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

[Vladimir Kozlov]

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