RFR(S): 8247824: CTW: C2 (Shenandoah) compilation fails with SEGV in SBC2Support::pin_and_expand

[Roman Kennke]

The change looks good.

We should consider if it's relevant for jdk14 and add the corresponding
affects-version in the bug.

Thank you!
Roman

On Thu, 2020-06-18 at 14:55 +0200, Roland Westrelin wrote:
> https://bugs.openjdk.java.net/browse/JDK-8247824
> http://cr.openjdk.java.net/~roland/8247824/webrev.00/
> 
> If a barrier is expanded in the outer loop of a strip mined loop
> nest,
> the outer loop head is changed to a new LoopNode so loop strip mining
> verification code doesn't trigger and fail. The crash occurs when
> there's 2 barriers in the outer loop and C2 attempts to transform the
> loop head twice. The second time, loop->_head points to a dead node.
> 
> Roland.