Expected performance difference between ConstantCallSite and unchanging MutableCallSite

[John Rose]

On May 15, 2011, at 5:36 PM, Charles Oliver Nutter wrote:

> Can I get a rough guesstimate from the JVM guys how much more overhead
> is involved in accessing a never-changed MutableCallSite versus a
> ConstantCallSite? I have a few places where I want to use
> invokedynamic to lazily initialize some literals. They'll never change
> after the initial construction, but I can't create them in the
> bootstrap because I need the state they pass in (e.g. the
> org.jruby.Ruby instance they're associated with). I'd like to bind a
> constant method handle into the site after the first call and never
> change it again.

At this point, the overhead is nil, because we don't special-case CCS yet.  :-(

When we do a pass of performance tuning, we'll fix that, and then the overhead (MCS/CCS) is likely to be a memory reference or two, plus a branch.

If you are building bytecodes which are specific to a particular org.jruby.Ruby instance, see if you can put a backpointer to the instance onto the Class containing the bytecodes, using ClassValue or a static field or a similar mechanism.  Then you can access the Ruby instance in the BSM, and use a CCS.  You are trying to do something pretty normal here, and I'm an intrigued that it is so hard to get the right info in the BSM; it shouldn't be.

-- John