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    1: \input texinfo    @c -*-texinfo-*-
    2: @comment %**start of header
    3: @setfilename vmgen.info
    4: @include version.texi
    5: @settitle Vmgen (Gforth @value{VERSION})
    6: @c @syncodeindex pg cp
    7: @comment %**end of header
    8: @copying
    9: This manual is for Vmgen
   10: (version @value{VERSION}, @value{UPDATED}),
   11: the virtual machine interpreter generator
   12: 
   13: Copyright @copyright{} 2002 Free Software Foundation, Inc.
   14: 
   15: @quotation
   16: Permission is granted to copy, distribute and/or modify this document
   17: under the terms of the GNU Free Documentation License, Version 1.1 or
   18: any later version published by the Free Software Foundation; with no
   19: Invariant Sections, with the Front-Cover texts being ``A GNU Manual,''
   20: and with the Back-Cover Texts as in (a) below.  A copy of the
   21: license is included in the section entitled ``GNU Free Documentation
   22: License.''
   23: 
   24: (a) The FSF's Back-Cover Text is: ``You have freedom to copy and modify
   25: this GNU Manual, like GNU software.  Copies published by the Free
   26: Software Foundation raise funds for GNU development.''
   27: @end quotation
   28: @end copying
   29: 
   30: @dircategory GNU programming tools
   31: @direntry
   32: * Vmgen: (vmgen).               Interpreter generator
   33: @end direntry
   34: 
   35: @titlepage
   36: @title Vmgen
   37: @subtitle for Gforth version @value{VERSION}, @value{UPDATED}
   38: @author M. Anton Ertl (@email{anton@@mips.complang.tuwien.ac.at})
   39: @page
   40: @vskip 0pt plus 1filll
   41: @insertcopying
   42: @end titlepage
   43: 
   44: @contents
   45: 
   46: @ifnottex
   47: @node Top, Introduction, (dir), (dir)
   48: @top Vmgen
   49: 
   50: @insertcopying
   51: @end ifnottex
   52: 
   53: @menu
   54: * Introduction::                What can Vmgen do for you?
   55: * Why interpreters?::           Advantages and disadvantages
   56: * Concepts::                    VM interpreter background
   57: * Invoking Vmgen::              
   58: * Example::                     
   59: * Input File Format::           
   60: * Using the generated code::    
   61: * Changes::                     from earlier versions
   62: * Contact::                     Bug reporting etc.
   63: * Copying This Manual::         Manual License
   64: * Index::                       
   65: 
   66: @detailmenu
   67:  --- The Detailed Node Listing ---
   68: 
   69: Concepts
   70: 
   71: * Front end and VM interpreter::  Modularizing an interpretive system
   72: * Data handling::               Stacks, registers, immediate arguments
   73: * Dispatch::                    From one VM instruction to the next
   74: 
   75: Example
   76: 
   77: * Example overview::            
   78: * Using profiling to create superinstructions::  
   79: 
   80: Input File Format
   81: 
   82: * Input File Grammar::          
   83: * Simple instructions::         
   84: * Superinstructions::           
   85: * Register Machines::           How to define register VM instructions
   86: 
   87: Simple instructions
   88: 
   89: * C Code Macros::               Macros recognized by Vmgen
   90: * C Code restrictions::         Vmgen makes assumptions about C code
   91: 
   92: Using the generated code
   93: 
   94: * VM engine::                   Executing VM code
   95: * VM instruction table::        
   96: * VM code generation::          Creating VM code (in the front-end)
   97: * Peephole optimization::       Creating VM superinstructions
   98: * VM disassembler::             for debugging the front end
   99: * VM profiler::                 for finding worthwhile superinstructions
  100: 
  101: Copying This Manual
  102: 
  103: * GNU Free Documentation License::  License for copying this manual.
  104: 
  105: @end detailmenu
  106: @end menu
  107: 
  108: @c @ifnottex
  109: @c This file documents Vmgen (Gforth @value{VERSION}).
  110: 
  111: @c ************************************************************
  112: @node Introduction, Why interpreters?, Top, Top
  113: @chapter Introduction
  114: 
  115: Vmgen is a tool for writing efficient interpreters.  It takes a simple
  116: virtual machine description and generates efficient C code for dealing
  117: with the virtual machine code in various ways (in particular, executing
  118: it).  The run-time efficiency of the resulting interpreters is usually
  119: within a factor of 10 of machine code produced by an optimizing
  120: compiler.
  121: 
  122: The interpreter design strategy supported by Vmgen is to divide the
  123: interpreter into two parts:
  124: 
  125: @itemize @bullet
  126: 
  127: @item The @emph{front end} takes the source code of the language to be
  128: implemented, and translates it into virtual machine code.  This is
  129: similar to an ordinary compiler front end; typically an interpreter
  130: front-end performs no optimization, so it is relatively simple to
  131: implement and runs fast.
  132: 
  133: @item The @emph{virtual machine interpreter} executes the virtual
  134: machine code.
  135: 
  136: @end itemize
  137: 
  138: Such a division is usually used in interpreters, for modularity as well
  139: as for efficiency.  The virtual machine code is typically passed between
  140: front end and virtual machine interpreter in memory, like in a
  141: load-and-go compiler; this avoids the complexity and time cost of
  142: writing the code to a file and reading it again.
  143: 
  144: A @emph{virtual machine} (VM) represents the program as a sequence of
  145: @emph{VM instructions}, following each other in memory, similar to real
  146: machine code.  Control flow occurs through VM branch instructions, like
  147: in a real machine.
  148: 
  149: @cindex functionality features overview
  150: In this setup, Vmgen can generate most of the code dealing with virtual
  151: machine instructions from a simple description of the virtual machine
  152: instructions (@pxref{Input File Format}), in particular:
  153: 
  154: @table @asis
  155: 
  156: @item VM instruction execution
  157: 
  158: @item VM code generation
  159: Useful in the front end.
  160: 
  161: @item VM code decompiler
  162: Useful for debugging the front end.
  163: 
  164: @item VM code tracing
  165: Useful for debugging the front end and the VM interpreter.  You will
  166: typically provide other means for debugging the user's programs at the
  167: source level.
  168: 
  169: @item VM code profiling
  170: Useful for optimizing the VM interpreter with superinstructions
  171: (@pxref{VM profiler}).
  172: 
  173: @end table
  174: 
  175: @cindex efficiency features overview
  176: @noindent
  177: Vmgen supports efficient interpreters though various optimizations, in
  178: particular
  179: 
  180: @itemize @bullet
  181: 
  182: @item Threaded code
  183: 
  184: @item Caching the top-of-stack in a register
  185: 
  186: @item Combining VM instructions into superinstructions
  187: 
  188: @item
  189: Replicating VM (super)instructions for better BTB prediction accuracy
  190: (not yet in vmgen-ex, but already in Gforth).
  191: 
  192: @end itemize
  193: 
  194: @cindex speed for JVM
  195: As a result, Vmgen-based interpreters are only about an order of
  196: magnitude slower than native code from an optimizing C compiler on small
  197: benchmarks; on large benchmarks, which spend more time in the run-time
  198: system, the slowdown is often less (e.g., the slowdown of a
  199: Vmgen-generated JVM interpreter over the best JVM JIT compiler we
  200: measured is only a factor of 2-3 for large benchmarks; some other JITs
  201: and all other interpreters we looked at were slower than our
  202: interpreter).
  203: 
  204: VMs are usually designed as stack machines (passing data between VM
  205: instructions on a stack), and Vmgen supports such designs especially
  206: well; however, you can also use Vmgen for implementing a register VM
  207: (@pxref{Register Machines}) and still benefit from most of the advantages
  208: offered by Vmgen.
  209: 
  210: There are many potential uses of the instruction descriptions that are
  211: not implemented at the moment, but we are open for feature requests, and
  212: we will implement new features if someone asks for them; so the feature
  213: list above is not exhaustive.
  214: 
  215: @c *********************************************************************
  216: @node Why interpreters?, Concepts, Introduction, Top
  217: @chapter Why interpreters?
  218: @cindex interpreters, advantages
  219: @cindex advantages of interpreters
  220: @cindex advantages of vmgen
  221: 
  222: Interpreters are a popular language implementation technique because
  223: they combine all three of the following advantages:
  224: 
  225: @itemize @bullet
  226: 
  227: @item Ease of implementation
  228: 
  229: @item Portability
  230: 
  231: @item Fast edit-compile-run cycle
  232: 
  233: @end itemize
  234: 
  235: Vmgen makes it even easier to implement interpreters.
  236: 
  237: @cindex speed of interpreters
  238: The main disadvantage of interpreters is their run-time speed.  However,
  239: there are huge differences between different interpreters in this area:
  240: the slowdown over optimized C code on programs consisting of simple
  241: operations is typically a factor of 10 for the more efficient
  242: interpreters, and a factor of 1000 for the less efficient ones (the
  243: slowdown for programs executing complex operations is less, because the
  244: time spent in libraries for executing complex operations is the same in
  245: all implementation strategies).
  246: 
  247: Vmgen supports techniques for building efficient interpreters.
  248: 
  249: @c ********************************************************************
  250: @node Concepts, Invoking Vmgen, Why interpreters?, Top
  251: @chapter Concepts
  252: 
  253: @menu
  254: * Front end and VM interpreter::  Modularizing an interpretive system
  255: * Data handling::               Stacks, registers, immediate arguments
  256: * Dispatch::                    From one VM instruction to the next
  257: @end menu
  258: 
  259: @c --------------------------------------------------------------------
  260: @node Front end and VM interpreter, Data handling, Concepts, Concepts
  261: @section Front end and VM interpreter
  262: @cindex modularization of interpreters
  263: 
  264: @cindex front-end
  265: Interpretive systems are typically divided into a @emph{front end} that
  266: parses the input language and produces an intermediate representation
  267: for the program, and an interpreter that executes the intermediate
  268: representation of the program.
  269: 
  270: @cindex virtual machine
  271: @cindex VM
  272: @cindex VM instruction
  273: @cindex instruction, VM
  274: @cindex VM branch instruction
  275: @cindex branch instruction, VM
  276: @cindex VM register
  277: @cindex register, VM
  278: @cindex opcode, VM instruction
  279: @cindex immediate argument, VM instruction
  280: For efficient interpreters the intermediate representation of choice is
  281: virtual machine code (rather than, e.g., an abstract syntax tree).
  282: @emph{Virtual machine} (VM) code consists of VM instructions arranged
  283: sequentially in memory; they are executed in sequence by the VM
  284: interpreter, but VM branch instructions can change the control flow and
  285: are used for implementing control structures.  The conceptual similarity
  286: to real machine code results in the name @emph{virtual machine}.
  287: Various terms similar to terms for real machines are used; e.g., there
  288: are @emph{VM registers} (like the instruction pointer and stack
  289: pointer(s)), and the VM instruction consists of an @emph{opcode} and
  290: @emph{immediate arguments}.
  291: 
  292: In this framework, Vmgen supports building the VM interpreter and any
  293: other component dealing with VM instructions.  It does not have any
  294: support for the front end, apart from VM code generation support.  The
  295: front end can be implemented with classical compiler front-end
  296: techniques, supported by tools like @command{flex} and @command{bison}.
  297: 
  298: The intermediate representation is usually just internal to the
  299: interpreter, but some systems also support saving it to a file, either
  300: as an image file, or in a full-blown linkable file format (e.g., JVM).
  301: Vmgen currently has no special support for such features, but the
  302: information in the instruction descriptions can be helpful, and we are
  303: open for feature requests and suggestions.
  304: 
  305: @c --------------------------------------------------------------------
  306: @node Data handling, Dispatch, Front end and VM interpreter, Concepts
  307: @section Data handling
  308: 
  309: @cindex stack machine
  310: @cindex register machine
  311: Most VMs use one or more stacks for passing temporary data between VM
  312: instructions.  Another option is to use a register machine architecture
  313: for the virtual machine; however, this option is either slower or
  314: significantly more complex to implement than a stack machine architecture.
  315: 
  316: Vmgen has special support and optimizations for stack VMs, making their
  317: implementation easy and efficient.
  318: 
  319: You can also implement a register VM with Vmgen (@pxref{Register
  320: Machines}), and you will still profit from most Vmgen features.
  321: 
  322: @cindex stack item size
  323: @cindex size, stack items
  324: Stack items all have the same size, so they typically will be as wide as
  325: an integer, pointer, or floating-point value.  Vmgen supports treating
  326: two consecutive stack items as a single value, but anything larger is
  327: best kept in some other memory area (e.g., the heap), with pointers to
  328: the data on the stack.
  329: 
  330: @cindex instruction stream
  331: @cindex immediate arguments
  332: Another source of data is immediate arguments VM instructions (in the VM
  333: instruction stream).  The VM instruction stream is handled similar to a
  334: stack in Vmgen.
  335: 
  336: @cindex garbage collection
  337: @cindex reference counting
  338: Vmgen has no built-in support for, nor restrictions against
  339: @emph{garbage collection}.  If you need garbage collection, you need to
  340: provide it in your run-time libraries.  Using @emph{reference counting}
  341: is probably harder, but might be possible (contact us if you are
  342: interested).
  343: @c reference counting might be possible by including counting code in 
  344: @c the conversion macros.
  345: 
  346: @c --------------------------------------------------------------------
  347: @node Dispatch,  , Data handling, Concepts
  348: @section Dispatch
  349: @cindex Dispatch of VM instructions
  350: @cindex main interpreter loop
  351: 
  352: Understanding this section is probably not necessary for using Vmgen,
  353: but it may help.  You may want to skip it now, and read it if you find statements about dispatch methods confusing.
  354: 
  355: After executing one VM instruction, the VM interpreter has to dispatch
  356: the next VM instruction (Vmgen calls the dispatch routine @samp{NEXT}).
  357: Vmgen supports two methods of dispatch:
  358: 
  359: @table @asis
  360: 
  361: @item switch dispatch
  362: @cindex switch dispatch
  363: In this method the VM interpreter contains a giant @code{switch}
  364: statement, with one @code{case} for each VM instruction.  The VM
  365: instruction opcodes are represented by integers (e.g., produced by an
  366: @code{enum}) in the VM code, and dispatch occurs by loading the next
  367: opcode, @code{switch}ing on it, and continuing at the appropriate
  368: @code{case}; after executing the VM instruction, the VM interpreter
  369: jumps back to the dispatch code.
  370: 
  371: @item threaded code
  372: @cindex threaded code
  373: This method represents a VM instruction opcode by the address of the
  374: start of the machine code fragment for executing the VM instruction.
  375: Dispatch consists of loading this address, jumping to it, and
  376: incrementing the VM instruction pointer.  Typically the threaded-code
  377: dispatch code is appended directly to the code for executing the VM
  378: instruction.  Threaded code cannot be implemented in ANSI C, but it can
  379: be implemented using GNU C's labels-as-values extension (@pxref{Labels
  380: as Values, , Labels as Values, gcc.info, GNU C Manual}).
  381: 
  382: @end table
  383: 
  384: Threaded code can be twice as fast as switch dispatch, depending on the
  385: interpreter, the benchmark, and the machine.
  386: 
  387: @c *************************************************************
  388: @node Invoking Vmgen, Example, Concepts, Top
  389: @chapter Invoking Vmgen
  390: @cindex Invoking Vmgen
  391: 
  392: The usual way to invoke Vmgen is as follows:
  393: 
  394: @example
  395: vmgen @var{infile}
  396: @end example
  397: 
  398: Here @var{infile} is the VM instruction description file, which usually
  399: ends in @file{.vmg}.  The output filenames are made by taking the
  400: basename of @file{infile} (i.e., the output files will be created in the
  401: current working directory) and replacing @file{.vmg} with @file{-vm.i},
  402: @file{-disasm.i}, @file{-gen.i}, @file{-labels.i}, @file{-profile.i},
  403: and @file{-peephole.i}.  E.g., @command{vmgen hack/foo.vmg} will create
  404: @file{foo-vm.i} etc.
  405: 
  406: The command-line options supported by Vmgen are
  407: 
  408: @table @option
  409: 
  410: @cindex -h, command-line option
  411: @cindex --help, command-line option
  412: @item --help
  413: @itemx -h
  414: Print a message about the command-line options
  415: 
  416: @cindex -v, command-line option
  417: @cindex --version, command-line option
  418: @item --version
  419: @itemx -v
  420: Print version and exit
  421: @end table
  422: 
  423: @c env vars GFORTHDIR GFORTHDATADIR
  424: 
  425: @c ****************************************************************
  426: @node Example, Input File Format, Invoking Vmgen, Top
  427: @chapter Example
  428: @cindex example of a Vmgen-based interpreter
  429: 
  430: @menu
  431: * Example overview::            
  432: * Using profiling to create superinstructions::  
  433: @end menu
  434: 
  435: @c --------------------------------------------------------------------
  436: @node Example overview, Using profiling to create superinstructions, Example, Example
  437: @section Example overview
  438: @cindex example overview
  439: @cindex @file{vmgen-ex}
  440: @cindex @file{vmgen-ex2}
  441: 
  442: There are two versions of the same example for using Vmgen:
  443: @file{vmgen-ex} and @file{vmgen-ex2} (you can also see Gforth as
  444: example, but it uses additional (undocumented) features, and also
  445: differs in some other respects).  The example implements @emph{mini}, a
  446: tiny Modula-2-like language with a small JavaVM-like virtual machine.
  447: 
  448: The difference between the examples is that @file{vmgen-ex} uses many
  449: casts, and @file{vmgen-ex2} tries to avoids most casts and uses unions
  450: instead.  In the rest of this manual we usually mention just files in
  451: @file{vmgen-ex}; if you want to use unions, use the equivalent file in
  452: @file{vmgen-ex2}.
  453: @cindex unions example
  454: @cindex casts example
  455: 
  456: The files provided with each example are:
  457: @cindex example files
  458: 
  459: @example
  460: Makefile
  461: README
  462: disasm.c           wrapper file
  463: engine.c           wrapper file
  464: peephole.c         wrapper file
  465: profile.c          wrapper file
  466: mini-inst.vmg      simple VM instructions
  467: mini-super.vmg     superinstructions (empty at first)
  468: mini.h             common declarations
  469: mini.l             scanner
  470: mini.y             front end (parser, VM code generator)
  471: support.c          main() and other support functions
  472: fib.mini           example mini program
  473: simple.mini        example mini program
  474: test.mini          example mini program (tests everything)
  475: test.out           test.mini output
  476: stat.awk           script for aggregating profile information
  477: peephole-blacklist list of instructions not allowed in superinstructions
  478: seq2rule.awk       script for creating superinstructions
  479: @end example
  480: 
  481: For your own interpreter, you would typically copy the following files
  482: and change little, if anything:
  483: @cindex wrapper files
  484: 
  485: @example
  486: disasm.c           wrapper file
  487: engine.c           wrapper file
  488: peephole.c         wrapper file
  489: profile.c          wrapper file
  490: stat.awk           script for aggregating profile information
  491: seq2rule.awk       script for creating superinstructions
  492: @end example
  493: 
  494: @noindent
  495: You would typically change much in or replace the following files:
  496: 
  497: @example
  498: Makefile
  499: mini-inst.vmg      simple VM instructions
  500: mini.h             common declarations
  501: mini.l             scanner
  502: mini.y             front end (parser, VM code generator)
  503: support.c          main() and other support functions
  504: peephole-blacklist list of instructions not allowed in superinstructions
  505: @end example
  506: 
  507: You can build the example by @code{cd}ing into the example's directory,
  508: and then typing @code{make}; you can check that it works with @code{make
  509: check}.  You can run run mini programs like this:
  510: 
  511: @example
  512: ./mini fib.mini
  513: @end example
  514: 
  515: To learn about the options, type @code{./mini -h}.
  516: 
  517: @c --------------------------------------------------------------------
  518: @node Using profiling to create superinstructions,  , Example overview, Example
  519: @section Using profiling to create superinstructions
  520: @cindex profiling example
  521: @cindex superinstructions example
  522: 
  523: I have not added rules for this in the @file{Makefile} (there are many
  524: options for selecting superinstructions, and I did not want to hardcode
  525: one into the @file{Makefile}), but there are some supporting scripts, and
  526: here's an example:
  527: 
  528: Suppose you want to use @file{fib.mini} and @file{test.mini} as training
  529: programs, you get the profiles like this:
  530: 
  531: @example
  532: make fib.prof test.prof #takes a few seconds
  533: @end example
  534: 
  535: You can aggregate these profiles with @file{stat.awk}:
  536: 
  537: @example
  538: awk -f stat.awk fib.prof test.prof
  539: @end example
  540: 
  541: The result contains lines like:
  542: 
  543: @example
  544:       2      16        36910041 loadlocal lit
  545: @end example
  546: 
  547: This means that the sequence @code{loadlocal lit} statically occurs a
  548: total of 16 times in 2 profiles, with a dynamic execution count of
  549: 36910041.
  550: 
  551: The numbers can be used in various ways to select superinstructions.
  552: E.g., if you just want to select all sequences with a dynamic
  553: execution count exceeding 10000, you would use the following pipeline:
  554: 
  555: @example
  556: awk -f stat.awk fib.prof test.prof|
  557: awk '$3>=10000'|                #select sequences
  558: fgrep -v -f peephole-blacklist| #eliminate wrong instructions
  559: awk -f seq2rule.awk|  #transform sequences into superinstruction rules
  560: sort -k 3 >mini-super.vmg       #sort sequences
  561: @end example
  562: 
  563: The file @file{peephole-blacklist} contains all instructions that
  564: directly access a stack or stack pointer (for mini: @code{call},
  565: @code{return}); the sort step is necessary to ensure that prefixes
  566: preceed larger superinstructions.
  567: 
  568: Now you can create a version of mini with superinstructions by just
  569: saying @samp{make}
  570: 
  571: 
  572: @c ***************************************************************
  573: @node Input File Format, Using the generated code, Example, Top
  574: @chapter Input File Format
  575: @cindex input file format
  576: @cindex format, input file
  577: 
  578: Vmgen takes as input a file containing specifications of virtual machine
  579: instructions.  This file usually has a name ending in @file{.vmg}.
  580: 
  581: Most examples are taken from the example in @file{vmgen-ex}.
  582: 
  583: @menu
  584: * Input File Grammar::          
  585: * Simple instructions::         
  586: * Superinstructions::           
  587: * Register Machines::           How to define register VM instructions
  588: @end menu
  589: 
  590: @c --------------------------------------------------------------------
  591: @node Input File Grammar, Simple instructions, Input File Format, Input File Format
  592: @section Input File Grammar
  593: @cindex grammar, input file
  594: @cindex input file grammar
  595: 
  596: The grammar is in EBNF format, with @code{@var{a}|@var{b}} meaning
  597: ``@var{a} or @var{b}'', @code{@{@var{c}@}} meaning 0 or more repetitions
  598: of @var{c} and @code{[@var{d}]} meaning 0 or 1 repetitions of @var{d}.
  599: 
  600: @cindex free-format, not
  601: Vmgen input is not free-format, so you have to take care where you put
  602: spaces and especially newlines; it's not as bad as makefiles, though:
  603: any sequence of spaces and tabs is equivalent to a single space.
  604: 
  605: @example
  606: description: @{instruction|comment|eval-escape@}
  607: 
  608: instruction: simple-inst|superinst
  609: 
  610: simple-inst: ident ' (' stack-effect ' )' newline c-code newline newline
  611: 
  612: stack-effect: @{ident@} ' --' @{ident@}
  613: 
  614: super-inst: ident ' =' ident @{ident@}  
  615: 
  616: comment:      '\ '  text newline
  617: 
  618: eval-escape:  '\e ' text newline
  619: @end example
  620: @c \+ \- \g \f \c
  621: 
  622: Note that the @code{\}s in this grammar are meant literally, not as
  623: C-style encodings for non-printable characters.
  624: 
  625: The C code in @code{simple-inst} must not contain empty lines (because
  626: Vmgen would mistake that as the end of the simple-inst.  The text in
  627: @code{comment} and @code{eval-escape} must not contain a newline.
  628: @code{Ident} must conform to the usual conventions of C identifiers
  629: (otherwise the C compiler would choke on the Vmgen output).
  630: 
  631: Vmgen understands a few extensions beyond the grammar given here, but
  632: these extensions are only useful for building Gforth.  You can find a
  633: description of the format used for Gforth in @file{prim}.
  634: 
  635: @subsection Eval escapes
  636: @cindex escape to Forth
  637: @cindex eval escape
  638: 
  639: @c woanders?
  640: The text in @code{eval-escape} is Forth code that is evaluated when
  641: Vmgen reads the line.  If you do not know (and do not want to learn)
  642: Forth, you can build the text according to the following grammar; these
  643: rules are normally all Forth you need for using Vmgen:
  644: 
  645: @example
  646: text: stack-decl|type-prefix-decl|stack-prefix-decl
  647: 
  648: stack-decl: 'stack ' ident ident ident
  649: type-prefix-decl: 
  650:     's" ' string '" ' ('single'|'double') ident 'type-prefix' ident
  651: stack-prefix-decl:  ident 'stack-prefix' string
  652: @end example
  653: 
  654: Note that the syntax of this code is not checked thoroughly (there are
  655: many other Forth program fragments that could be written there).
  656: 
  657: If you know Forth, the stack effects of the non-standard words involved
  658: are:
  659: @findex stack
  660: @findex type-prefix
  661: @findex single
  662: @findex double
  663: @findex stack-prefix
  664: @example
  665: stack        ( "name" "pointer" "type" -- )
  666:              ( name execution: -- stack )
  667: type-prefix  ( addr u xt1 xt2 n stack "prefix" -- )
  668: single       ( -- xt1 xt2 n )
  669: double       ( -- xt1 xt2 n )
  670: stack-prefix ( stack "prefix" -- )
  671: @end example
  672: 
  673: 
  674: @c --------------------------------------------------------------------
  675: @node Simple instructions, Superinstructions, Input File Grammar, Input File Format
  676: @section Simple instructions
  677: @cindex simple VM instruction
  678: @cindex instruction, simple VM
  679: 
  680: We will use the following simple VM instruction description as example:
  681: 
  682: @example
  683: sub ( i1 i2 -- i )
  684: i = i1-i2;
  685: @end example
  686: 
  687: The first line specifies the name of the VM instruction (@code{sub}) and
  688: its stack effect (@code{i1 i2 -- i}).  The rest of the description is
  689: just plain C code.
  690: 
  691: @cindex stack effect
  692: @cindex effect, stack
  693: The stack effect specifies that @code{sub} pulls two integers from the
  694: data stack and puts them in the C variables @code{i1} and @code{i2}
  695: (with the rightmost item (@code{i2}) taken from the top of stack;
  696: intuition: if you push @code{i1}, then @code{i2} on the stack, the
  697: resulting stack picture is @code{i1 i2}) and later pushes one integer
  698: (@code{i}) on the data stack (the rightmost item is on the top
  699: afterwards).
  700: 
  701: @cindex prefix, type
  702: @cindex type prefix
  703: @cindex default stack of a type prefix
  704: How do we know the type and stack of the stack items?  Vmgen uses
  705: prefixes, similar to Fortran; in contrast to Fortran, you have to
  706: define the prefix first:
  707: 
  708: @example
  709: \E s" Cell"   single data-stack type-prefix i
  710: @end example
  711: 
  712: This defines the prefix @code{i} to refer to the type @code{Cell}
  713: (defined as @code{long} in @file{mini.h}) and, by default, to the
  714: @code{data-stack}.  It also specifies that this type takes one stack
  715: item (@code{single}).  The type prefix is part of the variable name.
  716: 
  717: @cindex stack definition
  718: @cindex defining a stack
  719: Before we can use @code{data-stack} in this way, we have to define it:
  720: 
  721: @example
  722: \E stack data-stack sp Cell
  723: @end example
  724: @c !! use something other than Cell
  725: 
  726: @cindex stack basic type
  727: @cindex basic type of a stack
  728: @cindex type of a stack, basic
  729: @cindex stack growth direction
  730: This line defines the stack @code{data-stack}, which uses the stack
  731: pointer @code{sp}, and each item has the basic type @code{Cell}; other
  732: types have to fit into one or two @code{Cell}s (depending on whether the
  733: type is @code{single} or @code{double} wide), and are cast from and to
  734: Cells on accessing the @code{data-stack} with type cast macros
  735: (@pxref{VM engine}).  Stacks grow towards lower addresses in
  736: Vmgen-erated interpreters.
  737: 
  738: @cindex stack prefix
  739: @cindex prefix, stack
  740: We can override the default stack of a stack item by using a stack
  741: prefix.  E.g., consider the following instruction:
  742: 
  743: @example
  744: lit ( #i -- i )
  745: @end example
  746: 
  747: The VM instruction @code{lit} takes the item @code{i} from the
  748: instruction stream (indicated by the prefix @code{#}), and pushes it on
  749: the (default) data stack.  The stack prefix is not part of the variable
  750: name.  Stack prefixes are defined like this:
  751: 
  752: @example
  753: \E inst-stream stack-prefix #
  754: @end example
  755: 
  756: This definition defines that the stack prefix @code{#} specifies the
  757: ``stack'' @code{inst-stream}.  Since the instruction stream behaves a
  758: little differently than an ordinary stack, it is predefined, and you do
  759: not need to define it.
  760: 
  761: @cindex instruction stream
  762: The instruction stream contains instructions and their immediate
  763: arguments, so specifying that an argument comes from the instruction
  764: stream indicates an immediate argument.  Of course, instruction stream
  765: arguments can only appear to the left of @code{--} in the stack effect.
  766: If there are multiple instruction stream arguments, the leftmost is the
  767: first one (just as the intuition suggests).
  768: 
  769: @menu
  770: * C Code Macros::               Macros recognized by Vmgen
  771: * C Code restrictions::         Vmgen makes assumptions about C code
  772: @end menu
  773: 
  774: @c --------------------------------------------------------------------
  775: @node C Code Macros, C Code restrictions, Simple instructions, Simple instructions
  776: @subsection C Code Macros
  777: @cindex macros recognized by Vmgen
  778: @cindex basic block, VM level
  779: 
  780: Vmgen recognizes the following strings in the C code part of simple
  781: instructions:
  782: 
  783: @table @code
  784: 
  785: @item SET_IP
  786: @findex SET_IP
  787: As far as Vmgen is concerned, a VM instruction containing this ends a VM
  788: basic block (used in profiling to delimit profiled sequences).  On the C
  789: level, this also sets the instruction pointer.
  790: 
  791: @item SUPER_END
  792: @findex SUPER_END
  793: This ends a basic block (for profiling), even if the instruction
  794: contains no @code{SET_IP}.
  795: 
  796: @item TAIL;
  797: @findex TAIL;
  798: Vmgen replaces @samp{TAIL;} with code for ending a VM instruction and
  799: dispatching the next VM instruction.  Even without a @samp{TAIL;} this
  800: happens automatically when control reaches the end of the C code.  If
  801: you want to have this in the middle of the C code, you need to use
  802: @samp{TAIL;}.  A typical example is a conditional VM branch:
  803: 
  804: @example
  805: if (branch_condition) @{
  806:   SET_IP(target); TAIL;
  807: @}
  808: /* implicit tail follows here */
  809: @end example
  810: 
  811: In this example, @samp{TAIL;} is not strictly necessary, because there
  812: is another one implicitly after the if-statement, but using it improves
  813: branch prediction accuracy slightly and allows other optimizations.
  814: 
  815: @item SUPER_CONTINUE
  816: @findex SUPER_CONTINUE
  817: This indicates that the implicit tail at the end of the VM instruction
  818: dispatches the sequentially next VM instruction even if there is a
  819: @code{SET_IP} in the VM instruction.  This enables an optimization that
  820: is not yet implemented in the vmgen-ex code (but in Gforth).  The
  821: typical application is in conditional VM branches:
  822: 
  823: @example
  824: if (branch_condition) @{
  825:   SET_IP(target); TAIL; /* now this TAIL is necessary */
  826: @}
  827: SUPER_CONTINUE;
  828: @end example
  829: 
  830: @end table
  831: 
  832: Note that Vmgen is not smart about C-level tokenization, comments,
  833: strings, or conditional compilation, so it will interpret even a
  834: commented-out SUPER_END as ending a basic block (or, e.g.,
  835: @samp{RETAIL;} as @samp{TAIL;}).  Conversely, Vmgen requires the literal
  836: presence of these strings; Vmgen will not see them if they are hiding in
  837: a C preprocessor macro.
  838: 
  839: 
  840: @c --------------------------------------------------------------------
  841: @node C Code restrictions,  , C Code Macros, Simple instructions
  842: @subsection C Code restrictions
  843: @cindex C code restrictions
  844: @cindex restrictions on C code
  845: @cindex assumptions about C code
  846: 
  847: @cindex accessing stack (pointer)
  848: @cindex stack pointer, access
  849: @cindex instruction pointer, access
  850: Vmgen generates code and performs some optimizations under the
  851: assumption that the user-supplied C code does not access the stack
  852: pointers or stack items, and that accesses to the instruction pointer
  853: only occur through special macros.  In general you should heed these
  854: restrictions.  However, if you need to break these restrictions, read
  855: the following.
  856: 
  857: Accessing a stack or stack pointer directly can be a problem for several
  858: reasons: 
  859: @cindex stack caching, restriction on C code
  860: @cindex superinstructions, restrictions on components
  861: 
  862: @itemize @bullet
  863: 
  864: @item
  865: Vmgen optionally supports caching the top-of-stack item in a local
  866: variable (that is allocated to a register).  This is the most frequent
  867: source of trouble.  You can deal with it either by not using
  868: top-of-stack caching (slowdown factor 1-1.4, depending on machine), or
  869: by inserting flushing code (e.g., @samp{IF_spTOS(sp[...] = spTOS);}) at
  870: the start and reloading code (e.g., @samp{IF_spTOS(spTOS = sp[0])}) at
  871: the end of problematic C code.  Vmgen inserts a stack pointer update
  872: before the start of the user-supplied C code, so the flushing code has
  873: to use an index that corrects for that.  In the future, this flushing
  874: may be done automatically by mentioning a special string in the C code.
  875: @c sometimes flushing and/or reloading unnecessary
  876: 
  877: @item
  878: The Vmgen-erated code loads the stack items from stack-pointer-indexed
  879: memory into variables before the user-supplied C code, and stores them
  880: from variables to stack-pointer-indexed memory afterwards.  If you do
  881: any writes to the stack through its stack pointer in your C code, it
  882: will not affact the variables, and your write may be overwritten by the
  883: stores after the C code.  Similarly, a read from a stack using a stack
  884: pointer will not reflect computations of stack items in the same VM
  885: instruction.
  886: 
  887: @item
  888: Superinstructions keep stack items in variables across the whole
  889: superinstruction.  So you should not include VM instructions, that
  890: access a stack or stack pointer, as components of superinstructions
  891: (@pxref{VM profiler}).
  892: 
  893: @end itemize
  894: 
  895: You should access the instruction pointer only through its special
  896: macros (@samp{IP}, @samp{SET_IP}, @samp{IPTOS}); this ensure that these
  897: macros can be implemented in several ways for best performance.
  898: @samp{IP} points to the next instruction, and @samp{IPTOS} is its
  899: contents.
  900: 
  901: 
  902: @c --------------------------------------------------------------------
  903: @node Superinstructions, Register Machines, Simple instructions, Input File Format
  904: @section Superinstructions
  905: @cindex superinstructions, defining
  906: @cindex defining superinstructions
  907: 
  908: Note: don't invest too much work in (static) superinstructions; a future
  909: version of Vmgen will support dynamic superinstructions (see Ian
  910: Piumarta and Fabio Riccardi, @cite{Optimizing Direct Threaded Code by
  911: Selective Inlining}, PLDI'98), and static superinstructions have much
  912: less benefit in that context (preliminary results indicate only a factor
  913: 1.1 speedup).
  914: 
  915: Here is an example of a superinstruction definition:
  916: 
  917: @example
  918: lit_sub = lit sub
  919: @end example
  920: 
  921: @code{lit_sub} is the name of the superinstruction, and @code{lit} and
  922: @code{sub} are its components.  This superinstruction performs the same
  923: action as the sequence @code{lit} and @code{sub}.  It is generated
  924: automatically by the VM code generation functions whenever that sequence
  925: occurs, so if you want to use this superinstruction, you just need to
  926: add this definition (and even that can be partially automatized,
  927: @pxref{VM profiler}).
  928: 
  929: @cindex prefixes of superinstructions
  930: Vmgen requires that the component instructions are simple instructions
  931: defined before superinstructions using the components.  Currently, Vmgen
  932: also requires that all the subsequences at the start of a
  933: superinstruction (prefixes) must be defined as superinstruction before
  934: the superinstruction.  I.e., if you want to define a superinstruction
  935: 
  936: @example
  937: foo4 = load add sub mul
  938: @end example
  939: 
  940: you first have to define @code{load}, @code{add}, @code{sub} and
  941: @code{mul}, plus
  942: 
  943: @example
  944: foo2 = load add
  945: foo3 = load add sub
  946: @end example
  947: 
  948: Here, @code{sumof4} is the longest prefix of @code{sumof5}, and @code{sumof3}
  949: is the longest prefix of @code{sumof4}.
  950: 
  951: Note that Vmgen assumes that only the code it generates accesses stack
  952: pointers, the instruction pointer, and various stack items, and it
  953: performs optimizations based on this assumption.  Therefore, VM
  954: instructions where your C code changes the instruction pointer should
  955: only be used as last component; a VM instruction where your C code
  956: accesses a stack pointer should not be used as component at all.  Vmgen
  957: does not check these restrictions, they just result in bugs in your
  958: interpreter.
  959: 
  960: @c -------------------------------------------------------------------
  961: @node Register Machines,  , Superinstructions, Input File Format
  962: @section Register Machines
  963: @cindex Register VM
  964: @cindex Superinstructions for register VMs
  965: @cindex tracing of register VMs
  966: 
  967: If you want to implement a register VM rather than a stack VM with
  968: Vmgen, there are two ways to do it: Directly and through
  969: superinstructions.
  970: 
  971: If you use the direct way, you define instructions that take the
  972: register numbers as immediate arguments, like this:
  973: 
  974: @example
  975: add3 ( #src1 #src2 #dest -- )
  976: reg[dest] = reg[src1]+reg[src2];
  977: @end example
  978: 
  979: A disadvantage of this method is that during tracing you only see the
  980: register numbers, but not the register contents.  Actually, with an
  981: appropriate definition of @code{printarg_src} (@pxref{VM engine}), you
  982: can print the values of the source registers on entry, but you cannot
  983: print the value of the destination register on exit.
  984: 
  985: If you use superinstructions to define a register VM, you define simple
  986: instructions that use a stack, and then define superinstructions that
  987: have no overall stack effect, like this:
  988: 
  989: @example
  990: loadreg ( #src -- n )
  991: n = reg[src];
  992: 
  993: storereg ( n #dest -- )
  994: reg[dest] = n;
  995: 
  996: adds ( n1 n2 -- n )
  997: n = n1+n2;
  998: 
  999: add3 = loadreg loadreg adds storereg
 1000: @end example
 1001: 
 1002: An advantage of this method is that you see the values and not just the
 1003: register numbers in tracing.  A disadvantage of this method is that
 1004: currently you cannot generate superinstructions directly, but only
 1005: through generating a sequence of simple instructions (we might change
 1006: this in the future if there is demand).
 1007: 
 1008: Could the register VM support be improved, apart from the issues
 1009: mentioned above?  It is hard to see how to do it in a general way,
 1010: because there are a number of different designs that different people
 1011: mean when they use the term @emph{register machine} in connection with
 1012: VM interpreters.  However, if you have ideas or requests in that
 1013: direction, please let me know (@pxref{Contact}).
 1014: 
 1015: @c ********************************************************************
 1016: @node Using the generated code, Changes, Input File Format, Top
 1017: @chapter Using the generated code
 1018: @cindex generated code, usage
 1019: @cindex Using vmgen-erated code
 1020: 
 1021: The easiest way to create a working VM interpreter with Vmgen is
 1022: probably to start with @file{vmgen-ex}, and modify it for your purposes.
 1023: This chapter is just the reference manual for the macros etc. used by
 1024: the generated code, the other context expected by the generated code,
 1025: and what you can do with the various generated files.
 1026: 
 1027: @menu
 1028: * VM engine::                   Executing VM code
 1029: * VM instruction table::        
 1030: * VM code generation::          Creating VM code (in the front-end)
 1031: * Peephole optimization::       Creating VM superinstructions
 1032: * VM disassembler::             for debugging the front end
 1033: * VM profiler::                 for finding worthwhile superinstructions
 1034: @end menu
 1035: 
 1036: @c --------------------------------------------------------------------
 1037: @node VM engine, VM instruction table, Using the generated code, Using the generated code
 1038: @section VM engine
 1039: @cindex VM instruction execution
 1040: @cindex engine
 1041: @cindex executing VM code
 1042: @cindex @file{engine.c}
 1043: @cindex @file{-vm.i} output file
 1044: 
 1045: The VM engine is the VM interpreter that executes the VM code.  It is
 1046: essential for an interpretive system.
 1047: 
 1048: Vmgen supports two methods of VM instruction dispatch: @emph{threaded
 1049: code} (fast, but gcc-specific), and @emph{switch dispatch} (slow, but
 1050: portable across C compilers); you can use conditional compilation
 1051: (@samp{defined(__GNUC__)}) to choose between these methods, and our
 1052: example does so.
 1053: 
 1054: For both methods, the VM engine is contained in a C-level function.
 1055: Vmgen generates most of the contents of the function for you
 1056: (@file{@var{name}-vm.i}), but you have to define this function, and
 1057: macros and variables used in the engine, and initialize the variables.
 1058: In our example the engine function also includes
 1059: @file{@var{name}-labels.i} (@pxref{VM instruction table}).
 1060: 
 1061: @cindex tracing VM code
 1062: In addition to executing the code, the VM engine can optionally also
 1063: print out a trace of the executed instructions, their arguments and
 1064: results.  For superinstructions it prints the trace as if only component
 1065: instructions were executed; this allows to introduce new
 1066: superinstructions while keeping the traces comparable to old ones
 1067: (important for regression tests).
 1068: 
 1069: It costs significant performance to check in each instruction whether to
 1070: print tracing code, so we recommend producing two copies of the engine:
 1071: one for fast execution, and one for tracing.  See the rules for
 1072: @file{engine.o} and @file{engine-debug.o} in @file{vmgen-ex/Makefile}
 1073: for an example.
 1074: 
 1075: The following macros and variables are used in @file{@var{name}-vm.i}:
 1076: 
 1077: @table @code
 1078: 
 1079: @findex LABEL
 1080: @item LABEL(@var{inst_name})
 1081: This is used just before each VM instruction to provide a jump or
 1082: @code{switch} label (the @samp{:} is provided by Vmgen).  For switch
 1083: dispatch this should expand to @samp{case @var{label}}; for
 1084: threaded-code dispatch this should just expand to @samp{@var{label}}.
 1085: In either case @var{label} is usually the @var{inst_name} with some
 1086: prefix or suffix to avoid naming conflicts.
 1087: 
 1088: @findex LABEL2
 1089: @item LABEL2(@var{inst_name})
 1090: This will be used for dynamic superinstructions; at the moment, this
 1091: should expand to nothing.
 1092: 
 1093: @findex NAME
 1094: @item NAME(@var{inst_name_string})
 1095: Called on entering a VM instruction with a string containing the name of
 1096: the VM instruction as parameter.  In normal execution this should be a
 1097: noop, but for tracing this usually prints the name, and possibly other
 1098: information (several VM registers in our example).
 1099: 
 1100: @findex DEF_CA
 1101: @item DEF_CA
 1102: Usually empty.  Called just inside a new scope at the start of a VM
 1103: instruction.  Can be used to define variables that should be visible
 1104: during every VM instruction.  If you define this macro as non-empty, you
 1105: have to provide the finishing @samp{;} in the macro.
 1106: 
 1107: @findex NEXT_P0
 1108: @findex NEXT_P1
 1109: @findex NEXT_P2
 1110: @item NEXT_P0 NEXT_P1 NEXT_P2
 1111: The three parts of instruction dispatch.  They can be defined in
 1112: different ways for best performance on various processors (see
 1113: @file{engine.c} in the example or @file{engine/threaded.h} in Gforth).
 1114: @samp{NEXT_P0} is invoked right at the start of the VM instruction (but
 1115: after @samp{DEF_CA}), @samp{NEXT_P1} right after the user-supplied C
 1116: code, and @samp{NEXT_P2} at the end.  The actual jump has to be
 1117: performed by @samp{NEXT_P2}.
 1118: 
 1119: The simplest variant is if @samp{NEXT_P2} does everything and the other
 1120: macros do nothing.  Then also related macros like @samp{IP},
 1121: @samp{SET_IP}, @samp{IP}, @samp{INC_IP} and @samp{IPTOS} are very
 1122: straightforward to define.  For switch dispatch this code consists just
 1123: of a jump to the dispatch code (@samp{goto next_inst;} in our example);
 1124: for direct threaded code it consists of something like
 1125: @samp{(@{cfa=*ip++; goto *cfa;@})}.
 1126: 
 1127: Pulling code (usually the @samp{cfa=*ip++;}) up into @samp{NEXT_P1}
 1128: usually does not cause problems, but pulling things up into
 1129: @samp{NEXT_P0} usually requires changing the other macros (and, at least
 1130: for Gforth on Alpha, it does not buy much, because the compiler often
 1131: manages to schedule the relevant stuff up by itself).  An even more
 1132: extreme variant is to pull code up even further, into, e.g., NEXT_P1 of
 1133: the previous VM instruction (prefetching, useful on PowerPCs).
 1134: 
 1135: @findex INC_IP
 1136: @item INC_IP(@var{n})
 1137: This increments @code{IP} by @var{n}.
 1138: 
 1139: @findex SET_IP
 1140: @item SET_IP(@var{target})
 1141: This sets @code{IP} to @var{target}.
 1142: 
 1143: @cindex type cast macro
 1144: @findex vm_@var{A}2@var{B}
 1145: @item vm_@var{A}2@var{B}(a,b)
 1146: Type casting macro that assigns @samp{a} (of type @var{A}) to @samp{b}
 1147: (of type @var{B}).  This is mainly used for getting stack items into
 1148: variables and back.  So you need to define macros for every combination
 1149: of stack basic type (@code{Cell} in our example) and type-prefix types
 1150: used with that stack (in both directions).  For the type-prefix type,
 1151: you use the type-prefix (not the C type string) as type name (e.g.,
 1152: @samp{vm_Cell2i}, not @samp{vm_Cell2Cell}).  In addition, you have to
 1153: define a vm_@var{X}2@var{X} macro for the stack's basic type @var{X}
 1154: (used in superinstructions).
 1155: 
 1156: @cindex instruction stream, basic type
 1157: The stack basic type for the predefined @samp{inst-stream} is
 1158: @samp{Cell}.  If you want a stack with the same item size, making its
 1159: basic type @samp{Cell} usually reduces the number of macros you have to
 1160: define.
 1161: 
 1162: @cindex unions in type cast macros
 1163: @cindex casts in type cast macros
 1164: @cindex type casting between floats and integers
 1165: Here our examples differ a lot: @file{vmgen-ex} uses casts in these
 1166: macros, whereas @file{vmgen-ex2} uses union-field selection (or
 1167: assignment to union fields).  Note that casting floats into integers and
 1168: vice versa changes the bit pattern (and you do not want that).  In this
 1169: case your options are to use a (temporary) union, or to take the address
 1170: of the value, cast the pointer, and dereference that (not always
 1171: possible, and sometimes expensive).
 1172: 
 1173: @findex vm_two@var{A}2@var{B}
 1174: @findex vm_@var{B}2two@var{A}
 1175: @item vm_two@var{A}2@var{B}(a1,a2,b)
 1176: @item vm_@var{B}2two@var{A}(b,a1,a2)
 1177: Type casting between two stack items (@code{a1}, @code{a2}) and a
 1178: variable @code{b} of a type that takes two stack items.  This does not
 1179: occur in our small examples, but you can look at Gforth for examples
 1180: (see @code{vm_twoCell2d} in @file{engine/forth.h}).
 1181: 
 1182: @cindex stack pointer definition
 1183: @cindex instruction pointer definition
 1184: @item @var{stackpointer}
 1185: For each stack used, the stackpointer name given in the stack
 1186: declaration is used.  For a regular stack this must be an l-expression;
 1187: typically it is a variable declared as a pointer to the stack's basic
 1188: type.  For @samp{inst-stream}, the name is @samp{IP}, and it can be a
 1189: plain r-value; typically it is a macro that abstracts away the
 1190: differences between the various implementations of @code{NEXT_P*}.
 1191: 
 1192: @cindex top of stack caching
 1193: @cindex stack caching
 1194: @cindex TOS
 1195: @findex IPTOS
 1196: @item @var{stackpointer}TOS
 1197: The top-of-stack for the stack pointed to by @var{stackpointer}.  If you
 1198: are using top-of-stack caching for that stack, this should be defined as
 1199: variable; if you are not using top-of-stack caching for that stack, this
 1200: should be a macro expanding to @samp{@var{stackpointer}[0]}.  The stack
 1201: pointer for the predefined @samp{inst-stream} is called @samp{IP}, so
 1202: the top-of-stack is called @samp{IPTOS}.
 1203: 
 1204: @findex IF_@var{stackpointer}TOS
 1205: @item IF_@var{stackpointer}TOS(@var{expr})
 1206: Macro for executing @var{expr}, if top-of-stack caching is used for the
 1207: @var{stackpointer} stack.  I.e., this should do @var{expr} if there is
 1208: top-of-stack caching for @var{stackpointer}; otherwise it should do
 1209: nothing.
 1210: 
 1211: @findex SUPER_END
 1212: @item SUPER_END
 1213: This is used by the VM profiler (@pxref{VM profiler}); it should not do
 1214: anything in normal operation, and call @code{vm_count_block(IP)} for
 1215: profiling.
 1216: 
 1217: @findex SUPER_CONTINUE
 1218: @item SUPER_CONTINUE
 1219: This is just a hint to Vmgen and does nothing at the C level.
 1220: 
 1221: @findex VM_DEBUG
 1222: @item VM_DEBUG
 1223: If this is defined, the tracing code will be compiled in (slower
 1224: interpretation, but better debugging).  Our example compiles two
 1225: versions of the engine, a fast-running one that cannot trace, and one
 1226: with potential tracing and profiling.
 1227: 
 1228: @findex vm_debug
 1229: @item vm_debug
 1230: Needed only if @samp{VM_DEBUG} is defined.  If this variable contains
 1231: true, the VM instructions produce trace output.  It can be turned on or
 1232: off at any time.
 1233: 
 1234: @findex vm_out
 1235: @item vm_out
 1236: Needed only if @samp{VM_DEBUG} is defined.  Specifies the file on which
 1237: to print the trace output (type @samp{FILE *}).
 1238: 
 1239: @findex printarg_@var{type}
 1240: @item printarg_@var{type}(@var{value})
 1241: Needed only if @samp{VM_DEBUG} is defined.  Macro or function for
 1242: printing @var{value} in a way appropriate for the @var{type}.  This is
 1243: used for printing the values of stack items during tracing.  @var{Type}
 1244: is normally the type prefix specified in a @code{type-prefix} definition
 1245: (e.g., @samp{printarg_i}); in superinstructions it is currently the
 1246: basic type of the stack.
 1247: 
 1248: @end table
 1249: 
 1250: 
 1251: @c --------------------------------------------------------------------
 1252: @node VM instruction table, VM code generation, VM engine, Using the generated code
 1253: @section VM instruction table
 1254: @cindex instruction table
 1255: @cindex opcode definition
 1256: @cindex labels for threaded code
 1257: @cindex @code{vm_prim}, definition
 1258: @cindex @file{-labels.i} output file
 1259: 
 1260: For threaded code we also need to produce a table containing the labels
 1261: of all VM instructions.  This is needed for VM code generation
 1262: (@pxref{VM code generation}), and it has to be done in the engine
 1263: function, because the labels are not visible outside.  It then has to be
 1264: passed outside the function (and assigned to @samp{vm_prim}), to be used
 1265: by the VM code generation functions.
 1266: 
 1267: This means that the engine function has to be called first to produce
 1268: the VM instruction table, and later, after generating VM code, it has to
 1269: be called again to execute the generated VM code (yes, this is ugly).
 1270: In our example program, these two modes of calling the engine function
 1271: are differentiated by the value of the parameter ip0 (if it equals 0,
 1272: then the table is passed out, otherwise the VM code is executed); in our
 1273: example, we pass the table out by assigning it to @samp{vm_prim} and
 1274: returning from @samp{engine}.
 1275: 
 1276: In our example (@file{vmgen-ex/engine.c}), we also build such a table for
 1277: switch dispatch; this is mainly done for uniformity.
 1278: 
 1279: For switch dispatch, we also need to define the VM instruction opcodes
 1280: used as case labels in an @code{enum}.
 1281: 
 1282: For both purposes (VM instruction table, and enum), the file
 1283: @file{@var{name}-labels.i} is generated by Vmgen.  You have to define
 1284: the following macro used in this file:
 1285: 
 1286: @table @code
 1287: 
 1288: @findex INST_ADDR
 1289: @item INST_ADDR(@var{inst_name})
 1290: For switch dispatch, this is just the name of the switch label (the same
 1291: name as used in @samp{LABEL(@var{inst_name})}), for both uses of
 1292: @file{@var{name}-labels.i}.  For threaded-code dispatch, this is the
 1293: address of the label defined in @samp{LABEL(@var{inst_name})}); the
 1294: address is taken with @samp{&&} (@pxref{Labels as Values, , Labels as
 1295: Values, gcc.info, GNU C Manual}).
 1296: 
 1297: @end table
 1298: 
 1299: 
 1300: @c --------------------------------------------------------------------
 1301: @node VM code generation, Peephole optimization, VM instruction table, Using the generated code
 1302: @section VM code generation
 1303: @cindex VM code generation
 1304: @cindex code generation, VM
 1305: @cindex @file{-gen.i} output file
 1306: 
 1307: Vmgen generates VM code generation functions in @file{@var{name}-gen.i}
 1308: that the front end can call to generate VM code.  This is essential for
 1309: an interpretive system.
 1310: 
 1311: @findex gen_@var{inst}
 1312: For a VM instruction @samp{x ( #a b #c -- d )}, Vmgen generates a
 1313: function with the prototype
 1314: 
 1315: @example
 1316: void gen_x(Inst **ctp, a_type a, c_type c)
 1317: @end example
 1318: 
 1319: The @code{ctp} argument points to a pointer to the next instruction.
 1320: @code{*ctp} is increased by the generation functions; i.e., you should
 1321: allocate memory for the code to be generated beforehand, and start with
 1322: *ctp set at the start of this memory area.  Before running out of
 1323: memory, allocate a new area, and generate a VM-level jump to the new
 1324: area (this overflow handling is not implemented in our examples).
 1325: 
 1326: @cindex immediate arguments, VM code generation
 1327: The other arguments correspond to the immediate arguments of the VM
 1328: instruction (with their appropriate types as defined in the
 1329: @code{type_prefix} declaration.
 1330: 
 1331: The following types, variables, and functions are used in
 1332: @file{@var{name}-gen.i}:
 1333: 
 1334: @table @code
 1335: 
 1336: @findex Inst
 1337: @item Inst
 1338: The type of the VM instruction; if you use threaded code, this is
 1339: @code{void *}; for switch dispatch this is an integer type.
 1340: 
 1341: @cindex @code{vm_prim}, use
 1342: @item vm_prim
 1343: The VM instruction table (type: @code{Inst *}, @pxref{VM instruction table}).
 1344: 
 1345: @findex gen_inst
 1346: @item gen_inst(Inst **ctp, Inst i)
 1347: This function compiles the instruction @code{i}.  Take a look at it in
 1348: @file{vmgen-ex/peephole.c}.  It is trivial when you don't want to use
 1349: superinstructions (just the last two lines of the example function), and
 1350: slightly more complicated in the example due to its ability to use
 1351: superinstructions (@pxref{Peephole optimization}).
 1352: 
 1353: @findex genarg_@var{type_prefix}
 1354: @item genarg_@var{type_prefix}(Inst **ctp, @var{type} @var{type_prefix})
 1355: This compiles an immediate argument of @var{type} (as defined in a
 1356: @code{type-prefix} definition).  These functions are trivial to define
 1357: (see @file{vmgen-ex/support.c}).  You need one of these functions for
 1358: every type that you use as immediate argument.
 1359: 
 1360: @end table
 1361: 
 1362: @findex BB_BOUNDARY
 1363: In addition to using these functions to generate code, you should call
 1364: @code{BB_BOUNDARY} at every basic block entry point if you ever want to
 1365: use superinstructions (or if you want to use the profiling supported by
 1366: Vmgen; but this support is also useful mainly for selecting
 1367: superinstructions).  If you use @code{BB_BOUNDARY}, you should also
 1368: define it (take a look at its definition in @file{vmgen-ex/mini.y}).
 1369: 
 1370: You do not need to call @code{BB_BOUNDARY} after branches, because you
 1371: will not define superinstructions that contain branches in the middle
 1372: (and if you did, and it would work, there would be no reason to end the
 1373: superinstruction at the branch), and because the branches announce
 1374: themselves to the profiler.
 1375: 
 1376: 
 1377: @c --------------------------------------------------------------------
 1378: @node Peephole optimization, VM disassembler, VM code generation, Using the generated code
 1379: @section Peephole optimization
 1380: @cindex peephole optimization
 1381: @cindex superinstructions, generating
 1382: @cindex @file{peephole.c}
 1383: @cindex @file{-peephole.i} output file
 1384: 
 1385: You need peephole optimization only if you want to use
 1386: superinstructions.  But having the code for it does not hurt much if you
 1387: do not use superinstructions.
 1388: 
 1389: A simple greedy peephole optimization algorithm is used for
 1390: superinstruction selection: every time @code{gen_inst} compiles a VM
 1391: instruction, it checks if it can combine it with the last VM instruction
 1392: (which may also be a superinstruction resulting from a previous peephole
 1393: optimization); if so, it changes the last instruction to the combined
 1394: instruction instead of laying down @code{i} at the current @samp{*ctp}.
 1395: 
 1396: The code for peephole optimization is in @file{vmgen-ex/peephole.c}.
 1397: You can use this file almost verbatim.  Vmgen generates
 1398: @file{@var{file}-peephole.i} which contains data for the peephoile
 1399: optimizer.
 1400: 
 1401: @findex init_peeptable
 1402: You have to call @samp{init_peeptable()} after initializing
 1403: @samp{vm_prim}, and before compiling any VM code to initialize data
 1404: structures for peephole optimization.  After that, compiling with the VM
 1405: code generation functions will automatically combine VM instructions
 1406: into superinstructions.  Since you do not want to combine instructions
 1407: across VM branch targets (otherwise there will not be a proper VM
 1408: instruction to branch to), you have to call @code{BB_BOUNDARY}
 1409: (@pxref{VM code generation}) at branch targets.
 1410: 
 1411: 
 1412: @c --------------------------------------------------------------------
 1413: @node VM disassembler, VM profiler, Peephole optimization, Using the generated code
 1414: @section VM disassembler
 1415: @cindex VM disassembler
 1416: @cindex disassembler, VM code
 1417: @cindex @file{disasm.c}
 1418: @cindex @file{-disasm.i} output file
 1419: 
 1420: A VM code disassembler is optional for an interpretive system, but
 1421: highly recommended during its development and maintenance, because it is
 1422: very useful for detecting bugs in the front end (and for distinguishing
 1423: them from VM interpreter bugs).
 1424: 
 1425: Vmgen supports VM code disassembling by generating
 1426: @file{@var{file}-disasm.i}.  This code has to be wrapped into a
 1427: function, as is done in @file{vmgen-ex/disasm.c}.  You can use this file
 1428: almost verbatim.  In addition to @samp{vm_@var{A}2@var{B}(a,b)},
 1429: @samp{vm_out}, @samp{printarg_@var{type}(@var{value})}, which are
 1430: explained above, the following macros and variables are used in
 1431: @file{@var{file}-disasm.i} (and you have to define them):
 1432: 
 1433: @table @code
 1434: 
 1435: @item ip
 1436: This variable points to the opcode of the current VM instruction.
 1437: 
 1438: @cindex @code{IP}, @code{IPTOS} in disassmbler
 1439: @item IP IPTOS
 1440: @samp{IPTOS} is the first argument of the current VM instruction, and
 1441: @samp{IP} points to it; this is just as in the engine, but here
 1442: @samp{ip} points to the opcode of the VM instruction (in contrast to the
 1443: engine, where @samp{ip} points to the next cell, or even one further).
 1444: 
 1445: @findex VM_IS_INST
 1446: @item VM_IS_INST(Inst i, int n)
 1447: Tests if the opcode @samp{i} is the same as the @samp{n}th entry in the
 1448: VM instruction table.
 1449: 
 1450: @end table
 1451: 
 1452: 
 1453: @c --------------------------------------------------------------------
 1454: @node VM profiler,  , VM disassembler, Using the generated code
 1455: @section VM profiler
 1456: @cindex VM profiler
 1457: @cindex profiling for selecting superinstructions
 1458: @cindex superinstructions and profiling
 1459: @cindex @file{profile.c}
 1460: @cindex @file{-profile.i} output file
 1461: 
 1462: The VM profiler is designed for getting execution and occurence counts
 1463: for VM instruction sequences, and these counts can then be used for
 1464: selecting sequences as superinstructions.  The VM profiler is probably
 1465: not useful as profiling tool for the interpretive system.  I.e., the VM
 1466: profiler is useful for the developers, but not the users of the
 1467: interpretive system.
 1468: 
 1469: The output of the profiler is: for each basic block (executed at least
 1470: once), it produces the dynamic execution count of that basic block and
 1471: all its subsequences; e.g.,
 1472: 
 1473: @example
 1474:        9227465  lit storelocal 
 1475:        9227465  storelocal branch 
 1476:        9227465  lit storelocal branch 
 1477: @end example
 1478: 
 1479: I.e., a basic block consisting of @samp{lit storelocal branch} is
 1480: executed 9227465 times.
 1481: 
 1482: @cindex @file{stat.awk}
 1483: @cindex @file{seq2rule.awk}
 1484: This output can be combined in various ways.  E.g.,
 1485: @file{vmgen-ex/stat.awk} adds up the occurences of a given sequence wrt
 1486: dynamic execution, static occurence, and per-program occurence.  E.g.,
 1487: 
 1488: @example
 1489:       2      16        36910041 loadlocal lit 
 1490: @end example
 1491: 
 1492: @noindent
 1493: indicates that the sequence @samp{loadlocal lit} occurs in 2 programs,
 1494: in 16 places, and has been executed 36910041 times.  Now you can select
 1495: superinstructions in any way you like (note that compile time and space
 1496: typically limit the number of superinstructions to 100--1000).  After
 1497: you have done that, @file{vmgen/seq2rule.awk} turns lines of the form
 1498: above into rules for inclusion in a Vmgen input file.  Note that this
 1499: script does not ensure that all prefixes are defined, so you have to do
 1500: that in other ways.  So, an overall script for turning profiles into
 1501: superinstructions can look like this:
 1502: 
 1503: @example
 1504: awk -f stat.awk fib.prof test.prof|
 1505: awk '$3>=10000'|                #select sequences
 1506: fgrep -v -f peephole-blacklist| #eliminate wrong instructions
 1507: awk -f seq2rule.awk|            #turn into superinstructions
 1508: sort -k 3 >mini-super.vmg       #sort sequences
 1509: @end example
 1510: 
 1511: Here the dynamic count is used for selecting sequences (preliminary
 1512: results indicate that the static count gives better results, though);
 1513: the third line eliminates sequences containing instructions that must not
 1514: occur in a superinstruction, because they access a stack directly.  The
 1515: dynamic count selection ensures that all subsequences (including
 1516: prefixes) of longer sequences occur (because subsequences have at least
 1517: the same count as the longer sequences); the sort in the last line
 1518: ensures that longer superinstructions occur after their prefixes.
 1519: 
 1520: But before using this, you have to have the profiler.  Vmgen supports its
 1521: creation by generating @file{@var{file}-profile.i}; you also need the
 1522: wrapper file @file{vmgen-ex/profile.c} that you can use almost verbatim.
 1523: 
 1524: @cindex @code{SUPER_END} in profiling
 1525: @cindex @code{BB_BOUNDARY} in profiling
 1526: The profiler works by recording the targets of all VM control flow
 1527: changes (through @code{SUPER_END} during execution, and through
 1528: @code{BB_BOUNDARY} in the front end), and counting (through
 1529: @code{SUPER_END}) how often they were targeted.  After the program run,
 1530: the numbers are corrected such that each VM basic block has the correct
 1531: count (entering a block without executing a branch does not increase the
 1532: count, and the correction fixes that), then the subsequences of all
 1533: basic blocks are printed.  To get all this, you just have to define
 1534: @code{SUPER_END} (and @code{BB_BOUNDARY}) appropriately, and call
 1535: @code{vm_print_profile(FILE *file)} when you want to output the profile
 1536: on @code{file}.
 1537: 
 1538: @cindex @code{VM_IS_INST} in profiling
 1539: The @file{@var{file}-profile.i} is similar to the disassembler file, and
 1540: it uses variables and functions defined in @file{vmgen-ex/profile.c},
 1541: plus @code{VM_IS_INST} already defined for the VM disassembler
 1542: (@pxref{VM disassembler}).
 1543: 
 1544: 
 1545: @c **********************************************************
 1546: @node Changes, Contact, Using the generated code, Top
 1547: @chapter Changes
 1548: @cindex Changes from old versions
 1549: 
 1550: Users of the gforth-0.5.9-20010501 version of Vmgen need to change
 1551: several things in their source code to use the current version.  I
 1552: recommend keeping the gforth-0.5.9-20010501 version until you have
 1553: completed the change (note that you can have several versions of Gforth
 1554: installed at the same time).  I hope to avoid such incompatible changes
 1555: in the future.
 1556: 
 1557: The required changes are:
 1558: 
 1559: @table @code
 1560: 
 1561: @cindex @code{vm_@var{A}2@var{B}}, changes
 1562: @item vm_@var{A}2@var{B}
 1563: now takes two arguments.
 1564: 
 1565: @cindex @code{vm_two@var{A}2@var{B}}, changes
 1566: @item vm_two@var{A}2@var{B}(b,a1,a2);
 1567: changed to vm_two@var{A}2@var{B}(a1,a2,b) (note the absence of the @samp{;}).
 1568: 
 1569: @end table
 1570: 
 1571: Also some new macros have to be defined, e.g., @code{INST_ADDR}, and
 1572: @code{LABEL}; some macros have to be defined in new contexts, e.g.,
 1573: @code{VM_IS_INST} is now also needed in the disassembler.
 1574: 
 1575: @c *********************************************************
 1576: @node Contact, Copying This Manual, Changes, Top
 1577: @chapter Contact
 1578: 
 1579: @c ***********************************************************
 1580: @node Copying This Manual, Index, Contact, Top
 1581: @appendix Copying This Manual
 1582: 
 1583: @menu
 1584: * GNU Free Documentation License::  License for copying this manual.
 1585: @end menu
 1586: 
 1587: @include fdl.texi
 1588: 
 1589: 
 1590: @node Index,  , Copying This Manual, Top
 1591: @unnumbered Index
 1592: 
 1593: @printindex cp
 1594: 
 1595: @bye

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