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

[Vladimir Ivanov]

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