why this java code can not use escape analysis?

Question

my code is here =>

class PointHolder {
    private Point point;

    public PointHolder(Point point) {
        this.point = point;
    }
    //getter
}
class Point {
    private int x;
    private int y;

    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }
    //getter setter
}

public class Escape {
    public static void main(String[] args) {
        for (int i = 0; i < 20000; i++) {
            test4(i);
        }
    }
    static int foo;
    public static void test4(int x) {
        PointHolder pointHolder = new PointHolder(new Point(x + 2, 42));
        foo = pointHolder.getPoint().getX();
    }
}

use the jitwatch, i could see the pointHolder is not been allocated, but new Point(x+2, 42) is still been allocated.

i could not figure out why

can you show us the output. I am not fully understanding your issue here — roottraveller, Nov 03 '19 at 08:01
The posted code would not compile. `foo` is assigned a value but never declared, and a `getX()` method on the `Point` class is referenced but never defined. Please post your most current, up-to-date code if you expect us to help you. — mario_sunny, Nov 03 '19 at 08:06
@roottraveller hi，i upload the pic. The var pointHolder's allocation has been eliminated, but allocation of `new Point(x + 2,42)` not been eliminated. — lovezhy, Nov 03 '19 at 08:07
What do you mean "not been eliminated" - you mean not deallocated? — mario_sunny, Nov 03 '19 at 08:08
@mario_sunny according to the `escape analysis`, the scope of `new Point()` is in the test4 method, so the allocation of `new Point()` should be eliminated. — lovezhy, Nov 03 '19 at 08:11
Escape analysis is done as part of the JIT compilation, not java bytecode compilation. — Johannes Kuhn, Nov 03 '19 at 08:20
@JohannesKuhn yes，the pic is the jitwatch result after jit compilation. — lovezhy, Nov 03 '19 at 08:22

score 1 · Accepted Answer · answered Nov 04 '19 at 17:14

Escape Analysis in HotSpot C2 compiler is rather simple. It never attempted to detect all possible objects that do not escape the compilation scope. In particular, it does not currently handle a multi-level reference tree.

In your example, a reference to a newly allocated Point is assigned to an object field: PointHolder.point. JIT treats this assignment as an escape of Point object and thus does not eliminate the allocation.

One particular exception is boxing-unboxing: HotSpot handles boxing-unboxing methods specially. E.g. it would be still able to eliminate allocation of Integer object when it is assigned to an IntegerHolder field.

This problem is not something unsolvable though - it's rather a missed optimization opportunity. Graal JIT is better in this sence - in the given example it does eliminate both Point and PointHolder allocations.

why this java code can not use escape analysis?

1 Answers1