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

Tue Mar 6 20:23:33 UTC 2018

What would happen if it is volatile Load?

Vladimir

On 3/6/18 12:20 PM, Vladimir Kozlov wrote:
> 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/