Annotation of gforth/doc/vmgen.texi, revision 1.4

1.1       anton       1: @include version.texi
                      3: @c @ifnottex
                      4: This file documents vmgen (Gforth @value{VERSION}).
1.2       anton       6: @chapter Introduction
1.1       anton       7: 
                      8: Vmgen is a tool for writing efficient interpreters.  It takes a simple
                      9: virtual machine description and generates efficient C code for dealing
                     10: with the virtual machine code in various ways (in particular, executing
                     11: it).  The run-time efficiency of the resulting interpreters is usually
                     12: within a factor of 10 of machine code produced by an optimizing
                     13: compiler.
                     15: The interpreter design strategy supported by vmgen is to divide the
                     16: interpreter into two parts:
                     18: @itemize @bullet
                     20: @item The @emph{front end} takes the source code of the language to be
                     21: implemented, and translates it into virtual machine code.  This is
                     22: similar to an ordinary compiler front end; typically an interpreter
                     23: front-end performs no optimization, so it is relatively simple to
                     24: implement and runs fast.
                     26: @item The @emph{virtual machine interpreter} executes the virtual
                     27: machine code.
                     29: @end itemize
                     31: Such a division is usually used in interpreters, for modularity as well
                     32: as for efficiency reasons.  The virtual machine code is typically passed
                     33: between front end and virtual machine interpreter in memory, like in a
                     34: load-and-go compiler; this avoids the complexity and time cost of
                     35: writing the code to a file and reading it again.
                     37: A @emph{virtual machine} (VM) represents the program as a sequence of
                     38: @emph{VM instructions}, following each other in memory, similar to real
                     39: machine code.  Control flow occurs through VM branch instructions, like
                     40: in a real machine.
                     42: In this setup, vmgen can generate most of the code dealing with virtual
                     43: machine instructions from a simple description of the virtual machine
                     44: instructions (@pxref...), in particular:
                     46: @table @emph
                     48: @item VM instruction execution
                     50: @item VM code generation
                     51: Useful in the front end.
                     53: @item VM code decompiler
                     54: Useful for debugging the front end.
                     56: @item VM code tracing
                     57: Useful for debugging the front end and the VM interpreter.  You will
                     58: typically provide other means for debugging the user's programs at the
                     59: source level.
                     61: @item VM code profiling
                     62: Useful for optimizing the VM insterpreter with superinstructions
                     63: (@pxref...).
                     65: @end table
                     67: VMgen supports efficient interpreters though various optimizations, in
                     68: particular
                     70: @itemize
                     72: @item Threaded code
                     74: @item Caching the top-of-stack in a register
                     76: @item Combining VM instructions into superinstructions
                     78: @item
                     79: Replicating VM (super)instructions for better BTB prediction accuracy
                     80: (not yet in vmgen-ex, but already in Gforth).
                     82: @end itemize
                     84: As a result, vmgen-based interpreters are only about an order of
                     85: magintude slower than native code from an optimizing C compiler on small
                     86: benchmarks; on large benchmarks, which spend more time in the run-time
1.2       anton      87: system, the slowdown is often less (e.g., the slowdown of a
                     88: Vmgen-generated JVM interpreter over the best JVM JIT compiler we
                     89: measured is only a factor of 2-3 for large benchmarks; some other JITs
                     90: and all other interpreters we looked at were slower than our
                     91: interpreter).
1.1       anton      92: 
                     93: VMs are usually designed as stack machines (passing data between VM
                     94: instructions on a stack), and vmgen supports such designs especially
                     95: well; however, you can also use vmgen for implementing a register VM and
                     96: still benefit from most of the advantages offered by vmgen.
1.2       anton      98: There are many potential uses of the instruction descriptions that are
                     99: not implemented at the moment, but we are open for feature requests, and
                    100: we will implement new features if someone asks for them; so the feature
                    101: list above is not exhaustive.
1.1       anton     102: 
1.2       anton     103: @c *********************************************************************
                    104: @chapter Why interpreters?
                    106: Interpreters are a popular language implementation technique because
                    107: they combine all three of the following advantages:
                    109: @itemize
                    111: @item Ease of implementation
                    113: @item Portability
                    115: @item Fast edit-compile-run cycle
                    117: @end itemize
                    119: The main disadvantage of interpreters is their run-time speed.  However,
                    120: there are huge differences between different interpreters in this area:
                    121: the slowdown over optimized C code on programs consisting of simple
                    122: operations is typically a factor of 10 for the more efficient
                    123: interpreters, and a factor of 1000 for the less efficient ones (the
                    124: slowdown for programs executing complex operations is less, because the
                    125: time spent in libraries for executing complex operations is the same in
                    126: all implementation strategies).
                    128: Vmgen makes it even easier to implement interpreters.  It also supports
                    129: techniques for building efficient interpreters.
                    131: @c ********************************************************************
                    133: @chapter Concepts
                    135: @c --------------------------------------------------------------------
                    136: @section Front-end and virtual machine interpreter
                    138: @cindex front-end
                    139: Interpretive systems are typically divided into a @emph{front end} that
                    140: parses the input language and produces an intermediate representation
                    141: for the program, and an interpreter that executes the intermediate
                    142: representation of the program.
                    144: @cindex virtual machine
                    145: @cindex VM
                    146: @cindex instruction, VM
                    147: For efficient interpreters the intermediate representation of choice is
                    148: virtual machine code (rather than, e.g., an abstract syntax tree).
                    149: @emph{Virtual machine} (VM) code consists of VM instructions arranged
                    150: sequentially in memory; they are executed in sequence by the VM
                    151: interpreter, except for VM branch instructions, which implement control
                    152: structures.  The conceptual similarity to real machine code results in
                    153: the name @emph{virtual machine}.
                    155: In this framework, vmgen supports building the VM interpreter and any
                    156: other component dealing with VM instructions.  It does not have any
                    157: support for the front end, apart from VM code generation support.  The
                    158: front end can be implemented with classical compiler front-end
1.3       anton     159: techniques, supported by tools like @command{flex} and @command{bison}.
1.2       anton     160: 
                    161: The intermediate representation is usually just internal to the
                    162: interpreter, but some systems also support saving it to a file, either
                    163: as an image file, or in a full-blown linkable file format (e.g., JVM).
                    164: Vmgen currently has no special support for such features, but the
                    165: information in the instruction descriptions can be helpful, and we are
                    166: open for feature requests and suggestions.
1.3       anton     167: 
                    168: @section Data handling
                    170: @cindex stack machine
                    171: @cindex register machine
                    172: Most VMs use one or more stacks for passing temporary data between VM
                    173: instructions.  Another option is to use a register machine architecture
                    174: for the virtual machine; however, this option is either slower or
                    175: significantly more complex to implement than a stack machine architecture.
                    177: Vmgen has special support and optimizations for stack VMs, making their
                    178: implementation easy and efficient.
                    180: You can also implement a register VM with vmgen (@pxref{Register
                    181: Machines}), and you will still profit from most vmgen features.
                    183: @cindex stack item size
                    184: @cindex size, stack items
                    185: Stack items all have the same size, so they typically will be as wide as
                    186: an integer, pointer, or floating-point value.  Vmgen supports treating
                    187: two consecutive stack items as a single value, but anything larger is
                    188: best kept in some other memory area (e.g., the heap), with pointers to
                    189: the data on the stack.
                    191: @cindex instruction stream
                    192: @cindex immediate arguments
                    193: Another source of data is immediate arguments VM instructions (in the VM
                    194: instruction stream).  The VM instruction stream is handled similar to a
                    195: stack in vmgen.
                    197: @cindex garbage collection
                    198: @cindex reference counting
                    199: Vmgen has no built-in support for nor restrictions against @emph{garbage
                    200: collection}.  If you need garbage collection, you need to provide it in
                    201: your run-time libraries.  Using @emph{reference counting} is probably
                    202: harder, but might be possible (contact us if you are interested).
                    203: @c reference counting might be possible by including counting code in 
                    204: @c the conversion macros.
                    206: @c *************************************************************
                    207: @chapter Invoking vmgen
                    209: The usual way to invoke vmgen is as follows:
                    211: @example
                    212: vmgen @var{infile}
                    213: @end example
                    215: Here @var{infile} is the VM instruction description file, which usually
                    216: ends in @file{.vmg}.  The output filenames are made by taking the
                    217: basename of @file{infile} (i.e., the output files will be created in the
                    218: current working directory) and replacing @file{.vmg} with @file{-vm.i},
                    219: @file{-disasm.i}, @file{-gen.i}, @file{-labels.i}, @file{-profile.i},
                    220: and @file{-peephole.i}.  E.g., @command{bison hack/foo.vmg} will create
                    221: @file{foo-vm.i} etc.
                    223: The command-line options supported by vmgen are
                    225: @table @option
                    227: @cindex -h, command-line option
                    228: @cindex --help, command-line option
                    229: @item --help
                    230: @itemx -h
                    231: Print a message about the command-line options
                    233: @cindex -v, command-line option
                    234: @cindex --version, command-line option
                    235: @item --version
                    236: @itemx -v
                    237: Print version and exit
                    238: @end table
                    240: @c env vars GFORTHDIR GFORTHDATADIR
                    242: @c ***************************************************************
                    243: @chapter Input File Format
                    245: Vmgen takes as input a file containing specifications of virtual machine
                    246: instructions.  This file usually has a name ending in @file{.vmg}.
                    248: The examples are taken from the example in @file{vmgen-ex}.
                    250: @section Input File Grammar
                    252: The grammar is in EBNF format, with @code{@var{a}|@var{b}} meaning
                    253: ``@var{a} or @var{b}'', @code{@{@var{c}@}} meaning 0 or more repetitions
                    254: of @var{c} and @code{[@var{d}]} meaning 0 or 1 repetitions of @var{d}.
                    256: Vmgen input is not free-format, so you have to take care where you put
                    257: spaces and especially newlines; it's not as bad as makefiles, though:
                    258: any sequence of spaces and tabs is equivalent to a single space.
                    260: @example
                    261: description: {instruction|comment|eval-escape}
                    263: instruction: simple-inst|superinst
                    265: simple-inst: ident " (" stack-effect " )" newline c-code newline newline
                    267: stack-effect: {ident} " --" {ident}
                    269: super-inst: ident " =" ident {ident}  
                    271: comment:      "\ "  text newline
                    273: eval-escape:  "\e " text newline
                    274: @end example
                    275: @c \+ \- \g \f \c
                    277: Note that the @code{\}s in this grammar are meant literally, not as
                    278: C-style encodings for no-printable characters.
                    280: The C code in @code{simple-inst} must not contain empty lines (because
                    281: vmgen would mistake that as the end of the simple-inst.  The text in
                    282: @code{comment} and @code{eval-escape} must not contain a newline.
                    283: @code{Ident} must conform to the usual conventions of C identifiers
                    284: (otherwise the C compiler would choke on the vmgen output).
                    286: Vmgen understands a few extensions beyond the grammar given here, but
                    287: these extensions are only useful for building Gforth.  You can find a
                    288: description of the format used for Gforth in @file{prim}.
                    290: @subsection
                    291: @c woanders?
                    292: The text in @code{eval-escape} is Forth code that is evaluated when
                    293: vmgen reads the line.  If you do not know (and do not want to learn)
                    294: Forth, you can build the text according to the following grammar; these
                    295: rules are normally all Forth you need for using vmgen:
                    297: @example
                    298: text: stack-decl|type-prefix-decl|stack-prefix-decl
                    300: stack-decl: "stack " ident ident ident
                    301: type-prefix-decl: 
                    302:     's" ' string '" ' ("single"|"double") ident "type-prefix" ident
                    303: stack-prefix-decl:  ident "stack-prefix" string
                    304: @end example
                    306: Note that the syntax of this code is not checked thoroughly (there are
                    307: many other Forth program fragments that could be written there).
                    309: If you know Forth, the stack effects of the non-standard words involved
                    310: are:
                    312: @example
                    313: stack        ( "name" "pointer" "type" -- )
                    314:              ( name execution: -- stack )
                    315: type-prefix  ( addr u xt1 xt2 n stack "prefix" -- )
                    316: single       ( -- xt1 xt2 n )
                    317: double       ( -- xt1 xt2 n )
                    318: stack-prefix ( stack "prefix" -- )
                    319: @end example
                    321: @section Simple instructions
                    323: We will use the following simple VM instruction description as example:
                    325: @example
                    326: sub ( i1 i2 -- i )
                    327: i = i1-i2;
                    328: @end example
                    330: The first line specifies the name of the VM instruction (@code{sub}) and
                    331: its stack effect (@code{i1 i2 -- i}).  The rest of the description is
                    332: just plain C code.
                    334: @cindex stack effect
                    335: The stack effect specifies that @code{sub} pulls two integers from the
                    336: data stack and puts them in the C variable @code{i1} and @code{i2} (with
                    337: the rightmost item (@code{i2}) taken from the top of stack) and later
                    338: pushes one integer (@code{i)) on the data stack (the rightmost item is
                    339: on the top afterwards).
                    341: How do we know the type and stack of the stack items?  Vmgen uses
                    342: prefixes, similar to Fortran; in contrast to Fortran, you have to
                    343: define the prefix first:
                    345: @example
                    346: \E s" Cell"   single data-stack type-prefix i
                    347: @end example
                    349: This defines the prefix @code{i} to refer to the type @code{Cell}
                    350: (defined as @code{long} in @file{mini.h}) and, by default, to the
                    351: @code{data-stack}.  It also specifies that this type takes one stack
                    352: item (@code{single}).  The type prefix is part of the variable name.
                    354: Before we can use @code{data-stack} in this way, we have to define it:
                    356: @example
                    357: \E stack data-stack sp Cell
                    358: @end example
                    359: @c !! use something other than Cell
                    361: This line defines the stack @code{data-stack}, which uses the stack
                    362: pointer @code{sp}, and each item has the basic type @code{Cell}; other
                    363: types have to fit into one or two @code{Cell}s (depending on whether the
                    364: type is @code{single} or @code{double} wide), and are converted from and
                    365: to Cells on accessing the @code{data-stack) with conversion macros
                    366: (@pxref{Conversion macros}).  Stacks grow towards lower addresses in
                    367: vmgen.
                    369: We can override the default stack of a stack item by using a stack
                    370: prefix.  E.g., consider the following instruction:
                    372: @example
                    373: lit ( #i -- i )
                    374: @end example
                    376: The VM instruction @code{lit} takes the item @code{i} from the
                    377: instruction stream (indicated by the prefix @code{#}, and pushes it on
                    378: the (default) data stack.  The stack prefix is not part of the variable
                    379: name.  Stack prefixes are defined like this:
                    381: @example
                    382: \E inst-stream stack-prefix #
                    383: @end example
                    385: This definition defines that the stack prefix @code{#} to specifies the
                    386: ``stack'' @code{inst-stream}.  Since the instruction stream behaves a
                    387: little differently than an ordinary stack, it is predefined, and you do
                    388: not need to define it.
                    390: The instruction stream contains instructions and their immediate
                    391: arguments, so specifying that an argument comes from the instruction
                    392: stream indicates an immediate argument.  Of course, instruction stream
                    393: arguments can only appear to the left of @code{--} in the stack effect.
                    394: If there are multiple instruction stream arguments, the leftmost is the
                    395: first one (just as the intuition suggests).
                    397: @section Superinstructions
                    399: @section Stacks, types, and prefixes
1.2       anton     400: 
                    403: Invocation
                    405: Input Syntax
                    407: Concepts: Front end, VM, Stacks,  Types, input stream
                    409: Contact
1.4     ! anton     410: 
        !           411: 
        !           412: Required changes:
        !           413: vm_...2... -> two arguments
        !           414: "vm_two...2...(arg1,arg2,arg3);" -> "vm_two...2...(arg3,arg1,arg2)" (no ";").
        !           415: define INST_ADDR and LABEL
        !           416: define VM_IS_INST also for disassembler

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