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    1: @include version.texi
    2: 
    3: @c @ifnottex
    4: This file documents vmgen (Gforth @value{VERSION}).
    5: 
    6: @chapter Introduction
    7: 
    8: Vmgen is a tool for writing efficient interpreters.  It takes a simple
    9: virtual machine description and generates efficient C code for dealing
   10: with the virtual machine code in various ways (in particular, executing
   11: it).  The run-time efficiency of the resulting interpreters is usually
   12: within a factor of 10 of machine code produced by an optimizing
   13: compiler.
   14: 
   15: The interpreter design strategy supported by vmgen is to divide the
   16: interpreter into two parts:
   17: 
   18: @itemize @bullet
   19: 
   20: @item The @emph{front end} takes the source code of the language to be
   21: implemented, and translates it into virtual machine code.  This is
   22: similar to an ordinary compiler front end; typically an interpreter
   23: front-end performs no optimization, so it is relatively simple to
   24: implement and runs fast.
   25: 
   26: @item The @emph{virtual machine interpreter} executes the virtual
   27: machine code.
   28: 
   29: @end itemize
   30: 
   31: Such a division is usually used in interpreters, for modularity as well
   32: as for efficiency reasons.  The virtual machine code is typically passed
   33: between front end and virtual machine interpreter in memory, like in a
   34: load-and-go compiler; this avoids the complexity and time cost of
   35: writing the code to a file and reading it again.
   36: 
   37: A @emph{virtual machine} (VM) represents the program as a sequence of
   38: @emph{VM instructions}, following each other in memory, similar to real
   39: machine code.  Control flow occurs through VM branch instructions, like
   40: in a real machine.
   41: 
   42: In this setup, vmgen can generate most of the code dealing with virtual
   43: machine instructions from a simple description of the virtual machine
   44: instructions (@pxref...), in particular:
   45: 
   46: @table @emph
   47: 
   48: @item VM instruction execution
   49: 
   50: @item VM code generation
   51: Useful in the front end.
   52: 
   53: @item VM code decompiler
   54: Useful for debugging the front end.
   55: 
   56: @item VM code tracing
   57: Useful for debugging the front end and the VM interpreter.  You will
   58: typically provide other means for debugging the user's programs at the
   59: source level.
   60: 
   61: @item VM code profiling
   62: Useful for optimizing the VM insterpreter with superinstructions
   63: (@pxref...).
   64: 
   65: @end table
   66: 
   67: VMgen supports efficient interpreters though various optimizations, in
   68: particular
   69: 
   70: @itemize
   71: 
   72: @item Threaded code
   73: 
   74: @item Caching the top-of-stack in a register
   75: 
   76: @item Combining VM instructions into superinstructions
   77: 
   78: @item
   79: Replicating VM (super)instructions for better BTB prediction accuracy
   80: (not yet in vmgen-ex, but already in Gforth).
   81: 
   82: @end itemize
   83: 
   84: As a result, vmgen-based interpreters are only about an order of
   85: magintude slower than native code from an optimizing C compiler on small
   86: benchmarks; on large benchmarks, which spend more time in the run-time
   87: system, the slowdown is often less (e.g., the slowdown of a
   88: Vmgen-generated JVM interpreter over the best JVM JIT compiler we
   89: measured is only a factor of 2-3 for large benchmarks; some other JITs
   90: and all other interpreters we looked at were slower than our
   91: interpreter).
   92: 
   93: VMs are usually designed as stack machines (passing data between VM
   94: instructions on a stack), and vmgen supports such designs especially
   95: well; however, you can also use vmgen for implementing a register VM and
   96: still benefit from most of the advantages offered by vmgen.
   97: 
   98: There are many potential uses of the instruction descriptions that are
   99: not implemented at the moment, but we are open for feature requests, and
  100: we will implement new features if someone asks for them; so the feature
  101: list above is not exhaustive.
  102: 
  103: @c *********************************************************************
  104: @chapter Why interpreters?
  105: 
  106: Interpreters are a popular language implementation technique because
  107: they combine all three of the following advantages:
  108: 
  109: @itemize
  110: 
  111: @item Ease of implementation
  112: 
  113: @item Portability
  114: 
  115: @item Fast edit-compile-run cycle
  116: 
  117: @end itemize
  118: 
  119: The main disadvantage of interpreters is their run-time speed.  However,
  120: there are huge differences between different interpreters in this area:
  121: the slowdown over optimized C code on programs consisting of simple
  122: operations is typically a factor of 10 for the more efficient
  123: interpreters, and a factor of 1000 for the less efficient ones (the
  124: slowdown for programs executing complex operations is less, because the
  125: time spent in libraries for executing complex operations is the same in
  126: all implementation strategies).
  127: 
  128: Vmgen makes it even easier to implement interpreters.  It also supports
  129: techniques for building efficient interpreters.
  130: 
  131: @c ********************************************************************
  132: 
  133: @chapter Concepts
  134: 
  135: @c --------------------------------------------------------------------
  136: @section Front-end and virtual machine interpreter
  137: 
  138: @cindex front-end
  139: Interpretive systems are typically divided into a @emph{front end} that
  140: parses the input language and produces an intermediate representation
  141: for the program, and an interpreter that executes the intermediate
  142: representation of the program.
  143: 
  144: @cindex virtual machine
  145: @cindex VM
  146: @cindex instruction, VM
  147: For efficient interpreters the intermediate representation of choice is
  148: virtual machine code (rather than, e.g., an abstract syntax tree).
  149: @emph{Virtual machine} (VM) code consists of VM instructions arranged
  150: sequentially in memory; they are executed in sequence by the VM
  151: interpreter, except for VM branch instructions, which implement control
  152: structures.  The conceptual similarity to real machine code results in
  153: the name @emph{virtual machine}.
  154: 
  155: In this framework, vmgen supports building the VM interpreter and any
  156: other component dealing with VM instructions.  It does not have any
  157: support for the front end, apart from VM code generation support.  The
  158: front end can be implemented with classical compiler front-end
  159: techniques, supported by tools like @command{flex} and @command{bison}.
  160: 
  161: The intermediate representation is usually just internal to the
  162: interpreter, but some systems also support saving it to a file, either
  163: as an image file, or in a full-blown linkable file format (e.g., JVM).
  164: Vmgen currently has no special support for such features, but the
  165: information in the instruction descriptions can be helpful, and we are
  166: open for feature requests and suggestions.
  167: 
  168: @section Data handling
  169: 
  170: @cindex stack machine
  171: @cindex register machine
  172: Most VMs use one or more stacks for passing temporary data between VM
  173: instructions.  Another option is to use a register machine architecture
  174: for the virtual machine; however, this option is either slower or
  175: significantly more complex to implement than a stack machine architecture.
  176: 
  177: Vmgen has special support and optimizations for stack VMs, making their
  178: implementation easy and efficient.
  179: 
  180: You can also implement a register VM with vmgen (@pxref{Register
  181: Machines}), and you will still profit from most vmgen features.
  182: 
  183: @cindex stack item size
  184: @cindex size, stack items
  185: Stack items all have the same size, so they typically will be as wide as
  186: an integer, pointer, or floating-point value.  Vmgen supports treating
  187: two consecutive stack items as a single value, but anything larger is
  188: best kept in some other memory area (e.g., the heap), with pointers to
  189: the data on the stack.
  190: 
  191: @cindex instruction stream
  192: @cindex immediate arguments
  193: Another source of data is immediate arguments VM instructions (in the VM
  194: instruction stream).  The VM instruction stream is handled similar to a
  195: stack in vmgen.
  196: 
  197: @cindex garbage collection
  198: @cindex reference counting
  199: Vmgen has no built-in support for nor restrictions against @emph{garbage
  200: collection}.  If you need garbage collection, you need to provide it in
  201: your run-time libraries.  Using @emph{reference counting} is probably
  202: harder, but might be possible (contact us if you are interested).
  203: @c reference counting might be possible by including counting code in 
  204: @c the conversion macros.
  205: 
  206: @c *************************************************************
  207: @chapter Invoking vmgen
  208: 
  209: The usual way to invoke vmgen is as follows:
  210: 
  211: @example
  212: vmgen @var{infile}
  213: @end example
  214: 
  215: Here @var{infile} is the VM instruction description file, which usually
  216: ends in @file{.vmg}.  The output filenames are made by taking the
  217: basename of @file{infile} (i.e., the output files will be created in the
  218: current working directory) and replacing @file{.vmg} with @file{-vm.i},
  219: @file{-disasm.i}, @file{-gen.i}, @file{-labels.i}, @file{-profile.i},
  220: and @file{-peephole.i}.  E.g., @command{bison hack/foo.vmg} will create
  221: @file{foo-vm.i} etc.
  222: 
  223: The command-line options supported by vmgen are
  224: 
  225: @table @option
  226: 
  227: @cindex -h, command-line option
  228: @cindex --help, command-line option
  229: @item --help
  230: @itemx -h
  231: Print a message about the command-line options
  232: 
  233: @cindex -v, command-line option
  234: @cindex --version, command-line option
  235: @item --version
  236: @itemx -v
  237: Print version and exit
  238: @end table
  239: 
  240: @c env vars GFORTHDIR GFORTHDATADIR
  241: 
  242: @c ***************************************************************
  243: @chapter Input File Format
  244: 
  245: Vmgen takes as input a file containing specifications of virtual machine
  246: instructions.  This file usually has a name ending in @file{.vmg}.
  247: 
  248: The examples are taken from the example in @file{vmgen-ex}.
  249: 
  250: @section Input File Grammar
  251: 
  252: The grammar is in EBNF format, with @code{@var{a}|@var{b}} meaning
  253: ``@var{a} or @var{b}'', @code{@{@var{c}@}} meaning 0 or more repetitions
  254: of @var{c} and @code{[@var{d}]} meaning 0 or 1 repetitions of @var{d}.
  255: 
  256: Vmgen input is not free-format, so you have to take care where you put
  257: spaces and especially newlines; it's not as bad as makefiles, though:
  258: any sequence of spaces and tabs is equivalent to a single space.
  259: 
  260: @example
  261: description: {instruction|comment|eval-escape}
  262: 
  263: instruction: simple-inst|superinst
  264: 
  265: simple-inst: ident " (" stack-effect " )" newline c-code newline newline
  266: 
  267: stack-effect: {ident} " --" {ident}
  268: 
  269: super-inst: ident " =" ident {ident}  
  270: 
  271: comment:      "\ "  text newline
  272: 
  273: eval-escape:  "\e " text newline
  274: @end example
  275: @c \+ \- \g \f \c
  276: 
  277: Note that the @code{\}s in this grammar are meant literally, not as
  278: C-style encodings for no-printable characters.
  279: 
  280: The C code in @code{simple-inst} must not contain empty lines (because
  281: vmgen would mistake that as the end of the simple-inst.  The text in
  282: @code{comment} and @code{eval-escape} must not contain a newline.
  283: @code{Ident} must conform to the usual conventions of C identifiers
  284: (otherwise the C compiler would choke on the vmgen output).
  285: 
  286: Vmgen understands a few extensions beyond the grammar given here, but
  287: these extensions are only useful for building Gforth.  You can find a
  288: description of the format used for Gforth in @file{prim}.
  289: 
  290: @subsection
  291: @c woanders?
  292: The text in @code{eval-escape} is Forth code that is evaluated when
  293: vmgen reads the line.  If you do not know (and do not want to learn)
  294: Forth, you can build the text according to the following grammar; these
  295: rules are normally all Forth you need for using vmgen:
  296: 
  297: @example
  298: text: stack-decl|type-prefix-decl|stack-prefix-decl
  299: 
  300: stack-decl: "stack " ident ident ident
  301: type-prefix-decl: 
  302:     's" ' string '" ' ("single"|"double") ident "type-prefix" ident
  303: stack-prefix-decl:  ident "stack-prefix" string
  304: @end example
  305: 
  306: Note that the syntax of this code is not checked thoroughly (there are
  307: many other Forth program fragments that could be written there).
  308: 
  309: If you know Forth, the stack effects of the non-standard words involved
  310: are:
  311: 
  312: @example
  313: stack        ( "name" "pointer" "type" -- )
  314:              ( name execution: -- stack )
  315: type-prefix  ( addr u xt1 xt2 n stack "prefix" -- )
  316: single       ( -- xt1 xt2 n )
  317: double       ( -- xt1 xt2 n )
  318: stack-prefix ( stack "prefix" -- )
  319: @end example
  320: 
  321: @section Simple instructions
  322: 
  323: We will use the following simple VM instruction description as example:
  324: 
  325: @example
  326: sub ( i1 i2 -- i )
  327: i = i1-i2;
  328: @end example
  329: 
  330: The first line specifies the name of the VM instruction (@code{sub}) and
  331: its stack effect (@code{i1 i2 -- i}).  The rest of the description is
  332: just plain C code.
  333: 
  334: @cindex stack effect
  335: The stack effect specifies that @code{sub} pulls two integers from the
  336: data stack and puts them in the C variable @code{i1} and @code{i2} (with
  337: the rightmost item (@code{i2}) taken from the top of stack) and later
  338: pushes one integer (@code{i)) on the data stack (the rightmost item is
  339: on the top afterwards).
  340: 
  341: How do we know the type and stack of the stack items?  Vmgen uses
  342: prefixes, similar to Fortran; in contrast to Fortran, you have to
  343: define the prefix first:
  344: 
  345: @example
  346: \E s" Cell"   single data-stack type-prefix i
  347: @end example
  348: 
  349: This defines the prefix @code{i} to refer to the type @code{Cell}
  350: (defined as @code{long} in @file{mini.h}) and, by default, to the
  351: @code{data-stack}.  It also specifies that this type takes one stack
  352: item (@code{single}).  The type prefix is part of the variable name.
  353: 
  354: Before we can use @code{data-stack} in this way, we have to define it:
  355: 
  356: @example
  357: \E stack data-stack sp Cell
  358: @end example
  359: @c !! use something other than Cell
  360: 
  361: This line defines the stack @code{data-stack}, which uses the stack
  362: pointer @code{sp}, and each item has the basic type @code{Cell}; other
  363: types have to fit into one or two @code{Cell}s (depending on whether the
  364: type is @code{single} or @code{double} wide), and are converted from and
  365: to Cells on accessing the @code{data-stack) with conversion macros
  366: (@pxref{Conversion macros}).  Stacks grow towards lower addresses in
  367: vmgen.
  368: 
  369: We can override the default stack of a stack item by using a stack
  370: prefix.  E.g., consider the following instruction:
  371: 
  372: @example
  373: lit ( #i -- i )
  374: @end example
  375: 
  376: The VM instruction @code{lit} takes the item @code{i} from the
  377: instruction stream (indicated by the prefix @code{#}, and pushes it on
  378: the (default) data stack.  The stack prefix is not part of the variable
  379: name.  Stack prefixes are defined like this:
  380: 
  381: @example
  382: \E inst-stream stack-prefix #
  383: @end example
  384: 
  385: This definition defines that the stack prefix @code{#} to specifies the
  386: ``stack'' @code{inst-stream}.  Since the instruction stream behaves a
  387: little differently than an ordinary stack, it is predefined, and you do
  388: not need to define it.
  389: 
  390: The instruction stream contains instructions and their immediate
  391: arguments, so specifying that an argument comes from the instruction
  392: stream indicates an immediate argument.  Of course, instruction stream
  393: arguments can only appear to the left of @code{--} in the stack effect.
  394: If there are multiple instruction stream arguments, the leftmost is the
  395: first one (just as the intuition suggests).
  396: 
  397: @section Superinstructions
  398: 
  399: @section Stacks, types, and prefixes
  400: 
  401: 
  402: 
  403: Invocation
  404: 
  405: Input Syntax
  406: 
  407: Concepts: Front end, VM, Stacks,  Types, input stream
  408: 
  409: Contact

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