Development Sandbox: Java code inlining

Once in a while, it’s fun to take out the Java development tools and experiment. So I did just that. I went online and found a bytecode editor that does a quick job of parsing Java class files. I wanted to see what my code looked like in bytecode form. I wanted to test if a complicated if condition would get inlined. You know what I’m talking about. You’ve written that complicated condition fairly often. You need to validate that the variable you’re testing against is not null and is accessible.

Below is an example of such a condition:

arrayOfValues != null && arrayOfValues.length > 0 && arrayOfValues[0].equals(Integer.toString(anotherValue))

When a condition gets ugly I will often extract it into its own method to make the code easier to read. It also provides the opportunity to do some intense unit testing.

So here is the sample code I wrote that I would be testing:

public class Sample {
    // The number of iterations for the for loop
    static int iterations = 1000000;
    // How often do we print the iteration
    static int step = 100000;

    // The variable in our test
    static String[] arrayOfValues = {"1353"};

    public static void main(String... args) {
        // This is the big loop
        for (int i = 0; i < iterations; i++) {

            // Notify us of where we are in the loop
            if (i % step == 0) {
                System.out.println("Iteration: " + i);
            }

            // Notify us when we've match with the iteration
            if (isAMatch(i)) {
                System.out.println("Found: " + i);
            }
        }
    }

    // This is our test condition
    private static boolean isAMatch(int i) {
        return arrayOfValues != null && arrayOfValues.length > 0 && arrayOfValues[0].equals(Integer.toString(i));
    }
}

The code isn’t complicated so I won’t explain what it’s doing. But note the extracted method isAMatch() it’s an example of what most developers would code.

By default, the JIT will inline methods that are 34 bytes or shorter. So I used the Java Bytecode Editor to review the class file and see what’s what. According to the editor the function isAMatch() is just about 34 bytes long; just within the maximum limit. I’ll be honest, I thought Java would be able to fit more within 34 bytes. In a later post, I’ll experiment with different ways of writing the condition to see if I can get it shorter. But for now, it seems that the function is a good candidate for inlining.

javabytecodeeditor-1

On to testing

I compile the code using the Java 8 JDK with the following statement. Nothing extravagant.
javac Sample.java
The fun starts when executing the application, that’s when the JIT does all of its optimization magic. Below is the command I used to execute the class file.
java -XX:+UnlockDiagnosticVMOptions -XX:+PrintInlining Sample
This is what those options do:
  • UnlockDiagnosticVMOptions: This option it turns on the diagnostic tracking. Oddly enough, it is not listed on Oracle’s website even though it is required for PrintInlining to work. But the JIT will display an error if it’s not provided.
  • PrintInlining: This option will display a report on all the decisions the JIT makes in regards to inlining methods. It is impressive to see in action.
When you execute the class this is the output that you’ll get.

 

</pre>
@ 1 java.lang.Object::<init> (1 bytes)
@ 13 java/lang/StringIndexOutOfBoundsException::<init> (not loaded) not inlineable
@ 30 java/lang/StringIndexOutOfBoundsException::<init> (not loaded) not inlineable
@ 65 java/lang/StringIndexOutOfBoundsException::<init> (not loaded) not inlineable
@ 75 java.util.Arrays::copyOfRange (63 bytes) callee is too large
@ 16 java.lang.StringBuilder::<init> (7 bytes)
@ 3 java.lang.AbstractStringBuilder::<init> (12 bytes)
@ 1 java.lang.Object::<init> (1 bytes)
@ 20 java.lang.StringBuilder::append (8 bytes)
@ 2 java.lang.AbstractStringBuilder::append (62 bytes) callee is too large
@ 25 java.lang.StringBuilder::append (8 bytes)
@ 2 java.lang.AbstractStringBuilder::append (50 bytes) callee is too large
@ 29 java.lang.StringBuilder::append (8 bytes)
@ 2 java.lang.AbstractStringBuilder::append (62 bytes) callee is too large
@ 32 java.lang.StringBuilder::toString (17 bytes)
@ 13 java.lang.String::<init> (82 bytes) callee is too large
@ 35 java.lang.IllegalArgumentException::<init> (6 bytes) don't inline Throwable constructors
@ 54 java.lang.Math::min (11 bytes)
@ 57 java.lang.System::arraycopy (0 bytes) intrinsic
@ 66 java.lang.String::indexOfSupplementary (71 bytes) callee is too large
@ 18 java/lang/StringIndexOutOfBoundsException::<init> (not loaded) not inlineable
@ 4 java.lang.CharacterDataLatin1::getProperties (11 bytes)
@ 1 java.lang.Character::toUpperCase (9 bytes)
@ 1 java.lang.CharacterData::of (120 bytes) callee is too large
@ 5 java.lang.CharacterData::toUpperCase (0 bytes) no static binding
@ 1 java.lang.CharacterData::of (120 bytes) callee is too large
@ 5 java.lang.CharacterData::toUpperCase (0 bytes) no static binding
@ 1 java.lang.CharacterData::of (120 bytes) callee is too large
@ 5 java.lang.CharacterData::toLowerCase (0 bytes) no static binding
@ 4 java.lang.CharacterDataLatin1::getProperties (11 bytes)
@ 3 java.lang.String::indexOf (70 bytes) callee is too large
@ 17 java.lang.AbstractStringBuilder::newCapacity (39 bytes) callee is too large
@ 20 java.util.Arrays::copyOf (19 bytes)
@ 11 java.lang.Math::min (11 bytes)
@ 14 java.lang.System::arraycopy (0 bytes) intrinsic
@ 7 java.lang.AbstractStringBuilder::ensureCapacityInternal (27 bytes)
@ 17 java.lang.AbstractStringBuilder::newCapacity (39 bytes) callee is too large
@ 20 java.util.Arrays::copyOf (19 bytes)
@ 11 java.lang.Math::min (11 bytes)
@ 14 java.lang.System::arraycopy (0 bytes) intrinsic
@ 2 java.lang.AbstractStringBuilder::append (29 bytes)
@ 7 java.lang.AbstractStringBuilder::ensureCapacityInternal (27 bytes)
@ 17 java.lang.AbstractStringBuilder::newCapacity (39 bytes) callee is too large
@ 20 java.util.Arrays::copyOf (19 bytes)
@ 11 java.lang.Math::min (11 bytes)
@ 14 java.lang.System::arraycopy (0 bytes) intrinsic
@ 7 java.lang.AbstractStringBuilder::append (29 bytes)
@ 7 java.lang.AbstractStringBuilder::ensureCapacityInternal (27 bytes)
@ 17 java.lang.AbstractStringBuilder::newCapacity (39 bytes) callee is too large
@ 20 java.util.Arrays::copyOf (19 bytes)
@ 11 java.lang.Math::min (11 bytes)
@ 14 java.lang.System::arraycopy (0 bytes) intrinsic
@ 5 java.lang.AbstractStringBuilder::appendNull (56 bytes) callee is too large
@ 10 java.lang.String::length (6 bytes)
@ 21 java.lang.AbstractStringBuilder::ensureCapacityInternal (27 bytes)
@ 17 java.lang.AbstractStringBuilder::newCapacity (39 bytes) callee is too large
@ 20 java.util.Arrays::copyOf (19 bytes)
@ 11 java.lang.Math::min (11 bytes)
@ 14 java.lang.System::arraycopy (0 bytes) intrinsic
@ 35 java.lang.String::getChars (62 bytes) callee is too large
@ 2 java.lang.AbstractStringBuilder::append (50 bytes) callee is too large
Iteration: 0
@ 1 java.lang.Object::<init> (1 bytes)
@ 19 Found: 1353 java.lang.Integer::toString (48 bytes) callee is too large

@ 22 java.lang.String::equals (81 bytes) callee is too large
@ 15 java.lang.Integer::stringSize (21 bytes)
@ 24 java.lang.Integer::stringSize (21 bytes)
@ 35 java.lang.Integer::getChars (131 bytes) callee is too large
@ 44 java.lang.String::<init> (10 bytes)
@ 1 java.lang.Object::<init> (1 bytes)
@ 24 java.lang.Integer::stringSize (21 bytes) inline (hot)
@ 35 java.lang.Integer::getChars (131 bytes) inline (hot)
@ 44 java.lang.String::<init> (10 bytes) inline (hot)
@ 1 java.lang.Object::<init> (1 bytes) inline (hot)
@ 19 java.lang.Integer::toString (48 bytes) inline (hot)
@ 24 java.lang.Integer::stringSize (21 bytes) inline (hot)
@ 35 java.lang.Integer::getChars (131 bytes) inline (hot)
@ 44 java.lang.String::<init> (10 bytes) inline (hot)
@ 1 java.lang.Object::<init> (1 bytes) inline (hot)
@ 22 java.lang.String::equals (81 bytes) (intrinsic)
@ 24 java.lang.StringBuilder::<init> (7 bytes)
@ 3 java.lang.AbstractStringBuilder::<init> (12 bytes)
@ 1 java.lang.Object::<init> (1 bytes)
@ 29 java.lang.StringBuilder::append (8 bytes)
@ 2 java.lang.AbstractStringBuilder::append (50 bytes) callee is too large
@ 33 java.lang.StringBuilder::append (8 bytes)
@ 2 java.lang.AbstractStringBuilder::append (62 bytes) callee is too large
@ 36 java.lang.StringBuilder::toString (17 bytes)
@ 13 java.lang.String::<init> (82 bytes) callee is too large
!m @ 39 java.io.PrintStream::println (24 bytes)
@ 6 java.io.PrintStream::print (13 bytes)
!m @ 9 java.io.PrintStream::write (83 bytes) callee is too large
!m @ 10 java.io.PrintStream::newLine (73 bytes) callee is too large
@ 43 Sample::isAMatch (34 bytes)
@ 19 java.lang.Integer::toString (48 bytes) callee is too large
@ 22 java.lang.String::equals (81 bytes) callee is too large
@ 56 java.lang.StringBuilder::<init> (7 bytes)
@ 3 java.lang.AbstractStringBuilder::<init> (12 bytes)
@ 1 java.lang.Object::<init> (1 bytes)
@ 1 java.lang.Object::<init> (1 bytes) inline (hot)
@ 61 java.lang.StringBuilder::append (8 bytes)
@ 2 java.lang.AbstractStringBuilder::append (50 bytes) callee is too large
@ 65 java.lang.StringBuilder::append (8 bytes)
@ 2 java.lang.AbstractStringBuilder::append (62 bytes) callee is too large
@ 68 java.lang.StringBuilder::toString (17 bytes)
@ 13 java.lang.String::<init> (82 bytes) callee is too large
!m @ 71 java.io.PrintStream::println (24 bytes)
@ 6 java.io.PrintStream::print (13 bytes)
!m @ 9 java.io.PrintStream::write (83 bytes) callee is too large
!m @ 10 java.io.PrintStream::newLine (73 bytes) callee is too large
@ 15 java.lang.Integer::stringSize (21 bytes) inlining prohibited by policy
@ 24 java.lang.Integer::stringSize (21 bytes) inlining prohibited by policy
@ 35 java.lang.Integer::getChars (131 bytes) callee is too large
@ 44 java.lang.String::<init> (10 bytes)
@ 1 java.lang.Object::<init> (1 bytes)
@ 19 java.lang.Integer::toString (48 bytes) callee is too large
@ 22 java.lang.String::equals (81 bytes) callee is too large
@ 24 java.lang.StringBuilder::<init> (7 bytes)
@ 3 java.lang.AbstractStringBuilder::<init> (12 bytes)
@ 1 java.lang.Object::<init> (1 bytes)
@ 29 java.lang.StringBuilder::append (8 bytes)
@ 2 java.lang.AbstractStringBuilder::append (50 bytes) callee is too large
@ 33 java.lang.StringBuilder::append (8 bytes)
@ 2 java.lang.AbstractStringBuilder::append (62 bytes) callee is too large
@ 36 java.lang.StringBuilder::toString (17 bytes)
@ 13 java.lang.String::<init> (82 bytes) callee is too large
!m @ 39 java.io.PrintStream::println (24 bytes)
@ 6 java.io.PrintStream::print (13 bytes)
!m @ 9 java.io.PrintStream::write (83 bytes) callee is too large
!m @ 10 java.io.PrintStream::newLine (73 bytes) callee is too large
@ 43 Sample::isAMatch (34 bytes)
@ 19 java.lang.Integer::toString (48 bytes) callee is too large
@ 22 java.lang.String::equals (81 bytes) callee is too large
@ 56 java.lang.StringBuilder::<init> (7 bytes)
@ 3 java.lang.AbstractStringBuilder::<init> (12 bytes)
@ 1 java.lang.Object::<init> (1 bytes)
@ 61 java.lang.StringBuilder::append (8 bytes)
@ 2 java.lang.AbstractStringBuilder::append (50 bytes) callee is too large
@ 65 java.lang.StringBuilder::append (8 bytes)
@ 2 java.lang.AbstractStringBuilder::append (62 bytes) callee is too large
@ 68 java.lang.StringBuilder::toString (17 bytes)
@ 13 java.lang.String::<init> (82 bytes) callee is too large
!m @ 71 java.io.PrintStream::println (24 bytes)
@ 6 java.io.PrintStream::print (13 bytes)
!m @ 9 java.io.PrintStream::write (83 bytes) callee is too large
!m @ 10 java.io.PrintStream::newLine (73 bytes) callee is too large
@ 24 java.lang.Integer::stringSize (21 bytes) inline (hot)
@ 35 java.lang.Integer::getChars (131 bytes) inline (hot)
@ 44 java.lang.String::<init> (10 bytes) inline (hot)
@ 1 java.lang.Object::<init> (1 bytes) inline (hot)
Iteration: 100000
Iteration: 200000
@ 24 java.lang.StringBuilder::<init> (7 bytes) inline (hot)
@ 29 java.lang.StringBuilder::append (8 bytes) inline (hot)
@ 33 java.lang.StringBuilder::append (8 bytes) executed < MinInliningThreshold times
@ 36 java.lang.StringBuilder::toString (17 bytes) executed < MinInliningThreshold times
!m @ 39 java.io.PrintStream::println (24 bytes) executed < MinInliningThreshold times
@ 43 Sample::isAMatch (34 bytes) inline (hot)
@ 19 java.lang.Integer::toString (48 bytes) inline (hot)
@ 24 java.lang.Integer::stringSize (21 bytes) inline (hot)
@ 35 java.lang.Integer::getChars (131 bytes) inline (hot)
@ 44 java.lang.String::<init> (10 bytes) inline (hot)
@ 1 java.lang.Object::<init> (1 bytes) inline (hot)
@ 22 java.lang.String::equals (81 bytes) (intrinsic)
Iteration: 300000
Iteration: 400000
@ 19 java.lang.Integer::toString (48 bytes) inline (hot)
@ 24 java.lang.Integer::stringSize (21 bytes) inline (hot)
@ 35 java.lang.Integer::getChars (131 bytes) inline (hot)
@ 44 java.lang.String::<init> (10 bytes) inline (hot)
@ 1 java.lang.Object::<init> (1 bytes) inline (hot)
@ 22 java.lang.String::equals (81 bytes) (intrinsic)
Iteration: 500000
Iteration: 600000
Iteration: 700000
Iteration: 800000
Iteration: 900000

What I noticed is that the method isAMatch() is not inlined until it has gone through at least 200,000 iterations as noted by the label inline (hot) next to the method isAMatch() on line 160. To me that says three things.
First, the JIT thinks the method executes fast enough that it didn’t warrant inlining the first 200,000 iterations.
Second, a lot of smaller methods are inlined before the code reaches 100,000 iterations.
Third, I’m guessing that smaller methods are inlined first simply due to overhead in calling them. The overall performance of the application is what’s most important. So JIT thought that my method could wait.

It’s important to note that there are JIT options to control the maximum byte size of methods to inline. What I would like to investigate is how inlining plays out with regards to natively compiled code. And when does the JIT think it necessary to compile bytecode to native code. All fun stuff.  So stay tuned for more on this.