RFR(S): 8192992: Test8007294.java failed: attempted to spill a non-spillable item

[Nils Eliasson]

Thanks Tobias,

Take a look at version 3, it has the comments fixed.

I can add that I am running precheckin testing.

Best regards,
Nils

On 2018-03-14 12:06, Tobias Hartmann wrote:
> Hi Nils,
>
> very nice work! I also like the new debugging points.
>
> Small typo in gcm.cpp:711: "actully" -> "actually", "skill" -> "skip"?
>
> Best regards,
> Tobias
>
>
> On 14.03.2018 11:17, Nils Eliasson wrote:
>> Hi Vladimir,
>>
>> After taking a very close look I found that the anti-dependency checking that hoists the
>> testN_mem_reg from the jmpCon is broken, and that the hoisting is unnecessary. So this is not a case
>> where we need anti-depenency checking for loads before matching.
>>
>> Generally the insert_anti_dependences looks good, except the store->is_Phi() clause that is full of
>> holes (overly conservative). I don't think I fully understand how the graph looks when the clause is
>> needed, but it tries to find stores upwards that is otherwise unreachable from the downward memory
>> flow search.
>>
>> I found these three flaws:
>>
>> 1) A Phi in a block that is preceded by a store - even though the store is dominated by the loads
>> LCA it will force the testN up! We don't check where the stores are located.
>>
>> 2) A Phi that consumes the same memory as the load may force it up, even though no stores are involved.
>>
>> 3) A Phi that consumes a mergemem, which in itself has already has been processed and passed as
>> irrelevant, may force the testN up.
>>
>> One could add that any predecessor to the phi would have to be a store/call to affect the load
>> placement.
>>
>> I have also added some additional debugging printouts to the patch.
>>
>> http://cr.openjdk.java.net/~neliasso/8192992/webrev.03/
>> https://bugs.openjdk.java.net/browse/JDK-8192992
>>
>> Regards,
>>
>> // Nils
>>
>>
>>
>> On 2018-03-05 18:02, Vladimir Kozlov wrote:
>>> Hi Nils,
>>>
>>> Yes, it is legal workaround but this way you removed all subsuming loads in code.
>>>
>>> Should we do anti-dependency check for loads during matching when shared nodes are marked?:
>>>
>>> http://hg.openjdk.java.net/jdk/hs/file/4e82736053ae/src/hotspot/share/opto/matcher.cpp#l2159
>>>
>>> How expensive would be that?
>>>
>>> Vladimir
>>>
>>> On 3/5/18 7:50 AM, Nils Eliasson wrote:
>>>> Hi,
>>>>
>>>> This patch is a workaround for a scheduling problem encountered in some rare circumstances.
>>>> Instead of hitting the assert we retry the compilation without subsuming loads.
>>>>
>>>> To quote Tobias:
>>>>
>>>> "The crash happens because a testN_mem_reg0 (CmpN(LoadN(mem), NULL)) is scheduled in a different
>>>> block than its jmpCon user and the register allocator tries to spill the flag register. The
>>>> problem is that PhaseCFG::schedule_late() detects an anti-dependency for the testN_mem_reg0 on a
>>>> bottom memory Phi and therefore raises the LCA to the early block (see
>>>> PhaseCFG::insert_anti_dependences()) which is "far away" from its jmpCon user. "
>>>>
>>>> Thanks to Roland who suggested the workaround.
>>>>
>>>> https://bugs.openjdk.java.net/browse/JDK-8192992
>>>>
>>>> http://cr.openjdk.java.net/~neliasso/8192992/webrev.01/
>>>>
>>>> Regards,
>>>>
>>>> Nils
>>>>