test/hotspot/jtreg/compiler/loopopts/TestSplitIfNewIrreducibleLoop.java

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/*
 * @test
 * @bug 8348572
 * @summary Tests where new irreducible loop is introduced by split_if.
 *
 * @run driver compiler.loopopts.TestSplitIfNewIrreducibleLoop
 *
 * @run main/othervm -Xcomp -XX:PerMethodTrapLimit=0
 *                   -XX:CompileCommand=compileonly,compiler.loopopts.TestSplitIfNewIrreducibleLoop::test
 *                   compiler.loopopts.TestSplitIfNewIrreducibleLoop
 */

package compiler.loopopts;

public class TestSplitIfNewIrreducibleLoop {

    static class A {
        public A parent;
    }

    static class B extends A {}

    public static void main(String[] args) {
        // Instantiate one each: classes are loaded.
        A a = new A();
        B b = new B();
        test(b);
    }

    static int test(A parent) {
        do {
            if (parent instanceof B b) { return 1; }
            if (parent != null) { parent = parent.parent; }
            if (parent == null) { return 0; }
        } while (true);
    }

    // Before split_if it looks like this (the instanceof check has already been partial peeled):
    //
    // if (parent instanceof B b) { return 1; }
    // do {
    //     if (parent != null) { parent = parent.parent; }
    //     if (parent == null) { return 0; }
    //     if (parent instanceof B b) { return 1; }
    // } while (true);
    //
    //
    // Now, we want to split_if the first if in the loop body, like this:
    //
    // if (parent instanceof B b) { return 1; }
    // if (parent != null) { goto LOOP2; } else { goto LOOP1; }
    // do {
    //     :LOOP1
    //     parent = parent.parent;
    //     :LOOP2
    //     if (parent == null) { return 0; }
    //     if (parent instanceof B b) { return 1; }
    //     if (parent != null) { goto LOOP2; } else { goto LOOP1; }
    // } while (true);
    //
    // As the comment in ifnode.cpp / split_if says: we know that on the backedge
    // "parent" cannot be null, and so we would be able to split the if through
    // the region, and on the backedge it would constant fold away, and we would
    // only have to check the split if in the loop entry.
    //
    // Problem: we have introduced an irreducible loop!
}