[16] RFR(S): 8249603: C1: assert(has_error == false) failed: register allocation invalid

[Christian Hagedorn]

Thanks a lot Vladimir!

Best regards,
Christian

On 06.08.20 21:19, Vladimir Kozlov wrote:
> Fix looks good. And very nice description of the issue.
> 
> Thanks,
> Vladimir K
> 
> On 8/6/20 2:34 AM, Christian Hagedorn wrote:
>> Hi
>>
>> Please review the following patch:
>> https://bugs.openjdk.java.net/browse/JDK-8249603
>> http://cr.openjdk.java.net/~chagedorn/8249603/webrev.00/
>>
>> Register allocation fails in C1 in the testcase because two intervals 
>> overlap (they both have the same stack slot assigned). The problem can 
>> be traced back to the optimization to assign the same spill slot to 
>> non-intersecting intervals in 
>> LinearScanWalker::combine_spilled_intervals().
>>
>> In this method, we look at a split parent interval 'cur' and its 
>> register hint interval 'register_hint'. A register hint is present 
>> when the interval represents either the source or the target operand 
>> of a move operation and the register hint the target or source 
>> operand, respectively (the register hint is used to try to assign the 
>> same register to the source and target operand such that we can 
>> completely remove the move operation).
>>
>> If the register hint is set, then we do some additional checks and 
>> make sure that the split parent and the register hint do not 
>> intersect. If all checks pass, the split parent 'cur' gets the same 
>> spill slot as the register hint [1]. This means that both intervals 
>> get the same slot on the stack if they are spilled.
>>
>> The problem now is that we do not consider any split children of the 
>> register hint which all share the same spill slot with the register 
>> hint (their split parent). In the testcase, the split parent 'cur' 
>> does not intersect with the register hint but with one of its split 
>> children. As a result, they both get the same spill slot and are later 
>> indeed both spilled (i.e. both virtual registers/operands are put to 
>> the same stack location at the same time).
>>
>> The fix now additionally checks if the split parent 'cur' does not 
>> intersect any split children of the register hint in 
>> combine_spilled_intervals(). If there is such an intersection, then we 
>> bail out of the optimization.
>>
>> Some standard benchmark testing did not show any regressions.
>>
>> Thank you!
>>
>> Best regards,
>> Christian
>>
>>
>> [1] 
>> http://hg.openjdk.java.net/jdk/jdk/file/7a3522ab48b3/src/hotspot/share/c1/c1_LinearScan.cpp#l5728 
>>