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

[Vladimir Kozlov]

It is strange that next subgraph did not collapse - 
CheckCastPPNode::Identity() should remove second CheckCastPP:

 >         CheckCastPP
 >          A:NotNull
 >          /       \
 > CheckCastPP     |
 >   A:NotNull      |
 >           \     /
 >             Phi
 >              A

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);
   }

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
>