopMap_Node because the monitor object is still TOP.
>>>>
>>>> Before the fix for JDK-8139771, we had a check to verify that the type of the unique (uncasted) input is "at least as good" as the type of the PhiNode:
>>>>
>>>>    phase->type(uncasted_input)->higher_equal(type()))
>>>>
>>>> http://hg.openjdk.java.net/jdk9/hs-comp/hotspot/rev/9e17d9e4b59f#l4.79
>>>>
>>>> Re-adding this check, fixes the problem. However, I'm concerned that this check is not strong enough. For example, in the case where the type of the PhiNode is Object:
>>>>
>>>>           Parm (obj)
>>>>        TestPhiElimination
>>>>               |
>>>>             CastPP
>>>>    TestPhiElimination:NotNull
>>>>               |
>>>>          CheckCastPP
>>>>           A:NotNull
>>>>           /       \
>>>> CheckCastPP     |
>>>>    A:NotNull      |
>>>>            \     /
>>>>              Phi
>>>>             Object
>>>>
>>>> We would still replace the Phi because TestPhiElimination->higher_equal(Object) and again lose the NotNull information. I therefore added a slightly stronger check that also checks the types in-between. I had to remove the assert that Roland added.
>>>>
>>>> What do you think?
>>>>
>>>> Thanks,
>>>> Tobias
>>>>
>>>> [1] https://bugs.openjdk.java.net/browse/JDK-8139771
>>>> [1] https://bugs.openjdk.java.net/browse/JDK-8146999
>>>>