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

[Vladimir Kozlov]

Very good! Thank you for doing this analysis.
Please, run our usual mach5 set of tests.

Thanks,
Vladimir

On 3/14/18 3:17 AM, 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
>>>
>