crush depth

Mind Your Constants

A little known feature of javac is that it will inline constant references when compiling code. This can mean that it's possible to accidentally break binary compatibility with existing clients of a piece of code when changing the value of a constant. Worse, tools that analyze bytecode have no way of detecting a binary-incompatible change of this type.

For example, the following class defines a public constant called NAME:

public final class Constants
{
  public static final String NAME = "com.io7m.name";

  private Constants()
  {

  }
}

Another class refers to NAME directly:

public final class Main0
{
  public static void main(
    final String args[])
  {
    System.out.println(Constants.NAME);
  }
}

Now, let's assume that NAME actually becomes part of an API in some form; callers may pass NAME to API methods. Because we've taken the time to declare a global constant, it should be perfectly safe to change the value of NAME at a later date without having to recompile all clients of the API, yes? Well, no, unfortunately not. Take a look at the bytecode of Main0:

public final class Main0
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_FINAL, ACC_SUPER
Constant pool:
   #1 = Methodref          #7.#16         // java/lang/Object."<init>":()V
   #2 = Fieldref           #17.#18        // java/lang/System.out:Ljava/io/PrintStream;
   #3 = Class              #19            // Constants
   #4 = String             #20            // com.io7m.name
   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   #5 = Methodref          #21.#22        // java/io/PrintStream.println:(Ljava/lang/String;)V
   #6 = Class              #23            // Main0
   #7 = Class              #24            // java/lang/Object
  ...
  #19 = Utf8               Constants
  #20 = Utf8               com.io7m.name
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  #21 = Class              #28            // java/io/PrintStream
  #22 = NameAndType        #29:#30        // println:(Ljava/lang/String;)V
  ...
{
  public Main0();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 1: 0

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=1, args_size=1
         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
         3: ldc           #4                  // String com.io7m.name
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
         5: invokevirtual #5                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
         8: return
      LineNumberTable:
        line 6: 0
        line 7: 8
}

You can see that the value of the NAME constant has been inlined and inserted into the Main0 class's constant pool directly. This means that if you change the value of NAME in the Constants class at a later date, the Main0 class will need to be recompiled in order to see the change.

What can be done instead? Wrap the constant in a static method:

public final class ConstantsWrapped
{
  private static final String NAME = "com.io7m.name";

  public static final String name()
  {
    return NAME;
  }

  private ConstantsWrapped()
  {

  }
}

Call the method instead of referring to the constant directly:

public final class Main1
{
  public static void main(
    final String args[])
  {
    System.out.println(ConstantsWrapped.name());
  }
}

Now the resulting bytecode is:

public final class Main1
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_FINAL, ACC_SUPER
Constant pool:
   #1 = Methodref          #6.#15         // java/lang/Object."<init>":()V
   #2 = Fieldref           #16.#17        // java/lang/System.out:Ljava/io/PrintStream;
   #3 = Methodref          #18.#19        // ConstantsWrapped.name:()Ljava/lang/String;
   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   #4 = Methodref          #20.#21        // java/io/PrintStream.println:(Ljava/lang/String;)V
   #5 = Class              #22            // Main1
   #6 = Class              #23            // java/lang/Object
   #7 = Utf8               <init>
   #8 = Utf8               ()V
   #9 = Utf8               Code
  #10 = Utf8               LineNumberTable
  #11 = Utf8               main
  #12 = Utf8               ([Ljava/lang/String;)V
  #13 = Utf8               SourceFile
  #14 = Utf8               Main1.java
  #15 = NameAndType        #7:#8          // "<init>":()V
  #16 = Class              #24            // java/lang/System
  #17 = NameAndType        #25:#26        // out:Ljava/io/PrintStream;
  #18 = Class              #27            // ConstantsWrapped
  #19 = NameAndType        #28:#29        // name:()Ljava/lang/String;
  #20 = Class              #30            // java/io/PrintStream
  #21 = NameAndType        #31:#32        // println:(Ljava/lang/String;)V
  #22 = Utf8               Main1
  #23 = Utf8               java/lang/Object
  #24 = Utf8               java/lang/System
  #25 = Utf8               out
  #26 = Utf8               Ljava/io/PrintStream;
  #27 = Utf8               ConstantsWrapped
  #28 = Utf8               name
  #29 = Utf8               ()Ljava/lang/String;
  #30 = Utf8               java/io/PrintStream
  #31 = Utf8               println
  #32 = Utf8               (Ljava/lang/String;)V
{
  public Main1();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 1: 0

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=1, args_size=1
         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
         3: invokestatic  #3                  // Method ConstantsWrapped.name:()Ljava/lang/String;
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
         6: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
         9: return
      LineNumberTable:
        line 6: 0
        line 7: 9
}

This effectively solves the issue. The ldc opcode is changed to an invokestatic opcode, at no point does the string com.io7m.name appear directly in the Main1 class, and the value of the constant can be changed at a later date without breaking binary compatibility. Additionally, the JIT compiler will inline the invokestatic call at run-time, meaning that there's no performance degradation over using the constant directly.

FreeBSD ZFS Root

When I set up the initial FreeBSD install to host io7m.com, I didn't realize how trivial it was to use ZFS as the root partition. Having used this option several times since, I now wish I had done this for the io7m VPS. I might spin up a new VPS over the next few days with a ZFS root partition, copy the configuration data over to the new VPS, and then reconfigure DNS to point to the new system. If there's a mysterious outage, this will be the reason why.

Half Float Pain

Whilst working on smf, I ran into an issue when resampling 32-bit floating point mesh data to 16-bit floating point format. The issue turned out to be poor handling of subnormal values by my ieee754b16 package. I went looking for better implementations to borrow and found a nice paper by Jeroen van der Zijp called Fast Half Float Conversions. It uses precomputed lookup tables to perform conversions and appears to be drastically more accurate than my manual process (the mathematics of which I've almost entirely forgotten).

I decided to put together a simple C99 implementation in order to see how the code worked but am having some strange issues with some very specific values. My test suite basically tries to prove that packing a double value and then unpacking it should be an approximate identity operation. Essentially, ∀x. unpack(pack(x)) ≈ x. Unfortunately, some very specific values are failing. For some reason, my implementation yields these results:

unpack(pack(2048.0)) → 2048.0
unpack(pack(2047.0)) → -0.0
unpack(pack(2046.0)) → 2046.0
unpack(pack(16375.0)) → 16368.0
unpack(pack(16376.0)) → 0.0

All of the other values in the range [-32000, 32000] appear to be correct. The unusual 16375.0 → 16368.0 result is expected; the conversion is necessarily a lossy procedure and 16368.0 is simply the nearest representable value when converting down to 16-bits. However, the 0.0 values are utterly wrong. This suggests that there's an issue in implementation that's almost certainly caused by a mistake generating the conversion tables. It seems that packing is correct, but unpacking isn't. I've gone over the code several times, even going so far as to implement it twice in two different languages and have gotten the same results every time. I've spoken to Jeroen and he showed me some results from his own implementation and test suite that show that the above isn't a problem with the algorithm. So, assuming that I haven't managed to screw up the same implementation after some five clean-room attempts, there may be a transcription mistake in the paper. I'm waiting to hear more from Jeroen.

Maven Assembly Plugin Redux

A few months back, I filed a bug for the Maven Assembly Plugin. Karl Heinz Marbaise finally got back to me and solved the issue right away. Thanks again!

JEP 305 - Pattern Matching

http://openjdk.java.net/jeps/305

This is a good first step towards getting algebraic data types into Java.