RFR: 8265938: C2's conditional move optimization does not handle top Phi

[Vladimir Kozlov]

On Mon, 26 Apr 2021 06:46:55 GMT, Tobias Hartmann <thartmann at openjdk.org> wrote:

> C2's CCP optimization sets the type of a PhiNode to TOP because it became dead after loop unrolling. Now since that node is not reachable from the bottom due to an infinite loop at the end of the method, CCP does not enqueue the PhiNode for IGVN. As a result, the dead subgraph is not removed. During loop opts, the conditional move optimization fails on the PhiNode because its type is top. The expected behavior is to simply bail out with "COMPILE SKIPPED: infinite loop (retry at different tier)".
> 
> I suggest to fix this by simply handling a TOP Phi in the conditional move optimization code.
> 
> This issue was reported by John Jiang (johnsjiang at tencent.com).
> 
> Thanks,
> Tobias

The fix is good.

I think you need System.exit(0) (or something else) at the end of test. Otherwise the test runs forever due to presence of infinite loop in an other thread. I tried.
May be @iignatev can suggest graceful solution for timely exit in such case.

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Changes requested by kvn (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/3684