[9] RFR(S): 8034216: assert(false) failed: infinite loop in PhaseIterGVN::optimize
Vladimir Kozlov
vladimir.kozlov at oracle.com
Fri Apr 4 02:03:48 UTC 2014
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 :)
>
> 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.
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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