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  1. JDK
  2. JDK-8238765

PhaseCFG::schedule_pinned_nodes cannot handle precedence edges from unmatched CFG nodes correctly

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        The internal testing for JDK 14 revealed a case where a precedence edge from a not yet matched CFG node (i.e. a node in the middle of a block) was processed by PhaseCFG::schedule_pinned_nodes. As a result, the is_dominator() check returned a wrong answer which later resulted in a segfault due to a wrong block ordering produced by GCM together with StressGCM. In this case, a load was moved before the associated NULL check. This, however, was only observed by using ZGC but it appears to be unrelated to ZGC itself. With ZGC, an ArrayAllocation expands before a LoadP which is used for a LoadRange node later in such a way that PhaseCFG::schedule_pinned_nodes faces a precedence edge for that LoadRange node from a CFG node that is not yet machted (i.e. in the middle of a block).

        The internal test used -XX:+UseZGC and -XX:+StressGCM.

          1. dominator_check.png
            dominator_check.png
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          2. wrong_control_dependencies.png
            wrong_control_dependencies.png
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              chagedorn Christian Hagedorn
              chagedorn Christian Hagedorn
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