[9] RFR (S): 8146478: Node limit exceeded with -XX:AllocateInstancePrefetchLines=1073741823

[Vladimir Kozlov]

Where 4/2 number comes from? Some spec runs used higher number:

-XX:AllocatePrefetchLines=16

http://spec.org/jbb2005/results/res2009q1/jbb2005-20081203-00563.html

I would suggest something like 64 - I never see such number is used.

Also, please, limit AllocatePrefetchStepSize range. It corresponds to 
cache line size. 512 I would say for future proof - with assert that 
check that its setting in vm_Version_<arch>.cpp is in these range.

For the case AllocatePrefetchStyle == 2 number of lines is calculated as:

uint lines = AllocatePrefetchDistance / AllocatePrefetchStepSize;

Since AllocatePrefetchDistance limit is big you can get a lot of nodes 
again. May be also set the limit - 
AllocatePrefetchLines*AllocatePrefetchStepSize 64*32 = 2048.

Thanks,
Vladimir

On 1/26/16 8:43 AM, Zoltán Majó wrote:
> Hi,
>
>
> please review the patch for 8146478.
>
> https://bugs.openjdk.java.net/browse/JDK-8146478
>
> Problem: Setting a high value for AllocateInstancePrefetchLines can
> trigger an assert in the C2 compiler The reasons is that the number of
> live nodes exceeds the maximum node limit. The same problem can happen
> if AllocateInstanceLines is given a high value.
>
> Solution:
> Limit the range for AllocateInstancePrefetchLines/AllocateInstanceLines
> to 8. I picked the value 8 because
> - (1) the maximum possible value for theses flags is 4/2, so having a
> slightly higher value than 4/2 still allows for some experiments;
> - (2) the node_check() in PhaseMacroExpand::expand_macro_nodes() assumes
> that each macro node expansion will generate <75 new nodes. The number
> of nodes generated by expand_allocate_array()/expand_allocate() for 8
> prefetched lines closely fits into that margin (experimentally verified).
>
> In addition, I removed some code that is that is now unnecessary because
> of the range checks we have in place.
>
>
> Webrev:
> http://cr.openjdk.java.net/~zmajo/8146478/webrev.00/
>
> Testing:
> - JPRT: All JTREG hotspot tests, incl. TestOptionsWithRanges.java
>
> Thank you and best regards,
>
>
> Zoltan
>