[15] RFR(S): 8238438: SuperWord::co_locate_pack picks memory state of first instead of last load

[Tobias Hartmann]

+1

Best regards,
Tobias

On 28.02.20 19:21, Vladimir Kozlov wrote:
> Looks good.
> 
> Thanks,
> Vladimir
> 
> On 2/28/20 1:21 AM, Christian Hagedorn wrote:
>> As discussed with Vladimir K. and Tobias, I added an assert(false) in the code path that picks the
>> first memory state together with some node dumping:
>> http://cr.openjdk.java.net/~chagedorn/8238438/webrev.01/
>>
>> I suggest to file a follow-up RFE to either investigate further and/or to clean this code up
>> later, for example in JDK 16, if we have not seen this assert being hit. If it's hit at some point
>> and it turns out to be valid to use the first memory state, we can still remove the assert again
>> and leave the code about selecting the first memory state.
>>
>> Best regards,
>> Christian
>>
>> On 20.02.20 09:26, Christian Hagedorn wrote:
>>> Hi Vladimir
>>>
>>> Thank you for your review!
>>>
>>> On 20.02.20 03:04, Vladimir Kozlov wrote:
>>>> May be add -XX:+IgnoreUnrecognizedVMOptions flag to test command because LoopMaxUnroll is C2 flag.
>>>> Or use @requires vm.compiler2.enabled
>>>
>>> Fixed in place.
>>>
>>>> I assume you tested this fix with JDK-8233032 test case to make sure that bug did not return.
>>>
>>> Yes, I tested tier1-4 and as part of trying to figure out if the code path is dead on line 2289,
>>> as mentioned in Tobias' review, up to tier7 (was never reached):
>>>
>>> 2288       if (!independent(current, ld)) {
>>> 2289         return first_mem; // A later store depends on this load, pick memory state of first
>>> load
>>> 2290       }
>>>
>>> Best regards,
>>> Christian
>>>
>>>> On 2/17/20 8:32 AM, Christian Hagedorn wrote:
>>>>> Hi
>>>>>
>>>>> Please review the following patch:
>>>>> https://bugs.openjdk.java.net/browse/JDK-8238438
>>>>> http://cr.openjdk.java.net/~chagedorn/8238438/webrev.00/
>>>>>
>>>>> When processing a load pack in SuperWord::co_locate_pack(), we pick by default the memory state
>>>>> of the last load. But if we find a store that is dependent on an earlier load in the pack, then
>>>>> we need to pick the memory state of the first load.
>>>>>
>>>>> The current code, however, checks for each load 'l' if it has a dependency on an earlier store
>>>>> instead of checking if a later store is dependent on 'l' (regression since [1]). At [2], we
>>>>> start at the memory state of the current load and walk the memory graph to the memory state of
>>>>> the first load while checking dependency constraints. This wrongly checks for store->load
>>>>> instead of load->store dependencies. For example, in the test case [3], the load pack consists
>>>>> of LoadI 656, 652,... with its memory states 657 StoreI (first_mem) and 653 StoreI,
>>>>> respectively. When processing the second load in the pack, 652 LoadI, we go up the memory graph
>>>>> to first_mem (657 StoreI). The dependence graph now tells us that 652 LoadI is dependent on 657
>>>>> StoreI. From that we make the wrong conclusion that we need to pick the memory state of the
>>>>> first load which results in a wrong execution for that test case.
>>>>>
>>>>> The fix now first finds the last memory state. Afterwards, it walks the memory graph from the
>>>>> last memory state to the memory state of each load in the pack (but not beyond) while checking
>>>>> for any load->store dependency constraint (as done before [1]).
>>>>>
>>>>> Thank you!
>>>>>
>>>>> Best regards,
>>>>> Christian
>>>>>
>>>>>
>>>>> [1] https://bugs.openjdk.java.net/browse/JDK-8233032,
>>>>> https://hg.openjdk.java.net/jdk/jdk/rev/3b693618d084
>>>>> [2]
>>>>> http://hg.openjdk.java.net/jdk/jdk/file/27e87c000b16/src/hotspot/share/opto/superword.cpp#l2306
>>>>> [3] Part of the IR of the test case:
>>>>> https://bugs.openjdk.java.net/secure/attachment/86850/load_pack_memory.png