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Inlining “objects” in the JVM

[EDIT: It has been pointed out that the correct term for what I am talking about is Scalar Replacement. Also it seems that Java 7 already has some code to support it in the JIT compiler. Thanks.]

One of the interesting features of Scala is the ability to use implicit conversions to “add” methods to existing classes. A call to “x.func” will try to convert x to a type with a method func if it doesn’t naturally have one. The problem is that each time this happens it ends up allocating a temporary wrapper object. This is similar to Java idioms like:

long time = new Date().getTime();

In both cases an object is allocated and then is immediately ready to be garbage collected. Whenever I think about this I feel that the JVM could optimize it away. Maybe something like this.

The JVM discovers (during escape analysis, which I think it already does) that the object is short lived and local. It then inlines the small number of method calls (often just one, especially in the case of scala implicit conversions). When it sees that every method call on the object is inlined it inlines the constructor and converts any fields of the class into local variable and removes the allocation entirely.

This wouldn’t be easy. But it doesn’t seem like it would be too incredibly hard. Though I’m not a JVM programmer so I don’t really know.

Maybe I should give it a shot. I’ve never hacked on Hotspot and I’ve always kinda wanted to.

Any thoughts on this?

Posted in Java, JVM, Scala

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