Java是否会优化最大操作？

Question

Given the following Java code, will the JRE optimizer replace the call to max with a call to Math.max ?

Unoptimized code

public static void main(String[] args) {
  int max = max(3, 5);
}

public int max(int x, int y) {
  if (x < y) {
    return y;
  }
  else {
    return x;
  }
}

Optimized code

public static void main(String[] args) {
  int max = Math.max(3,5)
}

Answer 1

No. How would the compiler know they are the same?

What makes you think Math.max would be faster anyway? There is no reason it would perform faster than your function.

The compiler may well inline a simple function like that though - but that is down to the compiler.

Imagine how many libraries there are in a standard program. How much effort would the compiler need to go to to scan all those libraries for identical code fragments. Then see how tiny the gains from identifying them would be.

Now also consider the case of multiple libraries where both define this method. The compiler needs to leave both because otherwise one library becomes dependant on the other and what would happen if in the future one library changed or removed its definition of the method.

It's a huge messy can of worms for no real benefit in the end.

Answer 2

There's a very good reason for Math.max to be faster than a handwritten method: It's on the list of intrinsic methods. As it's a very simple operation, there might be no gain on a given CPU, but using it is probably a good idea.

For more complex operations there is a huge speed up from using intrinsics. For example, the Java code of Long.numberOfLeadingZeros(long) is pretty long and make take a dozen cycles (or more with some branch mispredictions). As the Hotspot JVM knows this method and there's a corresponding i86 instruction, you can get it in a single cycle (even leaving room for other instructions to be executed concurrently).