[9] RFR (S): 8164954: split_if creates empty phi and region nodes

[Nils Eliasson]

Thank you Vladimir and Vladimir!

Best regards,

Nils


On 2017-03-06 15:58, Vladimir Ivanov wrote:
> Looks good.
>
> Best regards,
> Vladimir Ivanov
>
> On 3/6/17 5:15 PM, Nils Eliasson wrote:
>> Hi,
>>
>> Please review.
>>
>> The test runtime/Metaspace/FragmentMetaspace.java triggered an assert
>> (assert(i < _max) failed: oob: i=1, _max=1) when Node::dominates reads
>> in[] out of bounds on a empty region node.
>>
>> The empty region node is created by split_if. Split_if tries to simplify
>> compares on a constant and a phi, where the phi has at least one
>> constant on an in edge. In this case the opportunity for optimization is
>> identified, and a phi and accompanying region node is created for all
>> remaining in-edges that aren't matching the constant. But here all the
>> in-edges to the phi matches the constant, and no edge will be split out,
>> leaving the new nodes without in-edges. Much later in the optimization
>> phase the empty region node will trigger the assert.
>>
>> Solution:
>> Abort split_if when all in-edges are the same constant - this is already
>> the desired state. It will be folded later.
>>
>> https://bugs.openjdk.java.net/browse/JDK-8164954
>> http://cr.openjdk.java.net/~neliasso/8164954/webrev/
>>
>> A big thank you to Richard Bäckman that helped me track this bug down.
>>
>> Regards,
>> Nils