Question

I wonder if JVM/javac is smart enough to turn

// This line...
string a = foo();

string foo()
{
  return bar();
}

string bar()
{
  return some-complicated-string computation;
}

into

string a = bar();

Or strip unnecessary call to foo() in release case (because unreachable code):

string a = foo(bar());

// bar is the same
...

string foo(string b)
{
  if (debug) do-something-with(b);
}

My feeling is yes for the first example and “not so sure” for the second one, but could anyone give me some pointers/links to confirm that?

Answer 1

javac will present bytecode that is a faithful representation of the original Java program that generated the bytecode (except in certain situations when it can optimize: constant folding and dead-code elimination). However, optimization may be performed by the JVM when it uses the JIT compiler.

For the first scenario it looks like the JVM supports inlining (see under Methods here and see here for an inlining example on the JVM).

I couldn’t find any examples of method inlining being performed by javac itself. I tried compiling a few sample programs (similar to the one you have described in your question) and none of them seemed to directly inline the method even when it was final. It would seem that these kind of optimizations are done by the JVM’s JIT compiler and not by javac. The “compiler” mentioned under Methods here seems to be the HotSpot JVM’s JIT compiler and not javac.

From what I can see, javac supports dead-code elimination (see the example for the second case) and constant folding. In constant folding, the compiler will precalculate constant expressions and use the calculated value instead of performing the calculation during runtime. For example:

public class ConstantFolding {

   private static final int a = 100;
   private static final int b = 200;

   public final void baz() {
      int c = a + b;
   }
}

compiles to the following bytecode:

Compiled from "ConstantFolding.java"
public class ConstantFolding extends java.lang.Object{
private static final int a;

private static final int b;

public ConstantFolding();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return

public final void baz();
  Code:
   0:   sipush  300
   3:   istore_1
   4:   return

}

Note that the bytecode has an sipush 300 instead of aload‘s getfields and an iadd. 300 is the calculated value. This is also the case for private final variables. If a and b were not static, the resulting bytecode will be:

Compiled from "ConstantFolding.java"
public class ConstantFolding extends java.lang.Object{
private final int a;

private final int b;

public ConstantFolding();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   aload_0
   5:   bipush  100
   7:   putfield    #2; //Field a:I
   10:  aload_0
   11:  sipush  200
   14:  putfield    #3; //Field b:I
   17:  return

public final void baz();
  Code:
   0:   sipush  300
   3:   istore_1
   4:   return

}

Here also, an sipush 300 is used.

For the second case (dead-code elimination), I used the following test program:

public class InlineTest {

   private static final boolean debug = false;

   private void baz() {
      if(debug) {
         String a = foo();
      }
   }

   private String foo() {
      return bar();
   }

   private String bar() {
      return "abc";
   }
}

which gives the following bytecode:

Compiled from "InlineTest.java"
public class InlineTest extends java.lang.Object{
private static final boolean debug;

public InlineTest();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return

private void baz();
  Code:
   0:   return

private java.lang.String foo();
  Code:
   0:   aload_0
   1:   invokespecial   #2; //Method bar:()Ljava/lang/String;
   4:   areturn

private java.lang.String bar();
  Code:
   0:   ldc #3; //String abc
   2:   areturn

}

As you can see, the foo is not called at all in baz because the code inside the if block is effectively “dead”.

Sun’s (now Oracle’s) HotSpot JVM combines interpretation of the bytecode as well as JIT compilation. When bytecode is presented to the JVM the code is initially interpreted, but the JVM will monitor the bytecode and pick out parts that are frequently executed. It coverts these parts into native code so that they will run faster. For piece of bytecode that are not used so frequently, this compilation is not done. This is just as well because compilation has some overhead. So it’s really a question of tradeoff. If you decide to compile all bytecode to nativecode, then the code can have a very long start-up delay.

In addition to monitoring the bytecode, the JVM can also perform static analysis of the bytecode as it is interpreting and loading it to perform further optimization.

If you want to know the specific kinds of optimizations that the JVM performs, this page at Oracle is pretty helpful. It describes the performance techniques used in the HotSpot JVM.