Question

First code:

public static int pitagoras(int a, int b)
{
    return (int) Math.sqrt(a*a + b*b);
}

public static int distance(int x, int y, int x2, int y2)
{
    return pitagoras(x - x2, y - y2);
}

distance is called very often. When I compiled it with javac and then decompiled with javap -c I got this bytecode:

public static int pitagoras(int, int);
  Code:
   0:   iload_0
   1:   iload_0
   2:   imul
   3:   iload_1
   4:   iload_1
   5:   imul
   6:   iadd
   7:   i2d
   8:   invokestatic    #24; //Method java/lang/Math.sqrt:(D)D
   11:  d2i
   12:  ireturn

public static int distance(int, int, int, int);
  Code:
   0:   iload_0
   1:   iload_2
   2:   isub
   3:   iload_1
   4:   iload_3
   5:   isub
   6:   invokestatic    #34; //Method pitagoras:(II)I
   9:   ireturn

It seems that javac hasn’t optimized second function, distance.

Second code, I think, faster:

public static int distance(int x, int y, int x2, int y2)
{
    return (int) Math.sqrt((x - x2) * (x - x2) + (y - y2) * (y - y2));
}

And its bytecode:

public static int distance(int, int, int, int);
  Code:
   0:   iload_0
   1:   iload_2
   2:   isub
   3:   iload_0
   4:   iload_2
   5:   isub
   6:   imul
   7:   iload_1
   8:   iload_3
   9:   isub
   10:  iload_1
   11:  iload_3
   12:  isub
   13:  imul
   14:  iadd
   15:  i2d
   16:  invokestatic    #24; //Method java/lang/Math.sqrt:(D)D
   19:  d2i
   20:  ireturn

Is invokestatic so fast that it’s the same as inlining static function? Why javac did not optimize this? Or maybe it is in fact optimized and these two codes will give the same, but I’m missing something?

Answer 1

javac doesn’t optimise. That’s the job of the JVM implementation (typically HotSpot).

There used to be a few optimisations in javac but they complicated the code and allegedly tended to arrange the code so that HotSpot optimisations were inhibited.

HotSpot optimisations are generally done dynamically after thousands of iterations (configurable, default dependent upon whether using “Client”, “Server” or tiered versions).

There are some things that javac is required to do by the language specification, such as inlining constants and combining literal strings.