jsr instructions in OpenJDK 6 classes

[Joe Darcy]

Hi Andrew,

Whether or not the jsr instruction is used is -target specific; version 
50.0 class files from -target 6 and later do not support jsr/ret since 
stackmaps are used.

I haven't delved into the OpenJDK 6 build before writing this email, but 
I suspect some of the classes in rt.jar are compiled with -target 5, but 
javac in OpenJDK 6 defaults to -target 6 so you wouldn't see jsr 
instructions from a stock compile.

Sufficiently recent versions of javap will emit the classfile version 
under the -verbose flag.

HTH,

-Joe

[1] "Source, target, class file version decoder ring,"
http://blogs.oracle.com/darcy/entry/source_target_class_file_version

On 02/02/2012 04:46 AM, Andrew Haley wrote:
> I'm sorry for the cross-post, but I don't know exactly who to
> address this to.
>
> Just to save you time reading all this: the javac in OpenJDK 6
> generates jsr instructions, but only if you compile a great many
> source files in one go.  This affects rt.jar.  I'm trying to find
> out why.
>
> If you build OpenJDK6 and do this:
>
> happy:jdk6 $ javap -c -bootclasspath build/linux-amd64/classes/ -classpath build/linux-amd64/classes/ java.util.concurrent/PriorityBlockingQueue | grep jsr
>     48:  jsr     62
>     56:  jsr     62
>     17:  jsr     28
>     23:  jsr     28
>     74:  jsr     87
>   ...
>
> you'll see lots of jsr instructions.  Why is this?  It seems that if
> you compile the .java file this doesn't happen:
>
> happy:jdk6 $ ./build/linux-amd64/j2sdk-image/jre/bin/java -Xmx896m -Xms128m -XX:PermSize=32m -XX:MaxPermSize=160m -Xbootclasspath/p:./build/linux-amd64/langtools/dist/bootstrap/lib/javac.jar -jar ./build/linux-amd64/langtools/dist/bootstrap/lib/javac.jar -g -source 1.5 -target 5 -encoding ascii -Xbootclasspath:/local/openjdk/jdk6/build/linux-amd64/classes -sourcepath /local/openjdk/jdk6/build/linux-amd64/gensrc:/local/openjdk/jdk6/jdk/src/solaris/classes:/local/openjdk/jdk6/jdk/src/share/classes -d /tmp
> /local/openjdk/jdk6/jdk/src/share/classes/java/util/concurrent/PriorityBlockingQueue.java
>
> happy:jdk6 $  javap -c -bootclasspath /tmp -classpath /tmp java.util.concurrent/PriorityBlockingQueue | grep jsr
>
> No jsr instructions this time.
>
> So, I try the compile again, but with the full list of classes in .classes.list:
>
>   $  $ /local/openjdk/jdk6/build/linux-amd64/j2sdk-image/jre/bin/java -Xmx896m -Xms128m -XX:PermSize=32m -XX:MaxPermSize=160m -Xbootclasspath/p:/local/openjdk/jdk6/build/linux-amd64/langtools/dist/bootstrap/lib/javac.jar -jar /local/openjdk/jdk6/build/linux-amd64/langtools/dist/bootstrap/lib/javac.jar -g -source 1.5 -target 5 -encoding ascii -Xbootclasspath:/local/openjdk/jdk6/build/linux-amd64/classes -sourcepath
> /local/openjdk/jdk6/build/linux-amd64/gensrc:/local/openjdk/jdk6/jdk/src/solaris/classes:/local/openjdk/jdk6/jdk/src/share/classes -d /tmp @/local/openjdk/jdk6/build/linux-amd64/tmp/java/java.lang/java/.classes.list
>
>   $  javap -c -bootclasspath /tmp -classpath /tmp java.util.concurrent/PriorityBlockingQueue | grep jsr | head -5
>     48:  jsr     62
>     56:  jsr     62
>     17:  jsr     28
>     23:  jsr     28
>     74:  jsr     87
>
> Lots of jsrs.
>
> Now the really weird bit: if I edit classes.list to contain only a
> small subset of the files and run the command again,
>
>   $ grep java.util.concurrent /local/openjdk/jdk6/build/linux-amd64/tmp/java/java.lang/java/.classes.list>  classes.concurrent
>
>   $ /local/openjdk/jdk6/build/linux-amd64/j2sdk-image/jre/bin/java -Xmx896m -Xms128m -XX:PermSize=32m -XX:MaxPermSize=160m -Xbootclasspath/p:/local/openjdk/jdk6/build/linux-amd64/langtools/dist/bootstrap/lib/javac.jar -jar /local/openjdk/jdk6/build/linux-amd64/langtools/dist/bootstrap/lib/javac.jar -g -source 1.5 -target 5 -encoding ascii -Xbootclasspath:/local/openjdk/jdk6/build/linux-amd64/classes -sourcepath
> /local/openjdk/jdk6/build/linux-amd64/gensrc:/local/openjdk/jdk6/jdk/src/solaris/classes:/local/openjdk/jdk6/jdk/src/share/classes -d /tmp @classes.concurrent
>
>   $  javap -c -bootclasspath /tmp -classpath /tmp java.util.concurrent/PriorityBlockingQueue | grep jsr | head -5
>
> ... no jsr instructions.
>
> I realize that OpenJDK 6 is nearing EOL, but this behaviour is so bizarre
> that I'd appreciate some input!
>
> Thanks,
> Andrew.