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@include version.texi

@c @ifnottex
This file documents vmgen (Gforth @value{VERSION}).

@section Introduction

Vmgen is a tool for writing efficient interpreters.  It takes a simple
virtual machine description and generates efficient C code for dealing
with the virtual machine code in various ways (in particular, executing
it).  The run-time efficiency of the resulting interpreters is usually
within a factor of 10 of machine code produced by an optimizing
compiler.

The interpreter design strategy supported by vmgen is to divide the
interpreter into two parts:

@itemize @bullet

@item The @emph{front end} takes the source code of the language to be
implemented, and translates it into virtual machine code.  This is
similar to an ordinary compiler front end; typically an interpreter
front-end performs no optimization, so it is relatively simple to
implement and runs fast.

@item The @emph{virtual machine interpreter} executes the virtual
machine code.

@end itemize

Such a division is usually used in interpreters, for modularity as well
as for efficiency reasons.  The virtual machine code is typically passed
between front end and virtual machine interpreter in memory, like in a
load-and-go compiler; this avoids the complexity and time cost of
writing the code to a file and reading it again.

A @emph{virtual machine} (VM) represents the program as a sequence of
@emph{VM instructions}, following each other in memory, similar to real
machine code.  Control flow occurs through VM branch instructions, like
in a real machine.

In this setup, vmgen can generate most of the code dealing with virtual
machine instructions from a simple description of the virtual machine
instructions (@pxref...), in particular:

@table @emph

@item VM instruction execution

@item VM code generation
Useful in the front end.

@item VM code decompiler
Useful for debugging the front end.

@item VM code tracing
Useful for debugging the front end and the VM interpreter.  You will
typically provide other means for debugging the user's programs at the
source level.

@item VM code profiling
Useful for optimizing the VM insterpreter with superinstructions
(@pxref...).

@end table

VMgen supports efficient interpreters though various optimizations, in
particular

@itemize

@item Threaded code

@item Caching the top-of-stack in a register

@item Combining VM instructions into superinstructions

@item
Replicating VM (super)instructions for better BTB prediction accuracy
(not yet in vmgen-ex, but already in Gforth).

@end itemize

As a result, vmgen-based interpreters are only about an order of
magintude slower than native code from an optimizing C compiler on small
benchmarks; on large benchmarks, which spend more time in the run-time
system, the slowdown is often less (e.g., the slowdown over the best JVM
JIT compiler we measured is only a factor of 2-3 for large benchmarks
(and some other JITs were slower than our interpreter).

VMs are usually designed as stack machines (passing data between VM
instructions on a stack), and vmgen supports such designs especially
well; however, you can also use vmgen for implementing a register VM and
still benefit from most of the advantages offered by vmgen.

@section Why interpreters?