RFR: 8274145: C2: Incorrect computation after JDK-8269752

[Vladimir Kozlov]

On Mon, 27 Sep 2021 08:57:25 GMT, Roland Westrelin <roland at openjdk.org> wrote:

> The bug happens because an If node that follows a CountedLoop is
> replaced by the CountedLoopEnd node of the main loop. Further
> unrolling happens after the If is replaced which causes the condition
> of the CountedLoopEnd node to change. This is made possible by
> JDK-8269752. The fix I propose is to detect that corner case and
> prevent the If to be replaced in that case.

src/hotspot/share/opto/loopopts.cpp line 1396:

> 1394:       while (dom != cutoff) {
> 1395:         if (dom->req() > 1 && dom->in(1) == bol && prevdom->in(0) == dom &&
> 1396:             safe_for_if_replacement(dom)) {

We skip replacement for counted loop but we still continue searching IDOMs. Which may find another dominating test before loop and replace node's control which may be not correct since we skipped loop with the same test.
And you said "The fix I propose is to detect that corner case and
prevent the If to be replaced in that case." We should bailout then.

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PR: https://git.openjdk.java.net/jdk/pull/5712