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@include version.texi

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

@chapter 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 of a
Vmgen-generated JVM interpreter over the best JVM JIT compiler we
measured is only a factor of 2-3 for large benchmarks; some other JITs
and all other interpreters we looked at 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.

There are many potential uses of the instruction descriptions that are
not implemented at the moment, but we are open for feature requests, and
we will implement new features if someone asks for them; so the feature
list above is not exhaustive.

@c *********************************************************************
@chapter Why interpreters?

Interpreters are a popular language implementation technique because
they combine all three of the following advantages:

@itemize

@item Ease of implementation

@item Portability

@item Fast edit-compile-run cycle

@end itemize

The main disadvantage of interpreters is their run-time speed.  However,
there are huge differences between different interpreters in this area:
the slowdown over optimized C code on programs consisting of simple
operations is typically a factor of 10 for the more efficient
interpreters, and a factor of 1000 for the less efficient ones (the
slowdown for programs executing complex operations is less, because the
time spent in libraries for executing complex operations is the same in
all implementation strategies).

Vmgen makes it even easier to implement interpreters.  It also supports
techniques for building efficient interpreters.

@c ********************************************************************

@chapter Concepts

@c --------------------------------------------------------------------
@section Front-end and virtual machine interpreter

@cindex front-end
Interpretive systems are typically divided into a @emph{front end} that
parses the input language and produces an intermediate representation
for the program, and an interpreter that executes the intermediate
representation of the program.

@cindex virtual machine
@cindex VM
@cindex instruction, VM
For efficient interpreters the intermediate representation of choice is
virtual machine code (rather than, e.g., an abstract syntax tree).
@emph{Virtual machine} (VM) code consists of VM instructions arranged
sequentially in memory; they are executed in sequence by the VM
interpreter, except for VM branch instructions, which implement control
structures.  The conceptual similarity to real machine code results in
the name @emph{virtual machine}.

In this framework, vmgen supports building the VM interpreter and any
other component dealing with VM instructions.  It does not have any
support for the front end, apart from VM code generation support.  The
front end can be implemented with classical compiler front-end
techniques, supported by tools like @command{flex} and @command{bison}.

The intermediate representation is usually just internal to the
interpreter, but some systems also support saving it to a file, either
as an image file, or in a full-blown linkable file format (e.g., JVM).
Vmgen currently has no special support for such features, but the
information in the instruction descriptions can be helpful, and we are
open for feature requests and suggestions.

@section Data handling

@cindex stack machine
@cindex register machine
Most VMs use one or more stacks for passing temporary data between VM
instructions.  Another option is to use a register machine architecture
for the virtual machine; however, this option is either slower or
significantly more complex to implement than a stack machine architecture.

Vmgen has special support and optimizations for stack VMs, making their
implementation easy and efficient.

You can also implement a register VM with vmgen (@pxref{Register
Machines}), and you will still profit from most vmgen features.

@cindex stack item size
@cindex size, stack items
Stack items all have the same size, so they typically will be as wide as
an integer, pointer, or floating-point value.  Vmgen supports treating
two consecutive stack items as a single value, but anything larger is
best kept in some other memory area (e.g., the heap), with pointers to
the data on the stack.

@cindex instruction stream
@cindex immediate arguments
Another source of data is immediate arguments VM instructions (in the VM
instruction stream).  The VM instruction stream is handled similar to a
stack in vmgen.

@cindex garbage collection
@cindex reference counting
Vmgen has no built-in support for nor restrictions against @emph{garbage
collection}.  If you need garbage collection, you need to provide it in
your run-time libraries.  Using @emph{reference counting} is probably
harder, but might be possible (contact us if you are interested).
@c reference counting might be possible by including counting code in 
@c the conversion macros.

@c *************************************************************
@chapter Invoking vmgen

The usual way to invoke vmgen is as follows:

@example
vmgen @var{infile}
@end example

Here @var{infile} is the VM instruction description file, which usually
ends in @file{.vmg}.  The output filenames are made by taking the
basename of @file{infile} (i.e., the output files will be created in the
current working directory) and replacing @file{.vmg} with @file{-vm.i},
@file{-disasm.i}, @file{-gen.i}, @file{-labels.i}, @file{-profile.i},
and @file{-peephole.i}.  E.g., @command{bison hack/foo.vmg} will create
@file{foo-vm.i} etc.

The command-line options supported by vmgen are

@table @option

@cindex -h, command-line option
@cindex --help, command-line option
@item --help
@itemx -h
Print a message about the command-line options

@cindex -v, command-line option
@cindex --version, command-line option
@item --version
@itemx -v
Print version and exit
@end table

@c env vars GFORTHDIR GFORTHDATADIR

@c ***************************************************************
@chapter Input File Format

Vmgen takes as input a file containing specifications of virtual machine
instructions.  This file usually has a name ending in @file{.vmg}.

The examples are taken from the example in @file{vmgen-ex}.

@section Input File Grammar

The grammar is in EBNF format, with @code{@var{a}|@var{b}} meaning
``@var{a} or @var{b}'', @code{@{@var{c}@}} meaning 0 or more repetitions
of @var{c} and @code{[@var{d}]} meaning 0 or 1 repetitions of @var{d}.

Vmgen input is not free-format, so you have to take care where you put
spaces and especially newlines; it's not as bad as makefiles, though:
any sequence of spaces and tabs is equivalent to a single space.

@example
description: {instruction|comment|eval-escape}

instruction: simple-inst|superinst

simple-inst: ident " (" stack-effect " )" newline c-code newline newline

stack-effect: {ident} " --" {ident}

super-inst: ident " =" ident {ident}  

comment:      "\ "  text newline

eval-escape:  "\e " text newline
@end example
@c \+ \- \g \f \c

Note that the @code{\}s in this grammar are meant literally, not as
C-style encodings for no-printable characters.

The C code in @code{simple-inst} must not contain empty lines (because
vmgen would mistake that as the end of the simple-inst.  The text in
@code{comment} and @code{eval-escape} must not contain a newline.
@code{Ident} must conform to the usual conventions of C identifiers
(otherwise the C compiler would choke on the vmgen output).

Vmgen understands a few extensions beyond the grammar given here, but
these extensions are only useful for building Gforth.  You can find a
description of the format used for Gforth in @file{prim}.

@subsection
@c woanders?
The text in @code{eval-escape} is Forth code that is evaluated when
vmgen reads the line.  If you do not know (and do not want to learn)
Forth, you can build the text according to the following grammar; these
rules are normally all Forth you need for using vmgen:

@example
text: stack-decl|type-prefix-decl|stack-prefix-decl

stack-decl: "stack " ident ident ident
type-prefix-decl: 
    's" ' string '" ' ("single"|"double") ident "type-prefix" ident
stack-prefix-decl:  ident "stack-prefix" string
@end example

Note that the syntax of this code is not checked thoroughly (there are
many other Forth program fragments that could be written there).

If you know Forth, the stack effects of the non-standard words involved
are:

@example
stack        ( "name" "pointer" "type" -- )
             ( name execution: -- stack )
type-prefix  ( addr u xt1 xt2 n stack "prefix" -- )
single       ( -- xt1 xt2 n )
double       ( -- xt1 xt2 n )
stack-prefix ( stack "prefix" -- )
@end example

@section Simple instructions

We will use the following simple VM instruction description as example:

@example
sub ( i1 i2 -- i )
i = i1-i2;
@end example

The first line specifies the name of the VM instruction (@code{sub}) and
its stack effect (@code{i1 i2 -- i}).  The rest of the description is
just plain C code.

@cindex stack effect
The stack effect specifies that @code{sub} pulls two integers from the
data stack and puts them in the C variable @code{i1} and @code{i2} (with
the rightmost item (@code{i2}) taken from the top of stack) and later
pushes one integer (@code{i)) on the data stack (the rightmost item is
on the top afterwards).

How do we know the type and stack of the stack items?  Vmgen uses
prefixes, similar to Fortran; in contrast to Fortran, you have to
define the prefix first:

@example
\E s" Cell"   single data-stack type-prefix i
@end example

This defines the prefix @code{i} to refer to the type @code{Cell}
(defined as @code{long} in @file{mini.h}) and, by default, to the
@code{data-stack}.  It also specifies that this type takes one stack
item (@code{single}).  The type prefix is part of the variable name.

Before we can use @code{data-stack} in this way, we have to define it:

@example
\E stack data-stack sp Cell
@end example
@c !! use something other than Cell

This line defines the stack @code{data-stack}, which uses the stack
pointer @code{sp}, and each item has the basic type @code{Cell}; other
types have to fit into one or two @code{Cell}s (depending on whether the
type is @code{single} or @code{double} wide), and are converted from and
to Cells on accessing the @code{data-stack) with conversion macros
(@pxref{Conversion macros}).  Stacks grow towards lower addresses in
vmgen.

We can override the default stack of a stack item by using a stack
prefix.  E.g., consider the following instruction:

@example
lit ( #i -- i )
@end example

The VM instruction @code{lit} takes the item @code{i} from the
instruction stream (indicated by the prefix @code{#}, and pushes it on
the (default) data stack.  The stack prefix is not part of the variable
name.  Stack prefixes are defined like this:

@example
\E inst-stream stack-prefix #
@end example

This definition defines that the stack prefix @code{#} to specifies the
``stack'' @code{inst-stream}.  Since the instruction stream behaves a
little differently than an ordinary stack, it is predefined, and you do
not need to define it.

The instruction stream contains instructions and their immediate
arguments, so specifying that an argument comes from the instruction
stream indicates an immediate argument.  Of course, instruction stream
arguments can only appear to the left of @code{--} in the stack effect.
If there are multiple instruction stream arguments, the leftmost is the
first one (just as the intuition suggests).

@section Superinstructions

@section Stacks, types, and prefixes



Invocation

Input Syntax

Concepts: Front end, VM, Stacks,  Types, input stream

Contact