AttoVM is a small virtual machine that we use for teaching concepts of
compilers, adaptive compilation, automatic memory management, and
other aspects of modern language runtimes.  To make it easily
accessible to students, the system is written almost completely in C,
with small bits of assembly and two Python-based conde generators.


HOMEPAGE
========
http://sepl.cs.uni-frankfurt.de/teaching/attovm.en.html


PLATFORM SUPPORT
================
AttoVM targets x86-64 machines exclusively.  We have tested it
extensively on Linux and have confirmed partial OS X support-- the
AttoVM itself works well on OS X, but the 2OPM debugger (see below)
does not.


CONTENTS AND FEATURES
=====================
- 2OPM: A custom, simplified version of x86-64 assembly language (see
  below) with assembler and debugger (no separate assembly/linkage
  supported at this time).
  
  This assembly language makes interacting with the x86-64 CPU easier
  for students familiar with MIPS or other RISC assembly languages, at
  a small performance cost compared to directly writing x86-64
  assembly.

- AttoL:  a small high-level language with support for dynamic array
  allocation, functions, classes, dynamic object creation, dynamic
  dispatch, dynamic typing, and simple static typing as input language.

- Automatic Memory Management: A Cheney Copying Garbage Collector.

- Dataflow analysis:  a framework for local dataflow analysis and
  optimisation.

- Adaptive compilation:  support for dynamic recompilation and
  sampling (not very generic yet).

- Simple AST representation: the IR is an annotated AST, with a
  handful of special nodes introduced during analysis.  Students don't
  have to learn an additional IR.  (Naturally, this also limits the
  opimisations that we can do.)


WHAT THIS IS GOOD FOR
=====================
This is a teaching tool.  We have found it useful to have students
understand how to implement garbage collectors, dataflow analyses, and
adaptive compilation.  We've also used it to demonstrate
Hindley-Milner type inference, though AttoL isn't the ideal language
to teach this.  We use it to teach

- basic compilation
- automatic memory management
- data flow analysis
- implementing dynamic dispatch
- boxing and unboxing
- adaptive compilation
- performance aspects of dynamic compilation


WHAT THIS IS NOT GOOD FOR
=========================
This is not a production-quality VM nor a full-fledged programming
language.  There's no register allocation or inlining support, and the
input language is desigend towards exposing interesting language
concepts rather than towards building scaleable programs.


2OPM
====
Our target assembly language is 2OPM (`two-operand pseudo-MIPS').  The
language looks very similar to MIPS but uses only 16 registers, and
most operations are two-operand ones.  This allows us to translate
2OPM into x86-64 machine code, often with little or no execution
overhead.  Check doc/asm-docs.tex for a full description; a (possibly
outdated) version is cached below:

  http://sepl.cs.uni-frankfurt.de/teaching/asm-docs.pdf

We've found 2OPM to be more palatable than Intel assembly, especially
to students brought up on MIPS.


ENGLISH vs GERMAN
=================
We use AttoVM both in English and in German-language classes.  For
that reason, comments can appear in either or both languages.  The
correct way of commenting is to use

  //e english comment
  //d german comment

  or

  /*e english comment */
  /*d german comment */

When you run `make dist', the system generates two tar packages, one
with the German comments removed, and one with the English comments
removed.  If you find stray comments that don't follow the above
rules, please feel free to adjust them (excluding copyright notices
and multi-lingual comments).  Some parts are commented only in German
or only in English; such is life.


BUILDING
========
Run `make'.

docs/overview.pdf has a description of the relevant files.  Note that
this file does not yet discuss some of the newer additions (such as
data flow analysis and automatic memory management) but does
accurately portray the files that it does explain.


CONTACT
=======
The current maintainer is Christoph Reichenbach, creichen at gmail.com
Please let us know if you found this tool to be useful for your own
teaching, or if you have made any extensions that might be useful to
others.