Compiler optimization of method calls

Question

Does the Java compiler apply some transformations to optimize method calls? Or does it always generate a faithful representation of the source with simple optimizations like removing dead code?

Specifically if we have the below example:

public static void main(String[] args) {
    System.out.println(foo());
    System.out.println(foo());
    System.out.println(foo());
    System.out.println(bar());
    System.out.println(bar());
    System.out.println(bar());
}

public static int foo() {
    int[] arr = {1, 2, 3, 4};
    return arr[0];
}

public static int bar() {
    return 10;
}

Does the compiler attempt to replace the call to bar (or even possibly foo) with a call to print the integer 10?

public static void main(String[] args) {
    System.out.println(1);
    System.out.println(1);
    System.out.println(1);
    System.out.println(10);
    System.out.println(10);
    System.out.println(10);
}

I know of a method called inlining that can lead to such transformations, but I was wondering if the Java compiler (at least the javac of the Oracle JDK) applies it, or if it's always deferred to the JVM.

The answer is 'it depends'. Java has a [JIT](http://en.wikipedia.org/wiki/Just-in-time_compilation), as well as doing some manner of optimizations when turning the source into byte-code. So basically, as long as it doesn't change the observable behavior of the code, Java can really do any kind of optimization at any time. — aruisdante, Oct 22 '14 at 20:06
See: http://stackoverflow.com/questions/2096361/are-there-inline-functions-in-java — aruisdante, Oct 22 '14 at 20:08
And: http://stackoverflow.com/questions/8256202/how-does-the-java-jit-compiler-optimize-my-code — aruisdante, Oct 22 '14 at 20:08

score 1 · Accepted Answer · answered Oct 22 '14 at 20:11

This type of optimization is called constant folding.

These (and other optimizations) that do not affect the observable result of programs are allowed by Java's specification.

However, whether or not a given compiler actually performs these optimizations is entirely up to the implementation.

Compiler optimization of method calls

1 Answers1