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

[Vladimir Kozlov]

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 are problem in 
05_after_matching.png.

Vladimir K

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/