* The Mono runtime

	The Mono runtime engine is considered feature complete.

	It implements a Just-in-Time compiler engine for the CIL
	virtual machine, the class loader, the garbage collector,
	threading system and metadata access libraries.

	We currently have two runtimes:

	<ul>
		* <b>mono:</b> Our Just-in-Time and Ahead-of-Time code
		  generator for maximum performance.  This supports
		  x86, PowerPC and SPARC cpus.
	
		* <b>mint:</b> The Mono interpreter.  This is an
		  easy-to-port runtime engine.
	</ul>

	We are using the Boehm conservative garbage collector.

	The Mono runtime can be used as a stand-alone process, or it
	can be <a href="embedded-api.html">embedded into applications</a> (see
	the documentation in mono/samples/embed for more details).

	Embedding the Mono runtime allows applications to be extended
	in C# while reusing all of the existing C and C++ code.  

	Paolo Molaro did a presentation on the current JIT engine and
	the new JIT engine.  You can find his <a
	href="http://primates.ximian.com/~lupus/slides/jit/">slides
	here</a>

** Current JIT Engine: technical details (<b>updated, June 28th, 2003</b>)

	We have re-written our JIT compiler. We wanted to support a
	number of features that were missing:

	<ul>
		* Ahead-of-time compilation.  

	     The idea is to allow developers to pre-compile their code
	     to native code to reduce startup time, and the working
	     set that is used at runtime in the just-in-time compiler.

	     Although in Mono this has not been a visible problem, we
	     wanted to pro-actively address this problem.

	     When an assembly (a Mono/.NET executable) is installed in
	     the system, it would then be possible to pre-compile the
	     code, and have the JIT compiler tune the generated code
	     to the particular CPU on which the software is
	     installed. 

	     This is done in the Microsoft.NET world with a tool
	     called ngen.exe

		* Have a good platform for doing code optimizations. 

	     The design called for a good architecture that would
	     enable various levels of optimizations: some
	     optimizations are better performed on high-level
	     intermediate representations, some on medium-level and
	     some at low-level representations.

	     Also it should be possible to conditionally turn these on
	     or off.  Some optimizations are too expensive to be used
	     in just-in-time compilation scenarios, but these
	     expensive optimizations can be turned on for
	     ahead-of-time compilations or when using profile-guided
	     optimizations on a subset of the executed methods.

		* Reduce the effort required to port the Mono code
             generator to new architectures.

	     For Mono to gain wide adoption in the Unix world, it is
	     necessary that the JIT engine works in most of today's
	     commercial hardware platforms. 
	</ul>

	The JIT engine implements a number of optimizations:

	<ul>
		* Opcode cost estimates (our architecture allows
	 	  us to generate different code paths depending
		  on the target CPU dynamically).
		  
		* Inlining.

		* Constant folding, copy propagation, dead code elimination.

		  Although compilers typically do
		  constant folding, the combination of inlining with
		  constant folding gives some very good results.

		* Linear scan register allocation.  In the past,
		  register allocation was our achilles heel, but now 
		  we have left this problem behind.

		* SSA-based framework.  Various optimizations are
		  implemented on top of this framework
	</ul>

	There are a couple of books that deal with this technique: "A
	Retargetable C Compiler" and "Advanced Compiler Design and
	Implementation" are good references.  You can also get a
        technical description of <a
        href="http://research.microsoft.com/copyright/accept.asp?path=http://www.research.microsoft.com/~drh/pubs/iburg.pdf&pub=ACM">lbrug</a>.

	The new JIT engines uses three intermediate representations:
	the source is the CIL which is transformed into a forest of
	trees; This is fed into a BURS instruction selector that
	generates the final low-level intermediate representation.

	The instruction selector is documented in the following
	papers:

	<ul>
		* <a href="http://research.microsoft.com/copyright/accept.asp?path=http://www.research.microsoft.com/~drh/pubs/interface.pdf&pub=wiley">A code generation interface for ANSI C</a>


		* <a href="http://research.microsoft.com/copyright/accept.asp?path=http://www.research.microsoft.com/~drh/pubs/iburg.pdf&pub=ACM">Engineering efficient code generators using tree matching and dynamic programming.</a>

	</ul>

** Garbage Collection

	We are using the Boehm conservative GC.  We might consider
	adopting other GC engines in the future, like the Intel ORP GC
	engine.  The Intel ORP GC engine as it provides a precise
	garbage collector engine, similar to what is available on the
	.NET environment.

	<ul>
		* Garbage collection list and FAQ:<br>
		  <a href="http://www.iecc.com/gclist/">http://www.iecc.com/gclist/</a>

		* "GC points in a Threaded Environment":<br>
		  <a href="http://research.sun.com/techrep/1998/abstract-70.html">
		  http://research.sun.com/techrep/1998/abstract-70.html</a>

		* "A Generational Mostly-concurrent Garbage Collector":
		  <a href="http://research.sun.com/techrep/2000/abstract-88.html">
		  http://research.sun.com/techrep/2000/abstract-88.html</a>

		* Details on The Microsoft .NET Garbage Collection Implementation:<br>
		  <a href="http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnmag00/html/GCI.asp">http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnmag00/html/GCI.asp</a>
		  <a href="http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnmag00/html/GCI2.asp">http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnmag00/html/GCI2.asp</a>
	</ul>

** IO and threading

	The ECMA runtime and the .NET runtime assume an IO model and a
	threading model that is very similar to the Win32 API.  

	Dick Porter has developed WAPI: the Mono abstraction layer
	that allows our runtime to execute code that depend on this
	behaviour.

** Useful links

	Paolo Molaro found a few interesting links:

	<ul>
		* On compilation of stack-based languages:<br>
		<a href="http://www.complang.tuwien.ac.at/projects/rafts.html">
		http://www.complang.tuwien.ac.at/projects/rafts.html</a>

		* A paper on fast JIT compilation of a stack-based language:<br>
		  <a href="http://www.research.microsoft.com/~cwfraser/pldi99codegen.pdf">
		  http://www.research.microsoft.com/~cwfraser/pldi99codegen.pdf</a>

		* Vmgen generates much of the code for efficient virtual machine (VM)
		  interpreters from simple descriptions of the VM instructions:<br>
		  <a href="http://www.complang.tuwien.ac.at/anton/vmgen/">
		  http://www.complang.tuwien.ac.at/anton/vmgen</a>
	</ul>

** PInvoke

	PInvoke is the mechanism we are using to wrap Unix API calls
	as well as talking to system libraries.

	Initially we used libffi, but it was fairly slow, so we have
	reused parts of the JIT work to create efficient PInvoke
	trampolines.

** Remoting

	Mono has support for remoting and proxy objects, just like
	.NET does.  The runtime provides these facilities.

** Porting

	If you are interested in porting the Mono runtime to other
	platforms, you might find the pre-compiled <a
	href="archive/mono-tests.tar.gz">Mono regression test
	suite</a> useful to debug your implementation.

* COM and XPCOM

	We plan on adding support for XPCOM on Unix and COM on Microsoft
	Windows later in our development process.