Re: JIT and class loaders

["Blair Wyman" <blairw@xxxxxxxxxx>]

David Morris wrote:

>Group,
>
>I searched the iSeries FAQs and can't find a clear description of when
>the JVM supports Jar and Class files created using CRTJVAPGM. I am
>assuming that if I use a custom class loader (almost any Servlet
>container) that the iSeries JVM will have to fall back to the JIT
>optimizations if available. Is that true? Also, is inlining possible
>with the JIT compiler and does the -O compile option have any affect?

Currently, any class loaded via a so-called "user classloader" will
be processed (by default) using the JIT.  This is an artifact of the
way our JVM finds the pre-compiled result of calling CRTJVAPGM, which
is using the file's inode.  When a user classloader is involved, the
JVM never gets to see the actual file (either by name or by inode),
and so there's no easy way to get to the JVAPGM.

The terminology "fall back" is a little misleading -- the JIT performance
is fast approaching the performance of CRTJVAPGM'd code, right on up to
level 40.  In some cases, even the most aggressive "static" analyses and
optimizations, done by CRTJVAPGM at high optimization levels, aren't as
effective as the optimizations possible using the JIT's complete
knowledge of the runtime environment.  (Static optimizations, due to
the dynamic nature of Java, have to be "guarded" -- the JIT is under
no such restriction.)

So, yes, the JIT can and does perform inlining, and in fact can do so
when CRTJVAPGM cannot.

The -O compile option might have some effect, but that's obviously all
happening before either the JIT or CRTJVAPGM get any chance, since it's
an option on the javac command.  (javac takes source code to bytecode.)
I haven't played with it much, but together with javap -c (to disassemble
a classfile) it could be interesting to see what it "tries."  :)

HTH.

-blair

Blair Wyman -- iSeries JVM -- (507) 253-2891
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
"I was born not knowing, and have had only a
little time to change that here and there."   -- Richard P. Feynman




                      "David Morris"
                      <David.Morris@plu        To:       
<JAVA400-L@midrange.com>
                      mcreek.com>              cc:
                      Sent by:                 Subject:  JIT and class loaders
                      java400-l-admin@m
                      idrange.com


                      01/08/2002 01:17
                      PM
                      Please respond to
                      java400-l






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