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

[Tobias Hartmann]

Hi Christian,

I agree with Vladimir, very nice analysis. Although I'm not too familiar with the C1 register
allocator, your explanation and fix makes sense to me.

Just wondering, do we hit this case with any of our existing tests?

Best regards,
Tobias

On 06.08.20 11:34, 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