RFC: C2: Anti-dependence on a load with a control in presence of a membar

Tue Mar 6 20:20:05 UTC 2018

I think we should remove control edge for Loads from *non-escaping* 
instances. Instance' pointer is not NULL and class is exact. And, as I 
said, such Loads can skip membars since their instance is not escaping.

It is not exception - we have other Load nodes without control edge.

Vladimir K

On 3/6/18 12:13 PM, Vladimir Ivanov wrote:
> 
> 
> On 3/6/18 10:26 PM, Vladimir Kozlov wrote:
>> On 3/6/18 11:21 AM, Vladimir Kozlov wrote:
>>> This changes everything. Load is associated with non-global-escaping 
>>> allocation #311 (iid is assigned only in such cases). It is allowed 
>>> its memory edge change in such way.
>>>
>>> Why GCM makes unschedulable graph? I don't see a problem in 
>>> 05_after_matching.png.
>>
>> Is it because Load's memory (#173) is above membar (#317) but the Load 
>> below because of control?
> 
> Exactly. Anti-dependences are added from membar (#317) to the loads 
> (#380/...) and it makes the graph unschedulable in LCM.
> 
> Best regards,
> Vladimir Ivanov
> 
>>> On 3/6/18 10:51 AM, Vladimir Ivanov wrote:
>>>>
>>>>> There were several bugs before when we had trouble with loads which 
>>>>> have control edge. As I remember we only require RAW loads to have 
>>>>> such edges. Meaning Load nodes should have only dependency on 
>>>>> memory state. Of cause, there could be exclusions.
>>>>>
>>>>> Originally EA can skip all membars for instance's load because it 
>>>>> assumes that it will end-up in Store node into allocated object 
>>>>> which should *follow* instance's allocation. And it can skip 
>>>>> membars (which follow allocation) because nobody see non-escaping 
>>>>> allocation.
>>>>>
>>>>> Load (#391) is not instance load from instance array (#363). It is 
>>>>> load from source Arraycopy (#255) (it is not allocation). So it 
>>>>> should not have bypass membars separating them:
>>>>>
>>>>> http://hg.openjdk.java.net/jdk/hs/file/4e82736053ae/src/hotspot/share/opto/escape.cpp#l2698 
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> Updated IR dump during before/after split_unique_types with wider 
>>>> context (and, unfortunately, different node ids):
>>>>
>>>> http://cr.openjdk.java.net/~vlivanov/misc/antidep/02_ea_split_unique_types_01.png 
>>>>
>>>>
>>>> One detail is missing in the original description: there's another 
>>>> AllocateArray (#311) dominating the ArrayCopy (#389) and loads 
>>>> access it directly.
>>>>
>>>> ArrayCopy uses #311 as destination, so ArrayCopyNode::may_modify() 
>>>> returns true and stops further analysis:
>>>>
>>>>
>>>> http://hg.openjdk.java.net/jdk/hs/file/edb65305d3ac/src/hotspot/share/opto/escape.cpp#l2705 
>>>>
>>>>
>>>>> So it is really some problem in step 2) in EA. Could be because 
>>>>> only one alias index (memory slice) is used for whole array access.
>>>>
>>>> Unlikely, since I don't see any interference between accesses to 
>>>> different elements during split_unique_types().
>>>>
>>>>> So what memory slice of Merge node (#379) was updated to bypass 
>>>>> membar?
>>>>
>>>> It updates instance memory slice corresponding to:
>>>>    bool[int:8]:NotNull:exact+any *,iid=311
>>>>
>>>> Best regards,
>>>> Vladimir Ivanov
>>>>
>>>>
>>>>> On 3/2/18 6:47 AM, Vladimir Ivanov wrote:
>>>>>> Hi,
>>>>>>
>>>>>> I'm seeing unschedulable graph being produced during GCM when 
>>>>>> adding anti-dependence to a load node with a control dependency. I 
>>>>>> found the root cause, but can't decide how to fix it.
>>>>>>
>>>>>> Here are steps which lead to the broken graph:
>>>>>>
>>>>>>   (1) The load causing problems (#391) is added as part of 
>>>>>> specializing ArrayCopy for small arrays (added as part of 
>>>>>> JDK-6912521 [1] in 9). Both control & memory are tied to 
>>>>>> AllocateArray. (IR [2])
>>>>>>
>>>>>>   (2) EA proves that AllocateArray (#363, destination) is scalar 
>>>>>> replaceable and during split_unique_types() updates corresponding 
>>>>>> MemoryMerge (#379) and it allows to directly use memory produced 
>>>>>> by ArrayCopy (#255, source) bypassing the allocation & membar 
>>>>>> (#348). (IR [3])
>>>>>>
>>>>>>   (3) After allocation elimination, the load control dependency is 
>>>>>> switched to MemBarCPUOrder (#348) which was immediate dominator of 
>>>>>> eliminated allocation (IR [4])
>>>>>>
>>>>>>   (4) After matching the load has control on the membar, but not 
>>>>>> memory (IR before [5] and after [6] matching.)
>>>>>>
>>>>>>   (5) During GCM, anti-dependence from membar (#317) to the load 
>>>>>> is added, but it makes the graph unschedulable which then triggers 
>>>>>> the assertion [7] during LCM.
>>>>>>
>>>>>> Relevant places in the code: [8]
>>>>>>
>>>>>> Everything looks fine, except updates of MergeMems in step #2:
>>>>>>
>>>>>>    * the load is pinned to the proper branch after deciding what 
>>>>>> direction to go;
>>>>>>
>>>>>>    * wide membars do need anti-dependences on loads
>>>>>>
>>>>>> So, as a fix I'd disable memory edge updates which bypass any 
>>>>>> membars. Does it sound reasonable or am I missing something 
>>>>>> important?
>>>>>>
>>>>>> Best regards,
>>>>>> Vladimir Ivanov
>>>>>>
>>>>>> [1] https://bugs.openjdk.java.net/browse/JDK-6912521
>>>>>>
>>>>>> [2] http://cr.openjdk.java.net/~vlivanov/misc/antidep/01_initial.png
>>>>>>
>>>>>> [3] 
>>>>>> http://cr.openjdk.java.net/~vlivanov/misc/antidep/02_ea_split_unique_types.png 
>>>>>>
>>>>>>
>>>>>> [4] 
>>>>>> http://cr.openjdk.java.net/~vlivanov/misc/antidep/03_after_alloc_elimination.png 
>>>>>>
>>>>>>
>>>>>> [5] 
>>>>>> http://cr.openjdk.java.net/~vlivanov/misc/antidep/04_before_matching.png 
>>>>>>
>>>>>>
>>>>>> [6] 
>>>>>> http://cr.openjdk.java.net/~vlivanov/misc/antidep/05_after_matching.png 
>>>>>>
>>>>>>
>>>>>> [7]
>>>>>> #  Internal Error 
>>>>>> (/Users/vlivanov/ws/jdk/panama-dev/open/src/hotspot/share/opto/lcm.cpp:1169), 
>>>>>> pid=90414, tid=14851
>>>>>> #  assert(false) failed: graph should be schedulable
>>>>>>
>>>>>>
>>>>>> [8] http://cr.openjdk.java.net/~vlivanov/misc/antidep/webrev/