[9] RFR(S): 8034216: assert(false) failed: infinite loop in PhaseIterGVN::optimize
Albert
albert.noll at oracle.com
Thu Apr 10 05:46:25 UTC 2014
Hi Vladimir,
thanks a lot for looking into this again. I will prepare a patch with
which removes CastPP node (this was Roland's idea). I will also run
SpecJVM98 to see if there
is an impact on performance.
Best,
Albert
On 04/09/2014 09:07 PM, Vladimir Kozlov wrote:
> Hi Albert,
>
> I am continue thinking about this problem.
> Your suggestion to process CastPP node first could be less intrusive
> than other solutions. It will do the same thing as current code if
> such nodes are first on worklist. It would be still nice to continue
> investigate the possibility to keep these nodes in graph after CCP but
> it should not be very high priority (file RFE).
>
> Note, you can't iterate the _worklist the way you do because remove()
> will put last element into current slot so you have to check it again.
> Also using remove(uint i) version will be faster.
>
> Thanks,
> Vladimir
>
> On 4/7/14 7:04 PM, Vladimir Kozlov wrote:
>> Hi Albert,
>>
>> Can you push the cleanup first under a different bug?
>> We may need to continue discussion how to fix the problem but I think
>> cleanup can be pushed now. Please, send a separate webrev for it under
>> new bug id.
>>
>> About the problem.
>>
>> It looks like 6916062 all over again :( See attached mail thread when I
>> did investigation:
>>
>> [217] CheckCastPP . . # java/util/ArrayList:NotNull*
>> [229] CheckCastPP . . #
>> java/util/Collections$UnmodifiableCollection:NotNull*
>> [292] Phi . [217] [229] # java/lang/Object:NotNull*
>> [11637] CastPP . [292] # _type = java/util/Collection* but
>> igvn.type(cast) = java/util/Collection:NotNull*
>> [11638] AddP . [11637][11637] + 8 # igvn.type(addp)
>> java/util/Collection:NotNull+8 *
>> [11639] LoadNKlass . . [11638]
>>
>> and we hit the assert in the 6916062 fix code:
>>
>> http://hg.openjdk.java.net/hsx/hsx19/baseline/rev/9a19ee0490e0
>>
>>
>> Types are converged during CCP phase when all CastPP nodes are still
>> present. Unfortunately, after CastPP nodes are removed, types could
>> degradate after your change:
>>
>> http://cr.openjdk.java.net/~anoll/8034216/webrev.01/
>>
>> In your case cached type for AddP (NotNull) will be replaced by input's
>> type because this is what AddPNode::Value() returns.
>>
>> I afraid this will affect types more than we want.
>>
>> Note, the only information about NotNull is located in the cached type
>> because corresponding CastPP was also removed.
>>
>>
>> Removing CastPP nodes first will replace cached type of AddP to even
>> more general j.l.Object (from Phi node).
>>
>> May be we should investigate if the next performance statement about
>> jvm98 is still true and solve it differently (in PhiNode::Ideal() for
>> example):
>>
>> // I tried to leave the CastPP's in. This makes the graph more
>> accurate in
>> // some sense; we get to keep around the knowledge that an oop is
>> not-null
>> // after some test. Alas, the CastPP's interfere with GVN (some
>> values are
>> // the regular oop, some are the CastPP of the oop, all merge at Phi's
>> which
>> // cannot collapse, etc). This cost us 10% on SpecJVM, even when I
>> removed
>> // some of the more trivial cases in the optimizer. Removing more
>> useless
>> // Phi's started allowing Loads to illegally float above null checks. I
>> gave
>> // up on this approach. CNC 10/20/2000
>>
>> Regards,
>> Vladimir
>>
>> On 4/4/14 8:09 AM, Albert wrote:
>>> Hi Vladimir,
>>>
>>> thanks for looking at this. Please see comments inline:
>>>
>>> On 04/04/2014 04:03 AM, Vladimir Kozlov wrote:
>>>> On 4/3/14 4:14 PM, Vladimir Kozlov wrote:
>>>>> Hi Albert,
>>>>>
>>>>> Thank you for doing this investigation. I hope it was good C2
>>>>> learning exercise :)
>>>>>
>>> Yes, I learned a lot. Many thanks to you and Roland for teaching me.
>>>
>>>>> I prefer the second solution, webrev.01. I don't like that we are
>>>>> loosing more concrete type with first solution.
>>>>
>>>> Actually it would be more correct and complete to have a solution
>>>> which does not put a load node on worklist again when there is no
>>>> progress with type. Even with webrev.01 we loose NotNull information.
>>> I have one general question here: Are types expected to converge at
>>> this
>>> point if everything would work as expected?
>>> Why do we loose NotNull information if types converge to
>>> java/util/Spliterator:NotNull+8 * ?
>>>
>>> I think that your suggestion, not pushing a node on the worklist if we
>>> don't make progress, will result in the type:
>>> java/util/Spliterator for the LoadKlass node - so the NotNull
>>> information would be lost in that case. If you think that
>>> this is the solution we should go for I'll look more into it.
>>>
>>> Here is a new webrev of the cleanup. I included all your suggestion. I
>>> deleted the two lines
>>> 1123 // Move users of node to worklist
>>> 1124 add_users_to_worklist( k );
>>>
>>> accidentally. Thanks for catching this.
>>> http://cr.openjdk.java.net/~anoll/8034216/webrev.03/
>>>
>>>>
>>>> Thanks,
>>>> Vladimir
>>>>
>>>>>
>>>>> It would be nice to detect such situations and bailout compilation in
>>>>> product VM and throw assert in debug VM.
>>>>>
>>>>> The clean up is nice, go for it. Note that you should put new methods
>>>>> under #ifndef PRODUCT instead of using
>>>>> PRODUCT_RETURN because they are called only in NOT_PRODUCT().
>>>>>
>>>>> Why you removed next code?:
>>>>>
>>>>> 1123 // Move users of node to worklist
>>>>> 1124 add_users_to_worklist( k );
>>>>>
>>>>> Next code should be under #ifdef ASSERT because TraceIterativeGVN is
>>>>> develop flag. So you don't need for NOT_PRODUCT()
>>>>> and DEBUG_ONLY() in the code:
>>>>>
>>>>> + if (TraceIterativeGVN && Verbose) {
>>>>> + tty->print(" Pop ");
>>>>> + NOT_PRODUCT(n->dump();)
>>>>> + DEBUG_ONLY(if( (num_processed++ % 100) == 0 )
>>>>> _worklist.print_set();)
>>>>> + }
>>>>>
>>>>> For next I also think is better to use #ifdef ASSERT:
>>>>>
>>>>> + DEBUG_ONLY(n->dump(4);)
>>>>> + DEBUG_ONLY(_worklist.dump();)
>>>>> + assert(false, "infinite loop in PhaseIterGVN::optimize");
>>>>>
>>>>> And #ifndef PRODUCT for next:
>>>>>
>>>>> + NOT_PRODUCT(trace_PhaseIterGVN(n, nn, oldtype));
>>>>> + NOT_PRODUCT(if (nn != n) verify_step((Node*) NULL);) //
>>>>> ignore n, it might be subsumed
>>>>>
>>>>>
>>>>> On 4/3/14 5:40 AM, Albert wrote:
>>>>>> Hi,
>>>>>>
>>>>>> could I get reviews for this patch?
>>>>>>
>>>>>> Bug:
>>>>>> ------
>>>>>> https://bugs.openjdk.java.net/browse/JDK-8034216
>>>>>>
>>>>>>
>>>>>> Problem:
>>>>>> ------------
>>>>>> The endless loop occurs, since there are 2 nodes of type
>>>>>> LoadKlass in
>>>>>> the worklist that keep pushing themselves
>>>>>> onto the worklist. The current implementation can only handle 1 node
>>>>>> that keeps pushing itself on the worklist.
>>>>>> The endless loop occurs in the IGVN pass that comes after CCP phase.
>>>>>>
>>>>>> This piece of code in memnode.cpp is responsible for the endless
>>>>>> loop:
>>>>>>
>>>>>> Node *MemNode::Ideal_common(PhaseGVN *phase, bool can_reshape) {
>>>>>> ...
>>>>>> if (can_reshape && igvn != NULL &&
>>>>>> (igvn->_worklist.member(address) ||
>>>>>> igvn->_worklist.size() > 0 && (t_adr != adr_type())) ) {
>>>>>> // The address's base and type may change when the address is
>>>>>> processed.
>>>>>> // Delay this mem node transformation until the address is
>>>>>> processed.
>>>>>> phase->is_IterGVN()->_worklist.push(this);
>>>>>> return NodeSentinel; // caller will return NULL
>>>>>> }
>>>>>> ...
>>>>>> }
>>>>>> As seen above, we keep pushing the current node onto the worklist if
>>>>>> t_adr != adr_type().
>>>>>> Unfortunately the types do not converge. See the following node
>>>>>> dump:
>>>>>>
>>>>>> 5559 272 b 4 java.util.concurrent.CountedCompleter::exec (6
>>>>>> bytes)
>>>>>> 2054 CheckCastPP === 4086 2023 [[ 2098 2064 2064 ]]
>>>>>> #java/util/Spliterators$DoubleArraySpliterator:NotNull:exact *
>>>>>> Oop:java/util/Spliterators$DoubleArraySpliterator:NotNull:exact *
>>>>>> !jvms:
>>>>>> AbstractTask::compute @ bci:6 CountedCompleter::exec @ bci:-1
>>>>>> 2044 CheckCastPP === 4090 2023 [[ 2098 2077 2077 ]]
>>>>>> #java/util/ArrayList$ArrayListSpliterator:NotNull:exact *
>>>>>> Oop:java/util/ArrayList$ArrayListSpliterator:NotNull:exact * !jvms:
>>>>>> AbstractTask::compute @ bci:6 CountedCompleter::exec @ bci:-1
>>>>>> 2095 Region === 2095 2085 2072 [[ 2095 2120 2096 2097 2098 2099 ]]
>>>>>> !jvms: AbstractTask::compute @ bci:6 CountedCompleter::exec @ bci:-1
>>>>>> 2169 CmpL === _ 2099 2164 [[ 2170 ]] !jvms:
>>>>>> AbstractTask::compute @
>>>>>> bci:29 CountedCompleter::exec @ bci:-1
>>>>>> 2121 IfTrue === 2120 [[ 2125 ]] #1 !jvms:
>>>>>> AbstractTask::getTargetSize
>>>>>> @ bci:8 AbstractTask::compute @ bci:14 CountedCompleter::exec @
>>>>>> bci:-1
>>>>>> 2122 IfFalse === 2120 [[ 2125 2162 ]] #0 !jvms:
>>>>>> AbstractTask::getTargetSize @ bci:8 AbstractTask::compute @ bci:14
>>>>>> CountedCompleter::exec @ bci:-1
>>>>>> 2098 Phi === 2095 2044 2054 [[ 2229 ]] #java/lang/Object:NotNull *
>>>>>> Oop:java/lang/Object:NotNull * !orig=2181 !jvms:
>>>>>> AbstractTask::compute @
>>>>>> bci:6 CountedCompleter::exec @ bci:-1
>>>>>> 2170 Bool === _ 2169 [[ 2171 ]] [le] !jvms:
>>>>>> AbstractTask::compute @
>>>>>> bci:29 CountedCompleter::exec @ bci:-1
>>>>>> 2125 Region === 2125 2122 2121 [[ 2125 2171 2163 2164 ]] !jvms:
>>>>>> AbstractTask::getTargetSize @ bci:24 AbstractTask::compute @ bci:14
>>>>>> CountedCompleter::exec @ bci:-1
>>>>>> 22 ConL === 0 [[ 24 2037 81 2230 288 ]] #long:8
>>>>>> 2229 CastPP === 2173 2098 [[ 2240 2240 2230 2230 2237 ]]
>>>>>> #java/util/Spliterator * Interface:java/util/Spliterator * !jvms:
>>>>>> AbstractTask::compute @ bci:33 CountedCompleter::exec @ bci:-1
>>>>>> 3 Start === 3 0 [[ 3 5 6 7 8 9 10 ]] #{0:control, 1:abIO,
>>>>>> 2:memory,
>>>>>> 3:rawptr:BotPTR, 4:return_address,
>>>>>> 5:java/util/concurrent/CountedCompleter:NotNull *}
>>>>>> 2171 If === 2125 2170 [[ 2172 2173 ]] P=0.500000, C=-1.000000
>>>>>> !jvms:
>>>>>> AbstractTask::compute @ bci:29 CountedCompleter::exec @ bci:-1
>>>>>> 2230 AddP === _ 2229 2229 22 [[ 2231 ]]
>>>>>> Interface:java/util/Spliterator+8 * !jvms: AbstractTask::compute @
>>>>>> bci:33 CountedCompleter::exec @ bci:-1
>>>>>> 7 Parm === 3 [[ 103 289 110 123 25 82 2231 2077 47 2064 2057
>>>>>> 2038 65
>>>>>> 2031 75 2023 ]] Memory Memory: @BotPTR *+bot, idx=Bot; !jvms:
>>>>>> CountedCompleter::exec @ bci:-1
>>>>>> 2173 IfFalse === 2171 [[ 4079 2231 2229 ]] #0 !orig=[2195] !jvms:
>>>>>> AbstractTask::compute @ bci:29 CountedCompleter::exec @ bci:-1
>>>>>> 2231 LoadKlass === 2173 7 2230 [[ 2232 ]] @java/lang/Object+8 *,
>>>>>> idx=4; #klass java/util/Spliterator: 0x0000000002252c98 *
>>>>>> Interface:klass java/util/Spliterator: 0x0000000002252c98 * !jvms
>>>>>>
>>>>>> t_adr: is the cached type of the Address input (2230 AddP):
>>>>>> java/util/Spliterator:NotNull
>>>>>> adr_type(): is the bottom type of the address input (2230 AddP):
>>>>>> java/util/Spliterator
>>>>>>
>>>>>> The bottom_type of the AddP node is determined by the bottom type of
>>>>>> it's address input node ( 2229 CastPP).
>>>>>> The bottom type of the CastPP node is set in the CCP phase:
>>>>>>
>>>>>> Node *PhaseCCP::transform_once( Node *n ) {
>>>>>> ...
>>>>>> Node *nn = n->Ideal_DU_postCCP(this);
>>>>>> ...
>>>>>> }
>>>>>>
>>>>>> In the concrete execution, the CCP phase sets the bottom type of the
>>>>>> castPP node equal to the type of its input (in(1)).
>>>>>> in(1) of the castPP node is a phi-node. Since the two types are
>>>>>> equal,
>>>>>> the castPP node is redundant and could be removed
>>>>>> in the subsequent IGVN phase. However, the castPP node is not
>>>>>> removed,
>>>>>> since another node in the worklist (a redundant
>>>>>> region node) is processed prior to the castPP node. Removing the
>>>>>> redundant region node results in a change of in(1) of the
>>>>>> castPP node. As a result, the type of in(1) of the castPP node
>>>>>> changes
>>>>>> as well and the castPP node is not removed. Not
>>>>>> removing the castPP node makes the types not converge as expected.
>>>>>>
>>>>>>
>>>>>> Solution:
>>>>>> -----------
>>>>>> Add a pass after CCP that removes the redundant castPP node. As a
>>>>>> result, the types converge to
>>>>>> java/lang/Object:NotNull+8 * and the endless loop does not occur.
>>>>>>
>>>>>> Here is the webrev:
>>>>>> http://cr.openjdk.java.net/~anoll/8034216/webrev.00/
>>>>>>
>>>>>>
>>>>>> Alternative solution:
>>>>>> --------------------------
>>>>>> Instead of adding the extra pass over the IR, we could raise the
>>>>>> bottom
>>>>>> type of a type node
>>>>>> in IGVN. As a result, the types converge to
>>>>>> java/util/Spliterator:NotNull+8 * and the endless loop
>>>>>> does not occur.
>>>>>>
>>>>>> Here is the webrev:
>>>>>> http://cr.openjdk.java.net/~anoll/8034216/webrev.01/
>>>>>>
>>>>>>
>>>>>> Cleanup:
>>>>>> ------------
>>>>>> In addition, I have worked on refactoring/cleaning up the affected
>>>>>> code.
>>>>>> This code cleanup comes
>>>>>> in a separate webrev to make it easier to figure out the actual
>>>>>> changes
>>>>>> that fix the bug. The code
>>>>>> cleanup is optional.
>>>>>>
>>>>>> Here is the webrev:
>>>>>> http://cr.openjdk.java.net/~anoll/8034216/webrev.02/
>>>>>>
>>>>>>
>>>>>> Testing:
>>>>>> ----------
>>>>>> Both solutions successfully passed JPRT and CTW (rt.jar).
>>>>>>
>>>>>> Many thanks in advance,
>>>>>> Albert
>>>
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