@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