Re: What CRTJVAPGM does

blairw@us.ibm.com wrote:
> 
> Larry Loen wrote:
> 
> > Starting in V4R5, we also added the more ordinary "just in time"
> > (JIT) compile process, which creates some on-the-fly compiled
> > code, but CRTJVAPGM OPTIMIZE(40) produces better
> > code than the "JIT" process does.  JIT and CRTJVAPGM objects
> > can interoperate as well.
> 
> Considering the superior code-gen of CRTJVAPGM, one might ask, "Why do a
> JIT at all?"  The answer is subtle, but its impacts are pervasive in
> server-side Java.

Blair --

An excellent explanation of the JIT/400 evolution.  So, here are my
questions:
1.  Considering only the runtime performance (not compile time), is the
runtime performance of CRTJVAPGM OPTIMIZE(40) with "guards" that much
better than the JIT with its runtime knowledge that it is worth it to
perform the CRTJVAPGM?  Do you see the runtime performance of the JIT
catching up to CRTJVAPGM?
2.  From a runtime perspective, at what optimization level, if any, is
the JIT code faster than CRTJVAPGM?
3.  Does the JIT do a "mixed mode" analysis to only JIT classes that
will be executed a lot or does it JIT compile everything?
4.  Is the compile time overhead of the JIT large enough to justify
maintaining both the JIT compiler and the CRTJVAPGM compiler going
forward?

If you have time, I think these answers would be interesting.  I really
have mixed feelings about CRTJVAPGM that I'm trying to overcome.  It
seems to be that if it's basically seamless for the .class files, it
would be nice if it was also seamless for the .jar/.zip files.  Then, I
wouldn't ever have to worry about it at all.  In other words, it would
appear to me as basically a JIT process that could persist across
invocations and would otherwise be invisible to me.

Thanks for your time,
Gary
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