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  1. JDK
  2. JDK-8131782

C1 Class.cast optimization breaks when Class is loaded from static final

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      b77

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        Description

        This is observed in VarHandles work for JDK-8130923.

        The reason is that C1 canonicalizer seems to assume a particular constant shape (InstanceConstant), while the Class instances coming through @Stable/static-final seem to be ObjectConstants. Accepting ObjectConstants as well fixes it:
          http://cr.openjdk.java.net/~shade/8131782/webrev.01/

        Simple benchmark articulates it clearly:
          http://cr.openjdk.java.net/~shade/8131782/ClassCastConstants.java

        (Runnable JAR: http://cr.openjdk.java.net/~shade/8131782/benchmarks.jar)

        Baseline result calls into Runtime::isInstance in "static final" case, experiencing the 2x hit:
          http://cr.openjdk.java.net/~shade/8131782/baseline.perfasm

        Patched VM behaves perfectly well in both cases:
          http://cr.openjdk.java.net/~shade/8131782/patched.perfasm

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                Assignee:
                shade Aleksey Shipilev
                Reporter:
                shade Aleksey Shipilev
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