Simple dynamic language using invokedynamic

[Rémi Forax]

Hi all, hi Charles,
Playing with juby and the backport ,  I've found  a mistake in the 
bootstrap code
(in SimpleJavaBootstrap.java).

The bootstrap method first install a method named fallback that will later
install the real method handle depending on the value of the first call
to fallback.

The problem lies in fallback, fallback can be decomposed in 3 steps,
first find the most specific method, then use invoke_1..10 to get
the return value and then set the target of the call site
to the most specific method.

The target is set *after* the call to the most specific method.

With fib, because the function is recursive, this mean that the next call
to fib will be done using the fallback and not the most specific method
because it is not already installed and will be only when fib will return.

In the particular case of the backport, because fallback is called often,
the JIT of the backport JITs fallback and when fib returns it deoptimizes
the call site to reoptimize it with the most specific method later.

If you just permute step2 and step3, the program is 10% faster
with client VM and more than 300% with server VM.

SimleJavaBootstrap with no modification:
[forax at localhost juby]$ time /usr/jdk/jdk1.6.0_11/bin/java 
-javaagent:../indy-backport/lib/jsr292-backport.jar -cp .:src examples.fib
9227465

real    0m4.036s
user    0m7.180s
sys    0m0.111s

SimleJavaBootstrap with step 2 and 3 permuted:
[forax at localhost juby]$ time /usr/jdk/jdk1.6.0_11/bin/java 
-javaagent:../indy-backport/lib/jsr292-backport.jar -cp .:src examples.fib
9227465

real    0m1.168s
user    0m1.621s
sys    0m0.059s

This values are with the backport but I am sure you will gain something 
with jdk7.

cheers,
Rémi