File:  [gforth] / gforth / doc / vmgen.texi
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Sun Sep 1 15:15:07 2002 UTC (17 years, 2 months ago) by anton
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disabled dynamic code generation for now (problems with gcc-3.2).
documentation changes

    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: * Error messages::              reported by Vmgen
   61: * Using the generated code::    
   62: * Hints::                       VM archictecture, efficiency
   63: * The future::                  
   64: * Changes::                     from earlier versions
   65: * Contact::                     Bug reporting etc.
   66: * Copying This Manual::         Manual License
   67: * Index::                       
   68: 
   69: @detailmenu
   70:  --- The Detailed Node Listing ---
   71: 
   72: Concepts
   73: 
   74: * Front end and VM interpreter::  Modularizing an interpretive system
   75: * Data handling::               Stacks, registers, immediate arguments
   76: * Dispatch::                    From one VM instruction to the next
   77: 
   78: Example
   79: 
   80: * Example overview::            
   81: * Using profiling to create superinstructions::  
   82: 
   83: Input File Format
   84: 
   85: * Input File Grammar::          
   86: * Simple instructions::         
   87: * Superinstructions::           
   88: * Store Optimization::          
   89: * Register Machines::           How to define register VM instructions
   90: 
   91: Input File Grammar
   92: 
   93: * Eval escapes::                what follows \E
   94: 
   95: Simple instructions
   96: 
   97: * C Code Macros::               Macros recognized by Vmgen
   98: * C Code restrictions::         Vmgen makes assumptions about C code
   99: 
  100: Using the generated code
  101: 
  102: * VM engine::                   Executing VM code
  103: * VM instruction table::        
  104: * VM code generation::          Creating VM code (in the front-end)
  105: * Peephole optimization::       Creating VM superinstructions
  106: * VM disassembler::             for debugging the front end
  107: * VM profiler::                 for finding worthwhile superinstructions
  108: 
  109: Hints
  110: 
  111: * Floating point::              and stacks
  112: 
  113: Copying This Manual
  114: 
  115: * GNU Free Documentation License::  License for copying this manual.
  116: 
  117: @end detailmenu
  118: @end menu
  119: 
  120: @c @ifnottex
  121: @c This file documents Vmgen (Gforth @value{VERSION}).
  122: 
  123: @c ************************************************************
  124: @node Introduction, Why interpreters?, Top, Top
  125: @chapter Introduction
  126: 
  127: Vmgen is a tool for writing efficient interpreters.  It takes a simple
  128: virtual machine description and generates efficient C code for dealing
  129: with the virtual machine code in various ways (in particular, executing
  130: it).  The run-time efficiency of the resulting interpreters is usually
  131: within a factor of 10 of machine code produced by an optimizing
  132: compiler.
  133: 
  134: The interpreter design strategy supported by Vmgen is to divide the
  135: interpreter into two parts:
  136: 
  137: @itemize @bullet
  138: 
  139: @item The @emph{front end} takes the source code of the language to be
  140: implemented, and translates it into virtual machine code.  This is
  141: similar to an ordinary compiler front end; typically an interpreter
  142: front-end performs no optimization, so it is relatively simple to
  143: implement and runs fast.
  144: 
  145: @item The @emph{virtual machine interpreter} executes the virtual
  146: machine code.
  147: 
  148: @end itemize
  149: 
  150: Such a division is usually used in interpreters, for modularity as well
  151: as for efficiency.  The virtual machine code is typically passed between
  152: front end and virtual machine interpreter in memory, like in a
  153: load-and-go compiler; this avoids the complexity and time cost of
  154: writing the code to a file and reading it again.
  155: 
  156: A @emph{virtual machine} (VM) represents the program as a sequence of
  157: @emph{VM instructions}, following each other in memory, similar to real
  158: machine code.  Control flow occurs through VM branch instructions, like
  159: in a real machine.
  160: 
  161: @cindex functionality features overview
  162: In this setup, Vmgen can generate most of the code dealing with virtual
  163: machine instructions from a simple description of the virtual machine
  164: instructions (@pxref{Input File Format}), in particular:
  165: 
  166: @table @strong
  167: 
  168: @item VM instruction execution
  169: 
  170: @item VM code generation
  171: Useful in the front end.
  172: 
  173: @item VM code decompiler
  174: Useful for debugging the front end.
  175: 
  176: @item VM code tracing
  177: Useful for debugging the front end and the VM interpreter.  You will
  178: typically provide other means for debugging the user's programs at the
  179: source level.
  180: 
  181: @item VM code profiling
  182: Useful for optimizing the VM interpreter with superinstructions
  183: (@pxref{VM profiler}).
  184: 
  185: @end table
  186: 
  187: To create parts of the interpretive system that do not deal with VM
  188: instructions, you have to use other tools (e.g., @command{bison}) and/or
  189: hand-code them.
  190: 
  191: @cindex efficiency features overview
  192: @noindent
  193: Vmgen supports efficient interpreters though various optimizations, in
  194: particular
  195: 
  196: @itemize @bullet
  197: 
  198: @item Threaded code
  199: 
  200: @item Caching the top-of-stack in a register
  201: 
  202: @item Combining VM instructions into superinstructions
  203: 
  204: @item
  205: Replicating VM (super)instructions for better BTB prediction accuracy
  206: (not yet in vmgen-ex, but already in Gforth).
  207: 
  208: @end itemize
  209: 
  210: @cindex speed for JVM
  211: As a result, Vmgen-based interpreters are only about an order of
  212: magnitude slower than native code from an optimizing C compiler on small
  213: benchmarks; on large benchmarks, which spend more time in the run-time
  214: system, the slowdown is often less (e.g., the slowdown of a
  215: Vmgen-generated JVM interpreter over the best JVM JIT compiler we
  216: measured is only a factor of 2-3 for large benchmarks; some other JITs
  217: and all other interpreters we looked at were slower than our
  218: interpreter).
  219: 
  220: VMs are usually designed as stack machines (passing data between VM
  221: instructions on a stack), and Vmgen supports such designs especially
  222: well; however, you can also use Vmgen for implementing a register VM
  223: (@pxref{Register Machines}) and still benefit from most of the advantages
  224: offered by Vmgen.
  225: 
  226: There are many potential uses of the instruction descriptions that are
  227: not implemented at the moment, but we are open for feature requests, and
  228: we will consider new features if someone asks for them; so the feature
  229: list above is not exhaustive.
  230: 
  231: @c *********************************************************************
  232: @node Why interpreters?, Concepts, Introduction, Top
  233: @chapter Why interpreters?
  234: @cindex interpreters, advantages
  235: @cindex advantages of interpreters
  236: @cindex advantages of vmgen
  237: 
  238: Interpreters are a popular language implementation technique because
  239: they combine all three of the following advantages:
  240: 
  241: @itemize @bullet
  242: 
  243: @item Ease of implementation
  244: 
  245: @item Portability
  246: 
  247: @item Fast edit-compile-run cycle
  248: 
  249: @end itemize
  250: 
  251: Vmgen makes it even easier to implement interpreters.
  252: 
  253: @cindex speed of interpreters
  254: The main disadvantage of interpreters is their run-time speed.  However,
  255: there are huge differences between different interpreters in this area:
  256: the slowdown over optimized C code on programs consisting of simple
  257: operations is typically a factor of 10 for the more efficient
  258: interpreters, and a factor of 1000 for the less efficient ones (the
  259: slowdown for programs executing complex operations is less, because the
  260: time spent in libraries for executing complex operations is the same in
  261: all implementation strategies).
  262: 
  263: Vmgen supports techniques for building efficient interpreters.
  264: 
  265: @c ********************************************************************
  266: @node Concepts, Invoking Vmgen, Why interpreters?, Top
  267: @chapter Concepts
  268: 
  269: @menu
  270: * Front end and VM interpreter::  Modularizing an interpretive system
  271: * Data handling::               Stacks, registers, immediate arguments
  272: * Dispatch::                    From one VM instruction to the next
  273: @end menu
  274: 
  275: @c --------------------------------------------------------------------
  276: @node Front end and VM interpreter, Data handling, Concepts, Concepts
  277: @section Front end and VM interpreter
  278: @cindex modularization of interpreters
  279: 
  280: @cindex front-end
  281: Interpretive systems are typically divided into a @emph{front end} that
  282: parses the input language and produces an intermediate representation
  283: for the program, and an interpreter that executes the intermediate
  284: representation of the program.
  285: 
  286: @cindex virtual machine
  287: @cindex VM
  288: @cindex VM instruction
  289: @cindex instruction, VM
  290: @cindex VM branch instruction
  291: @cindex branch instruction, VM
  292: @cindex VM register
  293: @cindex register, VM
  294: @cindex opcode, VM instruction
  295: @cindex immediate argument, VM instruction
  296: For efficient interpreters the intermediate representation of choice is
  297: virtual machine code (rather than, e.g., an abstract syntax tree).
  298: @emph{Virtual machine} (VM) code consists of VM instructions arranged
  299: sequentially in memory; they are executed in sequence by the VM
  300: interpreter, but VM branch instructions can change the control flow and
  301: are used for implementing control structures.  The conceptual similarity
  302: to real machine code results in the name @emph{virtual machine}.
  303: Various terms similar to terms for real machines are used; e.g., there
  304: are @emph{VM registers} (like the instruction pointer and stack
  305: pointer(s)), and the VM instruction consists of an @emph{opcode} and
  306: @emph{immediate arguments}.
  307: 
  308: In this framework, Vmgen supports building the VM interpreter and any
  309: other component dealing with VM instructions.  It does not have any
  310: support for the front end, apart from VM code generation support.  The
  311: front end can be implemented with classical compiler front-end
  312: techniques, supported by tools like @command{flex} and @command{bison}.
  313: 
  314: The intermediate representation is usually just internal to the
  315: interpreter, but some systems also support saving it to a file, either
  316: as an image file, or in a full-blown linkable file format (e.g., JVM).
  317: Vmgen currently has no special support for such features, but the
  318: information in the instruction descriptions can be helpful, and we are
  319: open to feature requests and suggestions.
  320: 
  321: @c --------------------------------------------------------------------
  322: @node Data handling, Dispatch, Front end and VM interpreter, Concepts
  323: @section Data handling
  324: 
  325: @cindex stack machine
  326: @cindex register machine
  327: Most VMs use one or more stacks for passing temporary data between VM
  328: instructions.  Another option is to use a register machine architecture
  329: for the virtual machine; we believe that using a stack architecture is
  330: usually both simpler and faster.
  331: 
  332: however, this option is slower or
  333: significantly more complex to implement than a stack machine architecture.
  334: 
  335: Vmgen has special support and optimizations for stack VMs, making their
  336: implementation easy and efficient.
  337: 
  338: You can also implement a register VM with Vmgen (@pxref{Register
  339: Machines}), and you will still profit from most Vmgen features.
  340: 
  341: @cindex stack item size
  342: @cindex size, stack items
  343: Stack items all have the same size, so they typically will be as wide as
  344: an integer, pointer, or floating-point value.  Vmgen supports treating
  345: two consecutive stack items as a single value, but anything larger is
  346: best kept in some other memory area (e.g., the heap), with pointers to
  347: the data on the stack.
  348: 
  349: @cindex instruction stream
  350: @cindex immediate arguments
  351: Another source of data is immediate arguments VM instructions (in the VM
  352: instruction stream).  The VM instruction stream is handled similar to a
  353: stack in Vmgen.
  354: 
  355: @cindex garbage collection
  356: @cindex reference counting
  357: Vmgen has no built-in support for, nor restrictions against
  358: @emph{garbage collection}.  If you need garbage collection, you need to
  359: provide it in your run-time libraries.  Using @emph{reference counting}
  360: is probably harder, but might be possible (contact us if you are
  361: interested).
  362: @c reference counting might be possible by including counting code in 
  363: @c the conversion macros.
  364: 
  365: @c --------------------------------------------------------------------
  366: @node Dispatch,  , Data handling, Concepts
  367: @section Dispatch
  368: @cindex Dispatch of VM instructions
  369: @cindex main interpreter loop
  370: 
  371: Understanding this section is probably not necessary for using Vmgen,
  372: but it may help.  You may want to skip it now, and read it if you find statements about dispatch methods confusing.
  373: 
  374: After executing one VM instruction, the VM interpreter has to dispatch
  375: the next VM instruction (Vmgen calls the dispatch routine @samp{NEXT}).
  376: Vmgen supports two methods of dispatch:
  377: 
  378: @table @strong
  379: 
  380: @item switch dispatch
  381: @cindex switch dispatch
  382: In this method the VM interpreter contains a giant @code{switch}
  383: statement, with one @code{case} for each VM instruction.  The VM
  384: instruction opcodes are represented by integers (e.g., produced by an
  385: @code{enum}) in the VM code, and dispatch occurs by loading the next
  386: opcode, @code{switch}ing on it, and continuing at the appropriate
  387: @code{case}; after executing the VM instruction, the VM interpreter
  388: jumps back to the dispatch code.
  389: 
  390: @item threaded code
  391: @cindex threaded code
  392: This method represents a VM instruction opcode by the address of the
  393: start of the machine code fragment for executing the VM instruction.
  394: Dispatch consists of loading this address, jumping to it, and
  395: incrementing the VM instruction pointer.  Typically the threaded-code
  396: dispatch code is appended directly to the code for executing the VM
  397: instruction.  Threaded code cannot be implemented in ANSI C, but it can
  398: be implemented using GNU C's labels-as-values extension (@pxref{Labels
  399: as Values, , Labels as Values, gcc.info, GNU C Manual}).
  400: 
  401: @c call threading
  402: @end table
  403: 
  404: Threaded code can be twice as fast as switch dispatch, depending on the
  405: interpreter, the benchmark, and the machine.
  406: 
  407: @c *************************************************************
  408: @node Invoking Vmgen, Example, Concepts, Top
  409: @chapter Invoking Vmgen
  410: @cindex Invoking Vmgen
  411: 
  412: The usual way to invoke Vmgen is as follows:
  413: 
  414: @example
  415: vmgen @var{inputfile}
  416: @end example
  417: 
  418: Here @var{inputfile} is the VM instruction description file, which
  419: usually ends in @file{.vmg}.  The output filenames are made by taking
  420: the basename of @file{inputfile} (i.e., the output files will be created
  421: in the current working directory) and replacing @file{.vmg} with
  422: @file{-vm.i}, @file{-disasm.i}, @file{-gen.i}, @file{-labels.i},
  423: @file{-profile.i}, and @file{-peephole.i}.  E.g., @command{vmgen
  424: hack/foo.vmg} will create @file{foo-vm.i}, @file{foo-disasm.i},
  425: @file{foo-gen.i}, @file{foo-labels.i}, @file{foo-profile.i} and
  426: @file{foo-peephole.i}.
  427: 
  428: The command-line options supported by Vmgen are
  429: 
  430: @table @option
  431: 
  432: @cindex -h, command-line option
  433: @cindex --help, command-line option
  434: @item --help
  435: @itemx -h
  436: Print a message about the command-line options
  437: 
  438: @cindex -v, command-line option
  439: @cindex --version, command-line option
  440: @item --version
  441: @itemx -v
  442: Print version and exit
  443: @end table
  444: 
  445: @c env vars GFORTHDIR GFORTHDATADIR
  446: 
  447: @c ****************************************************************
  448: @node Example, Input File Format, Invoking Vmgen, Top
  449: @chapter Example
  450: @cindex example of a Vmgen-based interpreter
  451: 
  452: @menu
  453: * Example overview::            
  454: * Using profiling to create superinstructions::  
  455: @end menu
  456: 
  457: @c --------------------------------------------------------------------
  458: @node Example overview, Using profiling to create superinstructions, Example, Example
  459: @section Example overview
  460: @cindex example overview
  461: @cindex @file{vmgen-ex}
  462: @cindex @file{vmgen-ex2}
  463: 
  464: There are two versions of the same example for using Vmgen:
  465: @file{vmgen-ex} and @file{vmgen-ex2} (you can also see Gforth as
  466: example, but it uses additional (undocumented) features, and also
  467: differs in some other respects).  The example implements @emph{mini}, a
  468: tiny Modula-2-like language with a small JavaVM-like virtual machine.
  469: 
  470: The difference between the examples is that @file{vmgen-ex} uses many
  471: casts, and @file{vmgen-ex2} tries to avoids most casts and uses unions
  472: instead.  In the rest of this manual we usually mention just files in
  473: @file{vmgen-ex}; if you want to use unions, use the equivalent file in
  474: @file{vmgen-ex2}.
  475: @cindex unions example
  476: @cindex casts example
  477: 
  478: The files provided with each example are:
  479: @cindex example files
  480: 
  481: @example
  482: Makefile
  483: README
  484: disasm.c           wrapper file
  485: engine.c           wrapper file
  486: peephole.c         wrapper file
  487: profile.c          wrapper file
  488: mini-inst.vmg      simple VM instructions
  489: mini-super.vmg     superinstructions (empty at first)
  490: mini.h             common declarations
  491: mini.l             scanner
  492: mini.y             front end (parser, VM code generator)
  493: support.c          main() and other support functions
  494: fib.mini           example mini program
  495: simple.mini        example mini program
  496: test.mini          example mini program (tests everything)
  497: test.out           test.mini output
  498: stat.awk           script for aggregating profile information
  499: peephole-blacklist list of instructions not allowed in superinstructions
  500: seq2rule.awk       script for creating superinstructions
  501: @end example
  502: 
  503: For your own interpreter, you would typically copy the following files
  504: and change little, if anything:
  505: @cindex wrapper files
  506: 
  507: @example
  508: disasm.c           wrapper file
  509: engine.c           wrapper file
  510: peephole.c         wrapper file
  511: profile.c          wrapper file
  512: stat.awk           script for aggregating profile information
  513: seq2rule.awk       script for creating superinstructions
  514: @end example
  515: 
  516: @noindent
  517: You would typically change much in or replace the following files:
  518: 
  519: @example
  520: Makefile
  521: mini-inst.vmg      simple VM instructions
  522: mini.h             common declarations
  523: mini.l             scanner
  524: mini.y             front end (parser, VM code generator)
  525: support.c          main() and other support functions
  526: peephole-blacklist list of instructions not allowed in superinstructions
  527: @end example
  528: 
  529: You can build the example by @code{cd}ing into the example's directory,
  530: and then typing @code{make}; you can check that it works with @code{make
  531: check}.  You can run run mini programs like this:
  532: 
  533: @example
  534: ./mini fib.mini
  535: @end example
  536: 
  537: To learn about the options, type @code{./mini -h}.
  538: 
  539: @c --------------------------------------------------------------------
  540: @node Using profiling to create superinstructions,  , Example overview, Example
  541: @section Using profiling to create superinstructions
  542: @cindex profiling example
  543: @cindex superinstructions example
  544: 
  545: I have not added rules for this in the @file{Makefile} (there are many
  546: options for selecting superinstructions, and I did not want to hardcode
  547: one into the @file{Makefile}), but there are some supporting scripts, and
  548: here's an example:
  549: 
  550: Suppose you want to use @file{fib.mini} and @file{test.mini} as training
  551: programs, you get the profiles like this:
  552: 
  553: @example
  554: make fib.prof test.prof #takes a few seconds
  555: @end example
  556: 
  557: You can aggregate these profiles with @file{stat.awk}:
  558: 
  559: @example
  560: awk -f stat.awk fib.prof test.prof
  561: @end example
  562: 
  563: The result contains lines like:
  564: 
  565: @example
  566:       2      16        36910041 loadlocal lit
  567: @end example
  568: 
  569: This means that the sequence @code{loadlocal lit} statically occurs a
  570: total of 16 times in 2 profiles, with a dynamic execution count of
  571: 36910041.
  572: 
  573: The numbers can be used in various ways to select superinstructions.
  574: E.g., if you just want to select all sequences with a dynamic
  575: execution count exceeding 10000, you would use the following pipeline:
  576: 
  577: @example
  578: awk -f stat.awk fib.prof test.prof|
  579: awk '$3>=10000'|                #select sequences
  580: fgrep -v -f peephole-blacklist| #eliminate wrong instructions
  581: awk -f seq2rule.awk|  #transform sequences into superinstruction rules
  582: sort -k 3 >mini-super.vmg       #sort sequences
  583: @end example
  584: 
  585: The file @file{peephole-blacklist} contains all instructions that
  586: directly access a stack or stack pointer (for mini: @code{call},
  587: @code{return}); the sort step is necessary to ensure that prefixes
  588: precede larger superinstructions.
  589: 
  590: Now you can create a version of mini with superinstructions by just
  591: saying @samp{make}
  592: 
  593: 
  594: @c ***************************************************************
  595: @node Input File Format, Error messages, Example, Top
  596: @chapter Input File Format
  597: @cindex input file format
  598: @cindex format, input file
  599: 
  600: Vmgen takes as input a file containing specifications of virtual machine
  601: instructions.  This file usually has a name ending in @file{.vmg}.
  602: 
  603: Most examples are taken from the example in @file{vmgen-ex}.
  604: 
  605: @menu
  606: * Input File Grammar::          
  607: * Simple instructions::         
  608: * Superinstructions::           
  609: * Store Optimization::          
  610: * Register Machines::           How to define register VM instructions
  611: @end menu
  612: 
  613: @c --------------------------------------------------------------------
  614: @node Input File Grammar, Simple instructions, Input File Format, Input File Format
  615: @section Input File Grammar
  616: @cindex grammar, input file
  617: @cindex input file grammar
  618: 
  619: The grammar is in EBNF format, with @code{@var{a}|@var{b}} meaning
  620: ``@var{a} or @var{b}'', @code{@{@var{c}@}} meaning 0 or more repetitions
  621: of @var{c} and @code{[@var{d}]} meaning 0 or 1 repetitions of @var{d}.
  622: 
  623: @cindex free-format, not
  624: @cindex newlines, significance in syntax
  625: Vmgen input is not free-format, so you have to take care where you put
  626: newlines (and, in a few cases, white space).
  627: 
  628: @example
  629: description: @{instruction|comment|eval-escape|c-escape@}
  630: 
  631: instruction: simple-inst|superinst
  632: 
  633: simple-inst: ident '(' stack-effect ')' newline c-code newline newline
  634: 
  635: stack-effect: @{ident@} '--' @{ident@}
  636: 
  637: super-inst: ident '=' ident @{ident@}  
  638: 
  639: comment:      '\ '  text newline
  640: 
  641: eval-escape:  '\E ' text newline
  642: 
  643: c-escape:     '\C ' text newline
  644: @end example
  645: @c \+ \- \g \f \c
  646: 
  647: Note that the @code{\}s in this grammar are meant literally, not as
  648: C-style encodings for non-printable characters.
  649: 
  650: There are two ways to delimit the C code in @code{simple-inst}:
  651: 
  652: @itemize @bullet
  653: 
  654: @item
  655: If you start it with a @samp{@{} at the start of a line (i.e., not even
  656: white space before it), you have to end it with a @samp{@}} at the start
  657: of a line (followed by a newline).  In this case you may have empty
  658: lines within the C code (typically used between variable definitions and
  659: statements).
  660: 
  661: @item
  662: You do not start it with @samp{@{}.  Then the C code ends at the first
  663: empty line, so you cannot have empty lines within this code.
  664: 
  665: @end itemize
  666: 
  667: The text in @code{comment}, @code{eval-escape} and @code{c-escape} must
  668: not contain a newline.  @code{Ident} must conform to the usual
  669: conventions of C identifiers (otherwise the C compiler would choke on
  670: the Vmgen output), except that idents in @code{stack-effect} may have a
  671: stack prefix (for stack prefix syntax, @pxref{Eval escapes}).
  672: 
  673: @cindex C escape
  674: @cindex @code{\C}
  675: @cindex conditional compilation of Vmgen output
  676: The @code{c-escape} passes the text through to each output file (without
  677: the @samp{\C}).  This is useful mainly for conditional compilation
  678: (i.e., you write @samp{\C #if ...} etc.).
  679: 
  680: @cindex sync lines
  681: @cindex @code{#line}
  682: In addition to the syntax given in the grammer, Vmgen also processes
  683: sync lines (lines starting with @samp{#line}), as produced by @samp{m4
  684: -s} (@pxref{Invoking m4, , Invoking m4, m4.info, GNU m4}) and similar
  685: tools.  This allows associating C compiler error messages with the
  686: original source of the C code.
  687: 
  688: Vmgen understands a few extensions beyond the grammar given here, but
  689: these extensions are only useful for building Gforth.  You can find a
  690: description of the format used for Gforth in @file{prim}.
  691: 
  692: @menu
  693: * Eval escapes::                what follows \E
  694: @end menu
  695: 
  696: @node Eval escapes,  , Input File Grammar, Input File Grammar
  697: @subsection Eval escapes
  698: @cindex escape to Forth
  699: @cindex eval escape
  700: @cindex @code{\E}
  701: 
  702: @c woanders?
  703: The text in @code{eval-escape} is Forth code that is evaluated when
  704: Vmgen reads the line.  You will normally use this feature to define
  705: stacks and types.
  706: 
  707: If you do not know (and do not want to learn) Forth, you can build the
  708: text according to the following grammar; these rules are normally all
  709: Forth you need for using Vmgen:
  710: 
  711: @example
  712: text: stack-decl|type-prefix-decl|stack-prefix-decl|set-flag
  713: 
  714: stack-decl: 'stack ' ident ident ident
  715: type-prefix-decl: 
  716:     's" ' string '" ' ('single'|'double') ident 'type-prefix' ident
  717: stack-prefix-decl:  ident 'stack-prefix' string
  718: set-flag: 'store-optimization' ('on'|'off')
  719: @end example
  720: 
  721: Note that the syntax of this code is not checked thoroughly (there are
  722: many other Forth program fragments that could be written in an
  723: eval-escape).
  724: 
  725: A stack prefix can contain letters, digits, or @samp{:}, and may start
  726: with an @samp{#}; e.g., in Gforth the return stack has the stack prefix
  727: @samp{R:}.  This restriction is not checked during the stack prefix
  728: definition, but it is enforced by the parsing rules for stack items
  729: later.
  730: 
  731: If you know Forth, the stack effects of the non-standard words involved
  732: are:
  733: @findex stack
  734: @findex type-prefix
  735: @findex single
  736: @findex double
  737: @findex stack-prefix
  738: @findex store-optimization
  739: @example
  740: stack              ( "name" "pointer" "type" -- )
  741:                    ( name execution: -- stack )
  742: type-prefix        ( addr u item-size stack "prefix" -- )
  743: single             ( -- item-size )
  744: double             ( -- item-size )
  745: stack-prefix       ( stack "prefix" -- )
  746: store-optimization ( -- addr )
  747: @end example
  748: 
  749: An @var{item-size} takes three cells on the stack.
  750: 
  751: @c --------------------------------------------------------------------
  752: @node Simple instructions, Superinstructions, Input File Grammar, Input File Format
  753: @section Simple instructions
  754: @cindex simple VM instruction
  755: @cindex instruction, simple VM
  756: 
  757: We will use the following simple VM instruction description as example:
  758: 
  759: @example
  760: sub ( i1 i2 -- i )
  761: i = i1-i2;
  762: @end example
  763: 
  764: The first line specifies the name of the VM instruction (@code{sub}) and
  765: its stack effect (@code{i1 i2 -- i}).  The rest of the description is
  766: just plain C code.
  767: 
  768: @cindex stack effect
  769: @cindex effect, stack
  770: The stack effect specifies that @code{sub} pulls two integers from the
  771: data stack and puts them in the C variables @code{i1} and @code{i2}
  772: (with the rightmost item (@code{i2}) taken from the top of stack;
  773: intuition: if you push @code{i1}, then @code{i2} on the stack, the
  774: resulting stack picture is @code{i1 i2}) and later pushes one integer
  775: (@code{i}) on the data stack (the rightmost item is on the top
  776: afterwards).
  777: 
  778: @cindex prefix, type
  779: @cindex type prefix
  780: @cindex default stack of a type prefix
  781: How do we know the type and stack of the stack items?  Vmgen uses
  782: prefixes, similar to Fortran; in contrast to Fortran, you have to
  783: define the prefix first:
  784: 
  785: @example
  786: \E s" Cell"   single data-stack type-prefix i
  787: @end example
  788: 
  789: This defines the prefix @code{i} to refer to the type @code{Cell}
  790: (defined as @code{long} in @file{mini.h}) and, by default, to the
  791: @code{data-stack}.  It also specifies that this type takes one stack
  792: item (@code{single}).  The type prefix is part of the variable name.
  793: 
  794: @cindex stack definition
  795: @cindex defining a stack
  796: Before we can use @code{data-stack} in this way, we have to define it:
  797: 
  798: @example
  799: \E stack data-stack sp Cell
  800: @end example
  801: @c !! use something other than Cell
  802: 
  803: @cindex stack basic type
  804: @cindex basic type of a stack
  805: @cindex type of a stack, basic
  806: @cindex stack growth direction
  807: This line defines the stack @code{data-stack}, which uses the stack
  808: pointer @code{sp}, and each item has the basic type @code{Cell}; other
  809: types have to fit into one or two @code{Cell}s (depending on whether the
  810: type is @code{single} or @code{double} wide), and are cast from and to
  811: Cells on accessing the @code{data-stack} with type cast macros
  812: (@pxref{VM engine}).  Stacks grow towards lower addresses in
  813: Vmgen-erated interpreters.
  814: 
  815: @cindex stack prefix
  816: @cindex prefix, stack
  817: We can override the default stack of a stack item by using a stack
  818: prefix.  E.g., consider the following instruction:
  819: 
  820: @example
  821: lit ( #i -- i )
  822: @end example
  823: 
  824: The VM instruction @code{lit} takes the item @code{i} from the
  825: instruction stream (indicated by the prefix @code{#}), and pushes it on
  826: the (default) data stack.  The stack prefix is not part of the variable
  827: name.  Stack prefixes are defined like this:
  828: 
  829: @example
  830: \E inst-stream stack-prefix #
  831: @end example
  832: 
  833: This definition defines that the stack prefix @code{#} specifies the
  834: ``stack'' @code{inst-stream}.  Since the instruction stream behaves a
  835: little differently than an ordinary stack, it is predefined, and you do
  836: not need to define it.
  837: 
  838: @cindex instruction stream
  839: The instruction stream contains instructions and their immediate
  840: arguments, so specifying that an argument comes from the instruction
  841: stream indicates an immediate argument.  Of course, instruction stream
  842: arguments can only appear to the left of @code{--} in the stack effect.
  843: If there are multiple instruction stream arguments, the leftmost is the
  844: first one (just as the intuition suggests).
  845: 
  846: @menu
  847: * C Code Macros::               Macros recognized by Vmgen
  848: * C Code restrictions::         Vmgen makes assumptions about C code
  849: @end menu
  850: 
  851: @c --------------------------------------------------------------------
  852: @node C Code Macros, C Code restrictions, Simple instructions, Simple instructions
  853: @subsection C Code Macros
  854: @cindex macros recognized by Vmgen
  855: @cindex basic block, VM level
  856: 
  857: Vmgen recognizes the following strings in the C code part of simple
  858: instructions:
  859: 
  860: @table @code
  861: 
  862: @item SET_IP
  863: @findex SET_IP
  864: As far as Vmgen is concerned, a VM instruction containing this ends a VM
  865: basic block (used in profiling to delimit profiled sequences).  On the C
  866: level, this also sets the instruction pointer.
  867: 
  868: @item SUPER_END
  869: @findex SUPER_END
  870: This ends a basic block (for profiling), even if the instruction
  871: contains no @code{SET_IP}.
  872: 
  873: @item INST_TAIL;
  874: @findex INST_TAIL;
  875: Vmgen replaces @samp{INST_TAIL;} with code for ending a VM instruction and
  876: dispatching the next VM instruction.  Even without a @samp{INST_TAIL;} this
  877: happens automatically when control reaches the end of the C code.  If
  878: you want to have this in the middle of the C code, you need to use
  879: @samp{INST_TAIL;}.  A typical example is a conditional VM branch:
  880: 
  881: @example
  882: if (branch_condition) @{
  883:   SET_IP(target); INST_TAIL;
  884: @}
  885: /* implicit tail follows here */
  886: @end example
  887: 
  888: In this example, @samp{INST_TAIL;} is not strictly necessary, because there
  889: is another one implicitly after the if-statement, but using it improves
  890: branch prediction accuracy slightly and allows other optimizations.
  891: 
  892: @item SUPER_CONTINUE
  893: @findex SUPER_CONTINUE
  894: This indicates that the implicit tail at the end of the VM instruction
  895: dispatches the sequentially next VM instruction even if there is a
  896: @code{SET_IP} in the VM instruction.  This enables an optimization that
  897: is not yet implemented in the vmgen-ex code (but in Gforth).  The
  898: typical application is in conditional VM branches:
  899: 
  900: @example
  901: if (branch_condition) @{
  902:   SET_IP(target); INST_TAIL; /* now this INST_TAIL is necessary */
  903: @}
  904: SUPER_CONTINUE;
  905: @end example
  906: 
  907: @end table
  908: 
  909: Note that Vmgen is not smart about C-level tokenization, comments,
  910: strings, or conditional compilation, so it will interpret even a
  911: commented-out SUPER_END as ending a basic block (or, e.g.,
  912: @samp{RESET_IP;} as @samp{SET_IP;}).  Conversely, Vmgen requires the literal
  913: presence of these strings; Vmgen will not see them if they are hiding in
  914: a C preprocessor macro.
  915: 
  916: 
  917: @c --------------------------------------------------------------------
  918: @node C Code restrictions,  , C Code Macros, Simple instructions
  919: @subsection C Code restrictions
  920: @cindex C code restrictions
  921: @cindex restrictions on C code
  922: @cindex assumptions about C code
  923: 
  924: @cindex accessing stack (pointer)
  925: @cindex stack pointer, access
  926: @cindex instruction pointer, access
  927: Vmgen generates code and performs some optimizations under the
  928: assumption that the user-supplied C code does not access the stack
  929: pointers or stack items, and that accesses to the instruction pointer
  930: only occur through special macros.  In general you should heed these
  931: restrictions.  However, if you need to break these restrictions, read
  932: the following.
  933: 
  934: Accessing a stack or stack pointer directly can be a problem for several
  935: reasons: 
  936: @cindex stack caching, restriction on C code
  937: @cindex superinstructions, restrictions on components
  938: 
  939: @itemize @bullet
  940: 
  941: @item
  942: Vmgen optionally supports caching the top-of-stack item in a local
  943: variable (that is allocated to a register).  This is the most frequent
  944: source of trouble.  You can deal with it either by not using
  945: top-of-stack caching (slowdown factor 1-1.4, depending on machine), or
  946: by inserting flushing code (e.g., @samp{IF_spTOS(sp[...] = spTOS);}) at
  947: the start and reloading code (e.g., @samp{IF_spTOS(spTOS = sp[0])}) at
  948: the end of problematic C code.  Vmgen inserts a stack pointer update
  949: before the start of the user-supplied C code, so the flushing code has
  950: to use an index that corrects for that.  In the future, this flushing
  951: may be done automatically by mentioning a special string in the C code.
  952: @c sometimes flushing and/or reloading unnecessary
  953: 
  954: @item
  955: The Vmgen-erated code loads the stack items from stack-pointer-indexed
  956: memory into variables before the user-supplied C code, and stores them
  957: from variables to stack-pointer-indexed memory afterwards.  If you do
  958: any writes to the stack through its stack pointer in your C code, it
  959: will not affect the variables, and your write may be overwritten by the
  960: stores after the C code.  Similarly, a read from a stack using a stack
  961: pointer will not reflect computations of stack items in the same VM
  962: instruction.
  963: 
  964: @item
  965: Superinstructions keep stack items in variables across the whole
  966: superinstruction.  So you should not include VM instructions, that
  967: access a stack or stack pointer, as components of superinstructions
  968: (@pxref{VM profiler}).
  969: 
  970: @end itemize
  971: 
  972: You should access the instruction pointer only through its special
  973: macros (@samp{IP}, @samp{SET_IP}, @samp{IPTOS}); this ensure that these
  974: macros can be implemented in several ways for best performance.
  975: @samp{IP} points to the next instruction, and @samp{IPTOS} is its
  976: contents.
  977: 
  978: 
  979: @c --------------------------------------------------------------------
  980: @node Superinstructions, Store Optimization, Simple instructions, Input File Format
  981: @section Superinstructions
  982: @cindex superinstructions, defining
  983: @cindex defining superinstructions
  984: 
  985: Note: don't invest too much work in (static) superinstructions; a future
  986: version of Vmgen will support dynamic superinstructions (see Ian
  987: Piumarta and Fabio Riccardi, @cite{Optimizing Direct Threaded Code by
  988: Selective Inlining}, PLDI'98), and static superinstructions have much
  989: less benefit in that context (preliminary results indicate only a factor
  990: 1.1 speedup).
  991: 
  992: Here is an example of a superinstruction definition:
  993: 
  994: @example
  995: lit_sub = lit sub
  996: @end example
  997: 
  998: @code{lit_sub} is the name of the superinstruction, and @code{lit} and
  999: @code{sub} are its components.  This superinstruction performs the same
 1000: action as the sequence @code{lit} and @code{sub}.  It is generated
 1001: automatically by the VM code generation functions whenever that sequence
 1002: occurs, so if you want to use this superinstruction, you just need to
 1003: add this definition (and even that can be partially automatized,
 1004: @pxref{VM profiler}).
 1005: 
 1006: @cindex prefixes of superinstructions
 1007: Vmgen requires that the component instructions are simple instructions
 1008: defined before superinstructions using the components.  Currently, Vmgen
 1009: also requires that all the subsequences at the start of a
 1010: superinstruction (prefixes) must be defined as superinstruction before
 1011: the superinstruction.  I.e., if you want to define a superinstruction
 1012: 
 1013: @example
 1014: foo4 = load add sub mul
 1015: @end example
 1016: 
 1017: you first have to define @code{load}, @code{add}, @code{sub} and
 1018: @code{mul}, plus
 1019: 
 1020: @example
 1021: foo2 = load add
 1022: foo3 = load add sub
 1023: @end example
 1024: 
 1025: Here, @code{sumof4} is the longest prefix of @code{sumof5}, and @code{sumof3}
 1026: is the longest prefix of @code{sumof4}.
 1027: 
 1028: Note that Vmgen assumes that only the code it generates accesses stack
 1029: pointers, the instruction pointer, and various stack items, and it
 1030: performs optimizations based on this assumption.  Therefore, VM
 1031: instructions where your C code changes the instruction pointer should
 1032: only be used as last component; a VM instruction where your C code
 1033: accesses a stack pointer should not be used as component at all.  Vmgen
 1034: does not check these restrictions, they just result in bugs in your
 1035: interpreter.
 1036: 
 1037: @c -------------------------------------------------------------------
 1038: @node  Store Optimization, Register Machines, Superinstructions, Input File Format
 1039: @section Store Optimization
 1040: @cindex store optimization
 1041: @cindex optimization, stack stores
 1042: @cindex stack stores, optimization
 1043: @cindex eliminating stack stores
 1044: 
 1045: This minor optimization (0.6\%--0.8\% reduction in executed instructions
 1046: for Gforth) puts additional requirements on the instruction descriptions
 1047: and is therefore disabled by default.
 1048: 
 1049: What does it do?  Consider an instruction like
 1050: 
 1051: @example
 1052: dup ( n -- n n )
 1053: @end example
 1054: 
 1055: For simplicity, also assume that we are not caching the top-of-stack in
 1056: a register.  Now, the C code for dup first loads @code{n} from the
 1057: stack, and then stores it twice to the stack, one time to the address
 1058: where it came from; that time is unnecessary, but gcc does not optimize
 1059: it away, so vmgen can do it instead (if you turn on the store
 1060: optimization).
 1061: 
 1062: Vmgen uses the stack item's name to determine if the stack item contains
 1063: the same value as it did at the start.  Therefore, if you use the store
 1064: optimization, you have to ensure that stack items that have the same
 1065: name on input and output also have the same value, and are not changed
 1066: in the C code you supply.  I.e., the following code could fail if you
 1067: turn on the store optimization:
 1068: 
 1069: @example
 1070: add1 ( n -- n )
 1071: n++;
 1072: @end example
 1073: 
 1074: Instead, you have to use different names, i.e.:
 1075: 
 1076: @example
 1077: add1 ( n1 -- n1 )
 1078: n2=n1+1;
 1079: @end example
 1080: 
 1081: To turn on the store optimization, write
 1082: 
 1083: @example
 1084: \E store-optimization on
 1085: @end example
 1086: 
 1087: at the start of the file.  You can turn this optimization on or off
 1088: between any two VM instruction descriptions.  For turning it off again,
 1089: you can use
 1090: 
 1091: @example
 1092: \E store-optimization off
 1093: @end example
 1094: 
 1095: @c -------------------------------------------------------------------
 1096: @node Register Machines,  , Store Optimization, Input File Format
 1097: @section Register Machines
 1098: @cindex Register VM
 1099: @cindex Superinstructions for register VMs
 1100: @cindex tracing of register VMs
 1101: 
 1102: If you want to implement a register VM rather than a stack VM with
 1103: Vmgen, there are two ways to do it: Directly and through
 1104: superinstructions.
 1105: 
 1106: If you use the direct way, you define instructions that take the
 1107: register numbers as immediate arguments, like this:
 1108: 
 1109: @example
 1110: add3 ( #src1 #src2 #dest -- )
 1111: reg[dest] = reg[src1]+reg[src2];
 1112: @end example
 1113: 
 1114: A disadvantage of this method is that during tracing you only see the
 1115: register numbers, but not the register contents.  Actually, with an
 1116: appropriate definition of @code{printarg_src} (@pxref{VM engine}), you
 1117: can print the values of the source registers on entry, but you cannot
 1118: print the value of the destination register on exit.
 1119: 
 1120: If you use superinstructions to define a register VM, you define simple
 1121: instructions that use a stack, and then define superinstructions that
 1122: have no overall stack effect, like this:
 1123: 
 1124: @example
 1125: loadreg ( #src -- n )
 1126: n = reg[src];
 1127: 
 1128: storereg ( n #dest -- )
 1129: reg[dest] = n;
 1130: 
 1131: adds ( n1 n2 -- n )
 1132: n = n1+n2;
 1133: 
 1134: add3 = loadreg loadreg adds storereg
 1135: @end example
 1136: 
 1137: An advantage of this method is that you see the values and not just the
 1138: register numbers in tracing.  A disadvantage of this method is that
 1139: currently you cannot generate superinstructions directly, but only
 1140: through generating a sequence of simple instructions (we might change
 1141: this in the future if there is demand).
 1142: 
 1143: Could the register VM support be improved, apart from the issues
 1144: mentioned above?  It is hard to see how to do it in a general way,
 1145: because there are a number of different designs that different people
 1146: mean when they use the term @emph{register machine} in connection with
 1147: VM interpreters.  However, if you have ideas or requests in that
 1148: direction, please let me know (@pxref{Contact}).
 1149: 
 1150: @c ********************************************************************
 1151: @node Error messages, Using the generated code, Input File Format, Top
 1152: @chapter Error messages
 1153: @cindex error messages
 1154: 
 1155: These error messages are created by Vmgen:
 1156: 
 1157: @table @code
 1158: 
 1159: @cindex @code{# can only be on the input side} error
 1160: @item # can only be on the input side
 1161: You have used an instruction-stream prefix (usually @samp{#}) after the
 1162: @samp{--} (the output side); you can only use it before (the input
 1163: side).
 1164: 
 1165: @cindex @code{prefix for this combination must be defined earlier} error
 1166: @item the prefix for this combination must be defined earlier
 1167: You have defined a superinstruction (e.g. @code{abc = a b c}) without
 1168: defining its direct prefix (e.g., @code{ab = a b}),
 1169: @xref{Superinstructions}.
 1170: 
 1171: @cindex @code{sync line syntax} error
 1172: @item sync line syntax
 1173: If you are using a preprocessor (e.g., @command{m4}) to generate Vmgen
 1174: input code, you may want to create @code{#line} directives (aka sync
 1175: lines).  This error indicates that such a line is not in th syntax
 1176: expected by Vmgen (this should not happen; please report the offending
 1177: line in a bug report).
 1178: 
 1179: @cindex @code{syntax error, wrong char} error
 1180: @cindex syntax error, wrong char
 1181: A syntax error.  If you do not see right away where the error is, it may
 1182: be helpful to check the following: Did you put an empty line in a VM
 1183: instruction where the C code is not delimited by braces (then the empty
 1184: line ends the VM instruction)?  If you used brace-delimited C code, did
 1185: you put the delimiting braces (and only those) at the start of the line,
 1186: without preceding white space?  Did you forget a delimiting brace?
 1187: 
 1188: @cindex @code{too many stacks} error
 1189: @item too many stacks
 1190: Vmgen currently supports 3 stacks (plus the instruction stream); if you
 1191: need more, let us know.
 1192: 
 1193: @cindex @code{unknown prefix} error
 1194: @item unknown prefix
 1195: The stack item does not match any defined type prefix (after stripping
 1196: away any stack prefix).  You should either declare the type prefix you
 1197: want for that stack item, or use a different type prefix
 1198: 
 1199: @item @code{unknown primitive} error
 1200: @item unknown primitive
 1201: You have used the name of a simple VM instruction in a superinstruction
 1202: definition without defining the simple VM instruction first.
 1203: 
 1204: @end table
 1205: 
 1206: In addition, the C compiler can produce errors due to code produced by
 1207: Vmgen; e.g., you need to define type cast functions.
 1208: 
 1209: @c ********************************************************************
 1210: @node Using the generated code, Hints, Error messages, Top
 1211: @chapter Using the generated code
 1212: @cindex generated code, usage
 1213: @cindex Using vmgen-erated code
 1214: 
 1215: The easiest way to create a working VM interpreter with Vmgen is
 1216: probably to start with @file{vmgen-ex}, and modify it for your purposes.
 1217: This chapter explains what the various wrapper and generated files do.
 1218: It also contains reference-manual style descriptions of the macros,
 1219: variables etc. used by the generated code, and you can skip that on
 1220: first reading.
 1221: 
 1222: @menu
 1223: * VM engine::                   Executing VM code
 1224: * VM instruction table::        
 1225: * VM code generation::          Creating VM code (in the front-end)
 1226: * Peephole optimization::       Creating VM superinstructions
 1227: * VM disassembler::             for debugging the front end
 1228: * VM profiler::                 for finding worthwhile superinstructions
 1229: @end menu
 1230: 
 1231: @c --------------------------------------------------------------------
 1232: @node VM engine, VM instruction table, Using the generated code, Using the generated code
 1233: @section VM engine
 1234: @cindex VM instruction execution
 1235: @cindex engine
 1236: @cindex executing VM code
 1237: @cindex @file{engine.c}
 1238: @cindex @file{-vm.i} output file
 1239: 
 1240: The VM engine is the VM interpreter that executes the VM code.  It is
 1241: essential for an interpretive system.
 1242: 
 1243: Vmgen supports two methods of VM instruction dispatch: @emph{threaded
 1244: code} (fast, but gcc-specific), and @emph{switch dispatch} (slow, but
 1245: portable across C compilers); you can use conditional compilation
 1246: (@samp{defined(__GNUC__)}) to choose between these methods, and our
 1247: example does so.
 1248: 
 1249: For both methods, the VM engine is contained in a C-level function.
 1250: Vmgen generates most of the contents of the function for you
 1251: (@file{@var{name}-vm.i}), but you have to define this function, and
 1252: macros and variables used in the engine, and initialize the variables.
 1253: In our example the engine function also includes
 1254: @file{@var{name}-labels.i} (@pxref{VM instruction table}).
 1255: 
 1256: @cindex tracing VM code
 1257: @cindex superinstructions and tracing
 1258: In addition to executing the code, the VM engine can optionally also
 1259: print out a trace of the executed instructions, their arguments and
 1260: results.  For superinstructions it prints the trace as if only component
 1261: instructions were executed; this allows to introduce new
 1262: superinstructions while keeping the traces comparable to old ones
 1263: (important for regression tests).
 1264: 
 1265: It costs significant performance to check in each instruction whether to
 1266: print tracing code, so we recommend producing two copies of the engine:
 1267: one for fast execution, and one for tracing.  See the rules for
 1268: @file{engine.o} and @file{engine-debug.o} in @file{vmgen-ex/Makefile}
 1269: for an example.
 1270: 
 1271: The following macros and variables are used in @file{@var{name}-vm.i}:
 1272: 
 1273: @table @code
 1274: 
 1275: @findex LABEL
 1276: @item LABEL(@var{inst_name})
 1277: This is used just before each VM instruction to provide a jump or
 1278: @code{switch} label (the @samp{:} is provided by Vmgen).  For switch
 1279: dispatch this should expand to @samp{case @var{label}:}; for
 1280: threaded-code dispatch this should just expand to @samp{@var{label}:}.
 1281: In either case @var{label} is usually the @var{inst_name} with some
 1282: prefix or suffix to avoid naming conflicts.
 1283: 
 1284: @findex LABEL2
 1285: @item LABEL2(@var{inst_name})
 1286: This will be used for dynamic superinstructions; at the moment, this
 1287: should expand to nothing.
 1288: 
 1289: @findex NAME
 1290: @item NAME(@var{inst_name_string})
 1291: Called on entering a VM instruction with a string containing the name of
 1292: the VM instruction as parameter.  In normal execution this should be
 1293: expand to nothing, but for tracing this usually prints the name, and
 1294: possibly other information (several VM registers in our example).
 1295: 
 1296: @findex DEF_CA
 1297: @item DEF_CA
 1298: Usually empty.  Called just inside a new scope at the start of a VM
 1299: instruction.  Can be used to define variables that should be visible
 1300: during every VM instruction.  If you define this macro as non-empty, you
 1301: have to provide the finishing @samp{;} in the macro.
 1302: 
 1303: @findex NEXT_P0
 1304: @findex NEXT_P1
 1305: @findex NEXT_P2
 1306: @item NEXT_P0 NEXT_P1 NEXT_P2
 1307: The three parts of instruction dispatch.  They can be defined in
 1308: different ways for best performance on various processors (see
 1309: @file{engine.c} in the example or @file{engine/threaded.h} in Gforth).
 1310: @samp{NEXT_P0} is invoked right at the start of the VM instruction (but
 1311: after @samp{DEF_CA}), @samp{NEXT_P1} right after the user-supplied C
 1312: code, and @samp{NEXT_P2} at the end.  The actual jump has to be
 1313: performed by @samp{NEXT_P2} (if you would do it earlier, important parts
 1314: of the VM instruction would not be executed).
 1315: 
 1316: The simplest variant is if @samp{NEXT_P2} does everything and the other
 1317: macros do nothing.  Then also related macros like @samp{IP},
 1318: @samp{SET_IP}, @samp{IP}, @samp{INC_IP} and @samp{IPTOS} are very
 1319: straightforward to define.  For switch dispatch this code consists just
 1320: of a jump to the dispatch code (@samp{goto next_inst;} in our example);
 1321: for direct threaded code it consists of something like
 1322: @samp{(@{cfa=*ip++; goto *cfa;@})}.
 1323: 
 1324: Pulling code (usually the @samp{cfa=*ip++;}) up into @samp{NEXT_P1}
 1325: usually does not cause problems, but pulling things up into
 1326: @samp{NEXT_P0} usually requires changing the other macros (and, at least
 1327: for Gforth on Alpha, it does not buy much, because the compiler often
 1328: manages to schedule the relevant stuff up by itself).  An even more
 1329: extreme variant is to pull code up even further, into, e.g., NEXT_P1 of
 1330: the previous VM instruction (prefetching, useful on PowerPCs).
 1331: 
 1332: @findex INC_IP
 1333: @item INC_IP(@var{n})
 1334: This increments @code{IP} by @var{n}.
 1335: 
 1336: @findex SET_IP
 1337: @item SET_IP(@var{target})
 1338: This sets @code{IP} to @var{target}.
 1339: 
 1340: @cindex type cast macro
 1341: @findex vm_@var{A}2@var{B}
 1342: @item vm_@var{A}2@var{B}(a,b)
 1343: Type casting macro that assigns @samp{a} (of type @var{A}) to @samp{b}
 1344: (of type @var{B}).  This is mainly used for getting stack items into
 1345: variables and back.  So you need to define macros for every combination
 1346: of stack basic type (@code{Cell} in our example) and type-prefix types
 1347: used with that stack (in both directions).  For the type-prefix type,
 1348: you use the type-prefix (not the C type string) as type name (e.g.,
 1349: @samp{vm_Cell2i}, not @samp{vm_Cell2Cell}).  In addition, you have to
 1350: define a vm_@var{X}2@var{X} macro for the stack's basic type @var{X}
 1351: (used in superinstructions).
 1352: 
 1353: @cindex instruction stream, basic type
 1354: The stack basic type for the predefined @samp{inst-stream} is
 1355: @samp{Cell}.  If you want a stack with the same item size, making its
 1356: basic type @samp{Cell} usually reduces the number of macros you have to
 1357: define.
 1358: 
 1359: @cindex unions in type cast macros
 1360: @cindex casts in type cast macros
 1361: @cindex type casting between floats and integers
 1362: Here our examples differ a lot: @file{vmgen-ex} uses casts in these
 1363: macros, whereas @file{vmgen-ex2} uses union-field selection (or
 1364: assignment to union fields).  Note that casting floats into integers and
 1365: vice versa changes the bit pattern (and you do not want that).  In this
 1366: case your options are to use a (temporary) union, or to take the address
 1367: of the value, cast the pointer, and dereference that (not always
 1368: possible, and sometimes expensive).
 1369: 
 1370: @findex vm_two@var{A}2@var{B}
 1371: @findex vm_@var{B}2two@var{A}
 1372: @item vm_two@var{A}2@var{B}(a1,a2,b)
 1373: @item vm_@var{B}2two@var{A}(b,a1,a2)
 1374: Type casting between two stack items (@code{a1}, @code{a2}) and a
 1375: variable @code{b} of a type that takes two stack items.  This does not
 1376: occur in our small examples, but you can look at Gforth for examples
 1377: (see @code{vm_twoCell2d} in @file{engine/forth.h}).
 1378: 
 1379: @cindex stack pointer definition
 1380: @cindex instruction pointer definition
 1381: @item @var{stackpointer}
 1382: For each stack used, the stackpointer name given in the stack
 1383: declaration is used.  For a regular stack this must be an l-expression;
 1384: typically it is a variable declared as a pointer to the stack's basic
 1385: type.  For @samp{inst-stream}, the name is @samp{IP}, and it can be a
 1386: plain r-value; typically it is a macro that abstracts away the
 1387: differences between the various implementations of @code{NEXT_P*}.
 1388: 
 1389: @cindex top of stack caching
 1390: @cindex stack caching
 1391: @cindex TOS
 1392: @findex IPTOS
 1393: @item @var{stackpointer}TOS
 1394: The top-of-stack for the stack pointed to by @var{stackpointer}.  If you
 1395: are using top-of-stack caching for that stack, this should be defined as
 1396: variable; if you are not using top-of-stack caching for that stack, this
 1397: should be a macro expanding to @samp{@var{stackpointer}[0]}.  The stack
 1398: pointer for the predefined @samp{inst-stream} is called @samp{IP}, so
 1399: the top-of-stack is called @samp{IPTOS}.
 1400: 
 1401: @findex IF_@var{stackpointer}TOS
 1402: @item IF_@var{stackpointer}TOS(@var{expr})
 1403: Macro for executing @var{expr}, if top-of-stack caching is used for the
 1404: @var{stackpointer} stack.  I.e., this should do @var{expr} if there is
 1405: top-of-stack caching for @var{stackpointer}; otherwise it should do
 1406: nothing.
 1407: 
 1408: @findex SUPER_END
 1409: @item SUPER_END
 1410: This is used by the VM profiler (@pxref{VM profiler}); it should not do
 1411: anything in normal operation, and call @code{vm_count_block(IP)} for
 1412: profiling.
 1413: 
 1414: @findex SUPER_CONTINUE
 1415: @item SUPER_CONTINUE
 1416: This is just a hint to Vmgen and does nothing at the C level.
 1417: 
 1418: @findex VM_DEBUG
 1419: @item VM_DEBUG
 1420: If this is defined, the tracing code will be compiled in (slower
 1421: interpretation, but better debugging).  Our example compiles two
 1422: versions of the engine, a fast-running one that cannot trace, and one
 1423: with potential tracing and profiling.
 1424: 
 1425: @findex vm_debug
 1426: @item vm_debug
 1427: Needed only if @samp{VM_DEBUG} is defined.  If this variable contains
 1428: true, the VM instructions produce trace output.  It can be turned on or
 1429: off at any time.
 1430: 
 1431: @findex vm_out
 1432: @item vm_out
 1433: Needed only if @samp{VM_DEBUG} is defined.  Specifies the file on which
 1434: to print the trace output (type @samp{FILE *}).
 1435: 
 1436: @findex printarg_@var{type}
 1437: @item printarg_@var{type}(@var{value})
 1438: Needed only if @samp{VM_DEBUG} is defined.  Macro or function for
 1439: printing @var{value} in a way appropriate for the @var{type}.  This is
 1440: used for printing the values of stack items during tracing.  @var{Type}
 1441: is normally the type prefix specified in a @code{type-prefix} definition
 1442: (e.g., @samp{printarg_i}); in superinstructions it is currently the
 1443: basic type of the stack.
 1444: 
 1445: @end table
 1446: 
 1447: 
 1448: @c --------------------------------------------------------------------
 1449: @node VM instruction table, VM code generation, VM engine, Using the generated code
 1450: @section VM instruction table
 1451: @cindex instruction table
 1452: @cindex opcode definition
 1453: @cindex labels for threaded code
 1454: @cindex @code{vm_prim}, definition
 1455: @cindex @file{-labels.i} output file
 1456: 
 1457: For threaded code we also need to produce a table containing the labels
 1458: of all VM instructions.  This is needed for VM code generation
 1459: (@pxref{VM code generation}), and it has to be done in the engine
 1460: function, because the labels are not visible outside.  It then has to be
 1461: passed outside the function (and assigned to @samp{vm_prim}), to be used
 1462: by the VM code generation functions.
 1463: 
 1464: This means that the engine function has to be called first to produce
 1465: the VM instruction table, and later, after generating VM code, it has to
 1466: be called again to execute the generated VM code (yes, this is ugly).
 1467: In our example program, these two modes of calling the engine function
 1468: are differentiated by the value of the parameter ip0 (if it equals 0,
 1469: then the table is passed out, otherwise the VM code is executed); in our
 1470: example, we pass the table out by assigning it to @samp{vm_prim} and
 1471: returning from @samp{engine}.
 1472: 
 1473: In our example (@file{vmgen-ex/engine.c}), we also build such a table for
 1474: switch dispatch; this is mainly done for uniformity.
 1475: 
 1476: For switch dispatch, we also need to define the VM instruction opcodes
 1477: used as case labels in an @code{enum}.
 1478: 
 1479: For both purposes (VM instruction table, and enum), the file
 1480: @file{@var{name}-labels.i} is generated by Vmgen.  You have to define
 1481: the following macro used in this file:
 1482: 
 1483: @table @code
 1484: 
 1485: @findex INST_ADDR
 1486: @item INST_ADDR(@var{inst_name})
 1487: For switch dispatch, this is just the name of the switch label (the same
 1488: name as used in @samp{LABEL(@var{inst_name})}), for both uses of
 1489: @file{@var{name}-labels.i}.  For threaded-code dispatch, this is the
 1490: address of the label defined in @samp{LABEL(@var{inst_name})}); the
 1491: address is taken with @samp{&&} (@pxref{Labels as Values, , Labels as
 1492: Values, gcc.info, GNU C Manual}).
 1493: 
 1494: @end table
 1495: 
 1496: 
 1497: @c --------------------------------------------------------------------
 1498: @node VM code generation, Peephole optimization, VM instruction table, Using the generated code
 1499: @section VM code generation
 1500: @cindex VM code generation
 1501: @cindex code generation, VM
 1502: @cindex @file{-gen.i} output file
 1503: 
 1504: Vmgen generates VM code generation functions in @file{@var{name}-gen.i}
 1505: that the front end can call to generate VM code.  This is essential for
 1506: an interpretive system.
 1507: 
 1508: @findex gen_@var{inst}
 1509: For a VM instruction @samp{x ( #a b #c -- d )}, Vmgen generates a
 1510: function with the prototype
 1511: 
 1512: @example
 1513: void gen_x(Inst **ctp, a_type a, c_type c)
 1514: @end example
 1515: 
 1516: The @code{ctp} argument points to a pointer to the next instruction.
 1517: @code{*ctp} is increased by the generation functions; i.e., you should
 1518: allocate memory for the code to be generated beforehand, and start with
 1519: *ctp set at the start of this memory area.  Before running out of
 1520: memory, allocate a new area, and generate a VM-level jump to the new
 1521: area (this overflow handling is not implemented in our examples).
 1522: 
 1523: @cindex immediate arguments, VM code generation
 1524: The other arguments correspond to the immediate arguments of the VM
 1525: instruction (with their appropriate types as defined in the
 1526: @code{type_prefix} declaration.
 1527: 
 1528: The following types, variables, and functions are used in
 1529: @file{@var{name}-gen.i}:
 1530: 
 1531: @table @code
 1532: 
 1533: @findex Inst
 1534: @item Inst
 1535: The type of the VM instruction; if you use threaded code, this is
 1536: @code{void *}; for switch dispatch this is an integer type.
 1537: 
 1538: @cindex @code{vm_prim}, use
 1539: @item vm_prim
 1540: The VM instruction table (type: @code{Inst *}, @pxref{VM instruction table}).
 1541: 
 1542: @findex gen_inst
 1543: @item gen_inst(Inst **ctp, Inst i)
 1544: This function compiles the instruction @code{i}.  Take a look at it in
 1545: @file{vmgen-ex/peephole.c}.  It is trivial when you don't want to use
 1546: superinstructions (just the last two lines of the example function), and
 1547: slightly more complicated in the example due to its ability to use
 1548: superinstructions (@pxref{Peephole optimization}).
 1549: 
 1550: @findex genarg_@var{type_prefix}
 1551: @item genarg_@var{type_prefix}(Inst **ctp, @var{type} @var{type_prefix})
 1552: This compiles an immediate argument of @var{type} (as defined in a
 1553: @code{type-prefix} definition).  These functions are trivial to define
 1554: (see @file{vmgen-ex/support.c}).  You need one of these functions for
 1555: every type that you use as immediate argument.
 1556: 
 1557: @end table
 1558: 
 1559: @findex BB_BOUNDARY
 1560: In addition to using these functions to generate code, you should call
 1561: @code{BB_BOUNDARY} at every basic block entry point if you ever want to
 1562: use superinstructions (or if you want to use the profiling supported by
 1563: Vmgen; but this support is also useful mainly for selecting
 1564: superinstructions).  If you use @code{BB_BOUNDARY}, you should also
 1565: define it (take a look at its definition in @file{vmgen-ex/mini.y}).
 1566: 
 1567: You do not need to call @code{BB_BOUNDARY} after branches, because you
 1568: will not define superinstructions that contain branches in the middle
 1569: (and if you did, and it would work, there would be no reason to end the
 1570: superinstruction at the branch), and because the branches announce
 1571: themselves to the profiler.
 1572: 
 1573: 
 1574: @c --------------------------------------------------------------------
 1575: @node Peephole optimization, VM disassembler, VM code generation, Using the generated code
 1576: @section Peephole optimization
 1577: @cindex peephole optimization
 1578: @cindex superinstructions, generating
 1579: @cindex @file{peephole.c}
 1580: @cindex @file{-peephole.i} output file
 1581: 
 1582: You need peephole optimization only if you want to use
 1583: superinstructions.  But having the code for it does not hurt much if you
 1584: do not use superinstructions.
 1585: 
 1586: A simple greedy peephole optimization algorithm is used for
 1587: superinstruction selection: every time @code{gen_inst} compiles a VM
 1588: instruction, it checks if it can combine it with the last VM instruction
 1589: (which may also be a superinstruction resulting from a previous peephole
 1590: optimization); if so, it changes the last instruction to the combined
 1591: instruction instead of laying down @code{i} at the current @samp{*ctp}.
 1592: 
 1593: The code for peephole optimization is in @file{vmgen-ex/peephole.c}.
 1594: You can use this file almost verbatim.  Vmgen generates
 1595: @file{@var{file}-peephole.i} which contains data for the peephoile
 1596: optimizer.
 1597: 
 1598: @findex init_peeptable
 1599: You have to call @samp{init_peeptable()} after initializing
 1600: @samp{vm_prim}, and before compiling any VM code to initialize data
 1601: structures for peephole optimization.  After that, compiling with the VM
 1602: code generation functions will automatically combine VM instructions
 1603: into superinstructions.  Since you do not want to combine instructions
 1604: across VM branch targets (otherwise there will not be a proper VM
 1605: instruction to branch to), you have to call @code{BB_BOUNDARY}
 1606: (@pxref{VM code generation}) at branch targets.
 1607: 
 1608: 
 1609: @c --------------------------------------------------------------------
 1610: @node VM disassembler, VM profiler, Peephole optimization, Using the generated code
 1611: @section VM disassembler
 1612: @cindex VM disassembler
 1613: @cindex disassembler, VM code
 1614: @cindex @file{disasm.c}
 1615: @cindex @file{-disasm.i} output file
 1616: 
 1617: A VM code disassembler is optional for an interpretive system, but
 1618: highly recommended during its development and maintenance, because it is
 1619: very useful for detecting bugs in the front end (and for distinguishing
 1620: them from VM interpreter bugs).
 1621: 
 1622: Vmgen supports VM code disassembling by generating
 1623: @file{@var{file}-disasm.i}.  This code has to be wrapped into a
 1624: function, as is done in @file{vmgen-ex/disasm.c}.  You can use this file
 1625: almost verbatim.  In addition to @samp{vm_@var{A}2@var{B}(a,b)},
 1626: @samp{vm_out}, @samp{printarg_@var{type}(@var{value})}, which are
 1627: explained above, the following macros and variables are used in
 1628: @file{@var{file}-disasm.i} (and you have to define them):
 1629: 
 1630: @table @code
 1631: 
 1632: @item ip
 1633: This variable points to the opcode of the current VM instruction.
 1634: 
 1635: @cindex @code{IP}, @code{IPTOS} in disassmbler
 1636: @item IP IPTOS
 1637: @samp{IPTOS} is the first argument of the current VM instruction, and
 1638: @samp{IP} points to it; this is just as in the engine, but here
 1639: @samp{ip} points to the opcode of the VM instruction (in contrast to the
 1640: engine, where @samp{ip} points to the next cell, or even one further).
 1641: 
 1642: @findex VM_IS_INST
 1643: @item VM_IS_INST(Inst i, int n)
 1644: Tests if the opcode @samp{i} is the same as the @samp{n}th entry in the
 1645: VM instruction table.
 1646: 
 1647: @end table
 1648: 
 1649: 
 1650: @c --------------------------------------------------------------------
 1651: @node VM profiler,  , VM disassembler, Using the generated code
 1652: @section VM profiler
 1653: @cindex VM profiler
 1654: @cindex profiling for selecting superinstructions
 1655: @cindex superinstructions and profiling
 1656: @cindex @file{profile.c}
 1657: @cindex @file{-profile.i} output file
 1658: 
 1659: The VM profiler is designed for getting execution and occurence counts
 1660: for VM instruction sequences, and these counts can then be used for
 1661: selecting sequences as superinstructions.  The VM profiler is probably
 1662: not useful as profiling tool for the interpretive system.  I.e., the VM
 1663: profiler is useful for the developers, but not the users of the
 1664: interpretive system.
 1665: 
 1666: The output of the profiler is: for each basic block (executed at least
 1667: once), it produces the dynamic execution count of that basic block and
 1668: all its subsequences; e.g.,
 1669: 
 1670: @example
 1671:        9227465  lit storelocal 
 1672:        9227465  storelocal branch 
 1673:        9227465  lit storelocal branch 
 1674: @end example
 1675: 
 1676: I.e., a basic block consisting of @samp{lit storelocal branch} is
 1677: executed 9227465 times.
 1678: 
 1679: @cindex @file{stat.awk}
 1680: @cindex @file{seq2rule.awk}
 1681: This output can be combined in various ways.  E.g.,
 1682: @file{vmgen-ex/stat.awk} adds up the occurences of a given sequence wrt
 1683: dynamic execution, static occurence, and per-program occurence.  E.g.,
 1684: 
 1685: @example
 1686:       2      16        36910041 loadlocal lit 
 1687: @end example
 1688: 
 1689: @noindent
 1690: indicates that the sequence @samp{loadlocal lit} occurs in 2 programs,
 1691: in 16 places, and has been executed 36910041 times.  Now you can select
 1692: superinstructions in any way you like (note that compile time and space
 1693: typically limit the number of superinstructions to 100--1000).  After
 1694: you have done that, @file{vmgen/seq2rule.awk} turns lines of the form
 1695: above into rules for inclusion in a Vmgen input file.  Note that this
 1696: script does not ensure that all prefixes are defined, so you have to do
 1697: that in other ways.  So, an overall script for turning profiles into
 1698: superinstructions can look like this:
 1699: 
 1700: @example
 1701: awk -f stat.awk fib.prof test.prof|
 1702: awk '$3>=10000'|                #select sequences
 1703: fgrep -v -f peephole-blacklist| #eliminate wrong instructions
 1704: awk -f seq2rule.awk|            #turn into superinstructions
 1705: sort -k 3 >mini-super.vmg       #sort sequences
 1706: @end example
 1707: 
 1708: Here the dynamic count is used for selecting sequences (preliminary
 1709: results indicate that the static count gives better results, though);
 1710: the third line eliminates sequences containing instructions that must not
 1711: occur in a superinstruction, because they access a stack directly.  The
 1712: dynamic count selection ensures that all subsequences (including
 1713: prefixes) of longer sequences occur (because subsequences have at least
 1714: the same count as the longer sequences); the sort in the last line
 1715: ensures that longer superinstructions occur after their prefixes.
 1716: 
 1717: But before using this, you have to have the profiler.  Vmgen supports its
 1718: creation by generating @file{@var{file}-profile.i}; you also need the
 1719: wrapper file @file{vmgen-ex/profile.c} that you can use almost verbatim.
 1720: 
 1721: @cindex @code{SUPER_END} in profiling
 1722: @cindex @code{BB_BOUNDARY} in profiling
 1723: The profiler works by recording the targets of all VM control flow
 1724: changes (through @code{SUPER_END} during execution, and through
 1725: @code{BB_BOUNDARY} in the front end), and counting (through
 1726: @code{SUPER_END}) how often they were targeted.  After the program run,
 1727: the numbers are corrected such that each VM basic block has the correct
 1728: count (entering a block without executing a branch does not increase the
 1729: count, and the correction fixes that), then the subsequences of all
 1730: basic blocks are printed.  To get all this, you just have to define
 1731: @code{SUPER_END} (and @code{BB_BOUNDARY}) appropriately, and call
 1732: @code{vm_print_profile(FILE *file)} when you want to output the profile
 1733: on @code{file}.
 1734: 
 1735: @cindex @code{VM_IS_INST} in profiling
 1736: The @file{@var{file}-profile.i} is similar to the disassembler file, and
 1737: it uses variables and functions defined in @file{vmgen-ex/profile.c},
 1738: plus @code{VM_IS_INST} already defined for the VM disassembler
 1739: (@pxref{VM disassembler}).
 1740: 
 1741: @c **********************************************************
 1742: @node Hints, The future, Using the generated code, Top
 1743: @chapter Hints
 1744: @cindex hints
 1745: 
 1746: @menu
 1747: * Floating point::              and stacks
 1748: @end menu
 1749: 
 1750: @c --------------------------------------------------------------------
 1751: @node Floating point,  , Hints, Hints
 1752: @section Floating point
 1753: 
 1754: How should you deal with floating point values?  Should you use the same
 1755: stack as for integers/pointers, or a different one?  This section
 1756: discusses this issue with a view on execution speed.
 1757: 
 1758: The simpler approach is to use a separate floating-point stack.  This
 1759: allows you to choose FP value size without considering the size of the
 1760: integers/pointers, and you avoid a number of performance problems.  The
 1761: main downside is that this needs an FP stack pointer (and that may not
 1762: fit in the register file on the 386 arhitecture, costing some
 1763: performance, but comparatively little if you take the other option into
 1764: account).  If you use a separate FP stack (with stack pointer @code{fp}),
 1765: using an fpTOS is helpful on most machines, but some spill the fpTOS
 1766: register into memory, and fpTOS should not be used there.
 1767: 
 1768: The other approach is to share one stack (pointed to by, say, @code{sp})
 1769: between integer/pointer and floating-point values.  This is ok if you do
 1770: not use @code{spTOS}.  If you do use @code{spTOS}, the compiler has to
 1771: decide whether to put that variable into an integer or a floating point
 1772: register, and the other type of operation becomes quite expensive on
 1773: most machines (because moving values between integer and FP registers is
 1774: quite expensive).  If a value of one type has to be synthesized out of
 1775: two values of the other type (@code{double} types), things are even more
 1776: interesting.
 1777: 
 1778: One way around this problem would be to not use the @code{spTOS}
 1779: supported by Vmgen, but to use explicit top-of-stack variables (one for
 1780: integers, one for FP values), and having a kind of accumulator+stack
 1781: architecture (e.g., Ocaml bytecode uses this approach); however, this is
 1782: a major change, and it's ramifications are not completely clear.
 1783: 
 1784: @c **********************************************************
 1785: @node The future, Changes, Hints, Top
 1786: @chapter The future
 1787: @cindex future ideas
 1788: 
 1789: We have a number of ideas for future versions of Gforth.  However, there
 1790: are so many possible things to do that we would like some feedback from
 1791: you.  What are you doing with Vmgen, what features are you missing, and
 1792: why?
 1793: 
 1794: One idea we are thinking about is to generate just one @file{.c} file
 1795: instead of letting you copy and adapt all the wrapper files (you would
 1796: still have to define stuff like the type-specific macros, and stack
 1797: pointers etc. somewhere).  The advantage would be that, if we change the
 1798: wrapper files between versions, you would not need to integrate your
 1799: changes and our changes to them; Vmgen would also be easier to use for
 1800: beginners.  The main disadvantage of that is that it would reduce the
 1801: flexibility of Vmgen a little (well, those who like flexibility could
 1802: still patch the resulting @file{.c} file, like they are now doing for
 1803: the wrapper files).  In any case, if you are doing things to the wrapper
 1804: files that would cause problems in a generated-@file{.c}-file approach,
 1805: please let us know.
 1806: 
 1807: @c **********************************************************
 1808: @node Changes, Contact, The future, Top
 1809: @chapter Changes
 1810: @cindex Changes from old versions
 1811: 
 1812: User-visible changes between 0.5.9-20020822 and 0.5.9-20020901:
 1813: 
 1814: The store optimization is now disabled by default, but can be enabled by
 1815: the user (@pxref{Store Optimization}).  Documentation for this
 1816: optimization is also new.
 1817: 
 1818: User-visible changes between 0.5.9-20010501 and 0.5.9-20020822:
 1819: 
 1820: There is now a manual (in info, HTML, Postscript, or plain text format).
 1821: 
 1822: There is the vmgen-ex2 variant of the vmgen-ex example; the new
 1823: variant uses a union type instead of lots of casting.
 1824: 
 1825: Both variants of the example can now be compiled with an ANSI C compiler
 1826: (using switch dispatch and losing quite a bit of performance); tested
 1827: with @command{lcc}.
 1828: 
 1829: Users of the gforth-0.5.9-20010501 version of Vmgen need to change
 1830: several things in their source code to use the current version.  I
 1831: recommend keeping the gforth-0.5.9-20010501 version until you have
 1832: completed the change (note that you can have several versions of Gforth
 1833: installed at the same time).  I hope to avoid such incompatible changes
 1834: in the future.
 1835: 
 1836: The required changes are:
 1837: 
 1838: @table @code
 1839: 
 1840: @cindex @code{TAIL;}, changes
 1841: @item TAIL;
 1842: has been renamed into @code{INST_TAIL;} (less chance of an accidental
 1843: match).
 1844: 
 1845: @cindex @code{vm_@var{A}2@var{B}}, changes
 1846: @item vm_@var{A}2@var{B}
 1847: now takes two arguments.
 1848: 
 1849: @cindex @code{vm_two@var{A}2@var{B}}, changes
 1850: @item vm_two@var{A}2@var{B}(b,a1,a2);
 1851: changed to vm_two@var{A}2@var{B}(a1,a2,b) (note the absence of the @samp{;}).
 1852: 
 1853: @end table
 1854: 
 1855: Also some new macros have to be defined, e.g., @code{INST_ADDR}, and
 1856: @code{LABEL}; some macros have to be defined in new contexts, e.g.,
 1857: @code{VM_IS_INST} is now also needed in the disassembler.
 1858: 
 1859: @c *********************************************************
 1860: @node Contact, Copying This Manual, Changes, Top
 1861: @chapter Contact
 1862: 
 1863: To report a bug, use
 1864: @url{https://savannah.gnu.org/bugs/?func=addbug&group_id=2672}.
 1865: 
 1866: For discussion on Vmgen (e.g., how to use it), use the mailing list
 1867: @email{bug-vmgen@@mail.freesoftware.fsf.org} (use
 1868: @url{http://mail.gnu.org/mailman/listinfo/help-vmgen} to subscribe).
 1869: 
 1870: You can find vmgen information at
 1871: @url{http://www.complang.tuwien.ac.at/anton/vmgen/}.
 1872: 
 1873: @c ***********************************************************
 1874: @node Copying This Manual, Index, Contact, Top
 1875: @appendix Copying This Manual
 1876: 
 1877: @menu
 1878: * GNU Free Documentation License::  License for copying this manual.
 1879: @end menu
 1880: 
 1881: @include fdl.texi
 1882: 
 1883: 
 1884: @node Index,  , Copying This Manual, Top
 1885: @unnumbered Index
 1886: 
 1887: @printindex cp
 1888: 
 1889: @bye

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