test/hotspot/jtreg/compiler/types/TestPhiElimination.java

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/**
 * @test
 * @bug 8150804
 * @summary Tests elimination of Phi nodes without losing type information.
 *
 * @run main/othervm -XX:-BackgroundCompilation -XX:-UseOnStackReplacement
 *                   compiler.types.TestPhiElimination
 */

package compiler.types;

public class TestPhiElimination {
    /*
       A::get() is inlined into test(obj) producing the following graph:

               Parm (obj)
            TestPhiElimination
                   |
                 CastPP
        TestPhiElimination:NotNull
                   |
              CheckCastPP
               A:NotNull
               /       \
       CheckCastPP     |
        A:NotNull      |
                \     /
                  Phi
                   A
                   |
               Safepoint

       PhiNode::ideal() then replaces the Phi by a CheckCastPP:

               Parm (obj)
            TestPhiElimination
                   |
              CheckCastPP
                   A
                   |
               Safepoint

       losing the :NotNull information. Therefore, we cannot prove that obj != null
       when accessing a field and add an uncommon trap. Since obj is used as monitor, we
       set it to TOP in the uncommon trap branch and later fail in Process_OopMap_Node
       because the monitor object is TOP.
    */
    public Object test(TestPhiElimination obj) {
        if (obj instanceof A) {
            return ((A) obj).get();
        }
        return null;
    }

    static public void main(String[] args) {
        TestPhiElimination t = new TestPhiElimination();

        // Warmup
        B b = new B();
        for (int i = 0; i < 1_000; ++i) {
            t.test(b);
        }

        // Compile
        A a = new A();
        for (int i = 0; i < 20_000; ++i) {
            if (i % 2 == 0) {
                a.f = null;
            }
            t.test(a);
        }
    }

    static class A extends TestPhiElimination {
        public Object f;

        public A create() {
            return new A();
        }

        public synchronized Object get() {
            if (f == null) {
                f = create();
            }
            return f;
        }
    }

    static class B extends A { }
}