"multianewarray" vs. "anewarray" (was: Re: Performance tweaks)

On Tuesday 22 February 2005 16:30, Oliver Jowett wrote:
> > I changed the line
> >
> >         byte[][] answer = new byte[l_nf][0];
> > to
> >         byte[][] answer = new byte[l_nf][];
> >
> > This gave ~1% increase on the benchmark I was running.
>
> Gah?! What JVM? Aren't the two forms equivalent?

javap says it's "multianewarray" vs. "anewarray":


 | $ cat Test1.java
 | public class Test1 {
 |     byte[][] answer = new byte[42][0];
 | }
 |
 | $ cat Test2.java
 | public class Test2 {
 |     byte[][] answer = new byte[42][];
 | }
 |
 | $ javap -c -classpath . Test1
 | Compiled from "Test1.java"
 | public class Test1 extends java.lang.Object{
 | byte[][] answer;
 |
 | public Test1();
 |   Code:
 |    0: aload_0
 |    1: invokespecial #1; //Method java/lang/Object."<init>":()V
 |    4: aload_0
 |    5: bipush 42
 |    7: iconst_0
 |    8: multianewarray #2,  2; //class "[[B"
 |    12: putfield #3; //Field answer:[[B
 |    15: return
 |
 | }
 |
 | $ javap -c -classpath . Test2
 | Compiled from "Test2.java"
 | public class Test2 extends java.lang.Object{
 | byte[][] answer;
 |
 | public Test2();
 |   Code:
 |    0: aload_0
 |    1: invokespecial #1; //Method java/lang/Object."<init>":()V
 |    4: aload_0
 |    5: bipush 42
 |    7: anewarray #2; //class "[B"
 |    10: putfield #3; //Field answer:[[B
 |    13: return
 |
 | }