PyPy - automatic generation of VMs for dynamic languages - JIT included
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PyPy is a framework written in Python for generating virtual machines
for dynamic languages.  The VMs are flexibly produced from a
high-level "specification" in the form of a simple interpreter for the
dynamic language. The interpreters are written in a high-level static
subset of Python.
 
Low-level details like memory allocation and object layout and stack
inspection are not encoded manually, but inserted by the VM generation process.
This allows us to produce VMs that run within a wide range of execution
environments (from C-like to JVM/.NET). 

The framework has the ability to automatically generate a dynamic compiler
from the interpreter too. A pragmatic application of partial evaluation
techniques guided by a few hints is used for the task.

Crucial for the effectiveness of dynamic compilation is
the use of run-time information to improve compilation results: in our
approach, a powerful primitive called "promotion" that
"promotes" run-time values to compile-time is used to that effect.