This is vmgen.info, produced by makeinfo version 4.2 from vmgen.texi.

This manual is for Vmgen (version 0.6.2, August 25, 2003), the virtual
machine interpreter generator

   Copyright (C) 2002, 03,2003 Free Software Foundation, Inc.

     Permission is granted to copy, distribute and/or modify this
     document under the terms of the GNU Free Documentation License,
     Version 1.1 or any later version published by the Free Software
     Foundation; with no Invariant Sections, with the Front-Cover texts
     being "A GNU Manual," and with the Back-Cover Texts as in (a)
     below.  A copy of the license is included in the section entitled
     "GNU Free Documentation License."

     (a) The FSF's Back-Cover Text is: "You have freedom to copy and
     modify this GNU Manual, like GNU software.  Copies published by
     the Free Software Foundation raise funds for GNU development."
   
INFO-DIR-SECTION Software development
START-INFO-DIR-ENTRY
* Vmgen: (vmgen).               Virtual machine interpreter generator
END-INFO-DIR-ENTRY


File: vmgen.info,  Node: VM instruction table,  Next: VM code generation,  Prev: VM engine,  Up: Using the generated code

VM instruction table
====================

   For threaded code we also need to produce a table containing the
labels of all VM instructions.  This is needed for VM code generation
(*note VM code generation::), and it has to be done in the engine
function, because the labels are not visible outside.  It then has to be
passed outside the function (and assigned to `vm_prim'), to be used by
the VM code generation functions.

   This means that the engine function has to be called first to produce
the VM instruction table, and later, after generating VM code, it has to
be called again to execute the generated VM code (yes, this is ugly).
In our example program, these two modes of calling the engine function
are differentiated by the value of the parameter ip0 (if it equals 0,
then the table is passed out, otherwise the VM code is executed); in our
example, we pass the table out by assigning it to `vm_prim' and
returning from `engine'.

   In our example (`vmgen-ex/engine.c'), we also build such a table for
switch dispatch; this is mainly done for uniformity.

   For switch dispatch, we also need to define the VM instruction
opcodes used as case labels in an `enum'.

   For both purposes (VM instruction table, and enum), the file
`NAME-labels.i' is generated by Vmgen.  You have to define the
following macro used in this file:

`INST_ADDR(INST_NAME)'
     For switch dispatch, this is just the name of the switch label
     (the same name as used in `LABEL(INST_NAME)'), for both uses of
     `NAME-labels.i'.  For threaded-code dispatch, this is the address
     of the label defined in `LABEL(INST_NAME)'); the address is taken
     with `&&' (*note Labels as Values: (gcc.info)Labels as Values.).


File: vmgen.info,  Node: VM code generation,  Next: Peephole optimization,  Prev: VM instruction table,  Up: Using the generated code

VM code generation
==================

   Vmgen generates VM code generation functions in `NAME-gen.i' that
the front end can call to generate VM code.  This is essential for an
interpretive system.

   For a VM instruction `x ( #a b #c -- d )', Vmgen generates a
function with the prototype

     void gen_x(Inst **ctp, a_type a, c_type c)

   The `ctp' argument points to a pointer to the next instruction.
`*ctp' is increased by the generation functions; i.e., you should
allocate memory for the code to be generated beforehand, and start with
*ctp set at the start of this memory area.  Before running out of
memory, allocate a new area, and generate a VM-level jump to the new
area (this overflow handling is not implemented in our examples).

   The other arguments correspond to the immediate arguments of the VM
instruction (with their appropriate types as defined in the
`type_prefix' declaration.

   The following types, variables, and functions are used in
`NAME-gen.i':

`Inst'
     The type of the VM instruction; if you use threaded code, this is
     `void *'; for switch dispatch this is an integer type.

`vm_prim'
     The VM instruction table (type: `Inst *', *note VM instruction
     table::).

`gen_inst(Inst **ctp, Inst i)'
     This function compiles the instruction `i'.  Take a look at it in
     `vmgen-ex/peephole.c'.  It is trivial when you don't want to use
     superinstructions (just the last two lines of the example
     function), and slightly more complicated in the example due to its
     ability to use superinstructions (*note Peephole optimization::).

`genarg_TYPE_PREFIX(Inst **ctp, TYPE TYPE_PREFIX)'
     This compiles an immediate argument of TYPE (as defined in a
     `type-prefix' definition).  These functions are trivial to define
     (see `vmgen-ex/support.c').  You need one of these functions for
     every type that you use as immediate argument.

   In addition to using these functions to generate code, you should
call `BB_BOUNDARY' at every basic block entry point if you ever want to
use superinstructions (or if you want to use the profiling supported by
Vmgen; but this support is also useful mainly for selecting
superinstructions).  If you use `BB_BOUNDARY', you should also define
it (take a look at its definition in `vmgen-ex/mini.y').

   You do not need to call `BB_BOUNDARY' after branches, because you
will not define superinstructions that contain branches in the middle
(and if you did, and it would work, there would be no reason to end the
superinstruction at the branch), and because the branches announce
themselves to the profiler.


File: vmgen.info,  Node: Peephole optimization,  Next: VM disassembler,  Prev: VM code generation,  Up: Using the generated code

Peephole optimization
=====================

   You need peephole optimization only if you want to use
superinstructions.  But having the code for it does not hurt much if you
do not use superinstructions.

   A simple greedy peephole optimization algorithm is used for
superinstruction selection: every time `gen_inst' compiles a VM
instruction, it checks if it can combine it with the last VM instruction
(which may also be a superinstruction resulting from a previous peephole
optimization); if so, it changes the last instruction to the combined
instruction instead of laying down `i' at the current `*ctp'.

   The code for peephole optimization is in `vmgen-ex/peephole.c'.  You
can use this file almost verbatim.  Vmgen generates `FILE-peephole.i'
which contains data for the peephole optimizer.

   You have to call `init_peeptable()' after initializing `vm_prim',
and before compiling any VM code to initialize data structures for
peephole optimization.  After that, compiling with the VM code
generation functions will automatically combine VM instructions into
superinstructions.  Since you do not want to combine instructions
across VM branch targets (otherwise there will not be a proper VM
instruction to branch to), you have to call `BB_BOUNDARY' (*note VM
code generation::) at branch targets.


File: vmgen.info,  Node: VM disassembler,  Next: VM profiler,  Prev: Peephole optimization,  Up: Using the generated code

VM disassembler
===============

   A VM code disassembler is optional for an interpretive system, but
highly recommended during its development and maintenance, because it is
very useful for detecting bugs in the front end (and for distinguishing
them from VM interpreter bugs).

   Vmgen supports VM code disassembling by generating `FILE-disasm.i'.
This code has to be wrapped into a function, as is done in
`vmgen-ex/disasm.c'.  You can use this file almost verbatim.  In
addition to `vm_A2B(a,b)', `vm_out', `printarg_TYPE(VALUE)', which are
explained above, the following macros and variables are used in
`FILE-disasm.i' (and you have to define them):

`ip'
     This variable points to the opcode of the current VM instruction.

`IP IPTOS'
     `IPTOS' is the first argument of the current VM instruction, and
     `IP' points to it; this is just as in the engine, but here `ip'
     points to the opcode of the VM instruction (in contrast to the
     engine, where `ip' points to the next cell, or even one further).

`VM_IS_INST(Inst i, int n)'
     Tests if the opcode `i' is the same as the `n'th entry in the VM
     instruction table.


File: vmgen.info,  Node: VM profiler,  Prev: VM disassembler,  Up: Using the generated code

VM profiler
===========

   The VM profiler is designed for getting execution and occurence
counts for VM instruction sequences, and these counts can then be used
for selecting sequences as superinstructions.  The VM profiler is
probably not useful as profiling tool for the interpretive system.
I.e., the VM profiler is useful for the developers, but not the users
of the interpretive system.

   The output of the profiler is: for each basic block (executed at
least once), it produces the dynamic execution count of that basic
block and all its subsequences; e.g.,

            9227465  lit storelocal
            9227465  storelocal branch
            9227465  lit storelocal branch

   I.e., a basic block consisting of `lit storelocal branch' is
executed 9227465 times.

   This output can be combined in various ways.  E.g.,
`vmgen-ex/stat.awk' adds up the occurences of a given sequence wrt
dynamic execution, static occurence, and per-program occurence.  E.g.,

           2      16        36910041 loadlocal lit

indicates that the sequence `loadlocal lit' occurs in 2 programs, in 16
places, and has been executed 36910041 times.  Now you can select
superinstructions in any way you like (note that compile time and space
typically limit the number of superinstructions to 100-1000).  After
you have done that, `vmgen/seq2rule.awk' turns lines of the form above
into rules for inclusion in a Vmgen input file.  Note that this script
does not ensure that all prefixes are defined, so you have to do that
in other ways.  So, an overall script for turning profiles into
superinstructions can look like this:

     awk -f stat.awk fib.prof test.prof|
     awk '$3>=10000'|                #select sequences
     fgrep -v -f peephole-blacklist| #eliminate wrong instructions
     awk -f seq2rule.awk|            #turn into superinstructions
     sort -k 3 >mini-super.vmg       #sort sequences

   Here the dynamic count is used for selecting sequences (preliminary
results indicate that the static count gives better results, though);
the third line eliminates sequences containing instructions that must
not occur in a superinstruction, because they access a stack directly.
The dynamic count selection ensures that all subsequences (including
prefixes) of longer sequences occur (because subsequences have at least
the same count as the longer sequences); the sort in the last line
ensures that longer superinstructions occur after their prefixes.

   But before using this, you have to have the profiler.  Vmgen
supports its creation by generating `FILE-profile.i'; you also need the
wrapper file `vmgen-ex/profile.c' that you can use almost verbatim.

   The profiler works by recording the targets of all VM control flow
changes (through `SUPER_END' during execution, and through
`BB_BOUNDARY' in the front end), and counting (through `SUPER_END') how
often they were targeted.  After the program run, the numbers are
corrected such that each VM basic block has the correct count (entering
a block without executing a branch does not increase the count, and the
correction fixes that), then the subsequences of all basic blocks are
printed.  To get all this, you just have to define `SUPER_END' (and
`BB_BOUNDARY') appropriately, and call `vm_print_profile(FILE *file)'
when you want to output the profile on `file'.

   The `FILE-profile.i' is similar to the disassembler file, and it
uses variables and functions defined in `vmgen-ex/profile.c', plus
`VM_IS_INST' already defined for the VM disassembler (*note VM
disassembler::).


File: vmgen.info,  Node: Hints,  Next: The future,  Prev: Using the generated code,  Up: Top

Hints
*****

* Menu:

* Floating point::              and stacks


File: vmgen.info,  Node: Floating point,  Prev: Hints,  Up: Hints

Floating point
==============

   How should you deal with floating point values?  Should you use the
same stack as for integers/pointers, or a different one?  This section
discusses this issue with a view on execution speed.

   The simpler approach is to use a separate floating-point stack.  This
allows you to choose FP value size without considering the size of the
integers/pointers, and you avoid a number of performance problems.  The
main downside is that this needs an FP stack pointer (and that may not
fit in the register file on the 386 arhitecture, costing some
performance, but comparatively little if you take the other option into
account).  If you use a separate FP stack (with stack pointer `fp'),
using an fpTOS is helpful on most machines, but some spill the fpTOS
register into memory, and fpTOS should not be used there.

   The other approach is to share one stack (pointed to by, say, `sp')
between integer/pointer and floating-point values.  This is ok if you do
not use `spTOS'.  If you do use `spTOS', the compiler has to decide
whether to put that variable into an integer or a floating point
register, and the other type of operation becomes quite expensive on
most machines (because moving values between integer and FP registers is
quite expensive).  If a value of one type has to be synthesized out of
two values of the other type (`double' types), things are even more
interesting.

   One way around this problem would be to not use the `spTOS'
supported by Vmgen, but to use explicit top-of-stack variables (one for
integers, one for FP values), and having a kind of accumulator+stack
architecture (e.g., Ocaml bytecode uses this approach); however, this is
a major change, and it's ramifications are not completely clear.


File: vmgen.info,  Node: The future,  Next: Changes,  Prev: Hints,  Up: Top

The future
**********

   We have a number of ideas for future versions of Vmgen.  However,
there are so many possible things to do that we would like some
feedback from you.  What are you doing with Vmgen, what features are
you missing, and why?

   One idea we are thinking about is to generate just one `.c' file
instead of letting you copy and adapt all the wrapper files (you would
still have to define stuff like the type-specific macros, and stack
pointers etc. somewhere).  The advantage would be that, if we change the
wrapper files between versions, you would not need to integrate your
changes and our changes to them; Vmgen would also be easier to use for
beginners.  The main disadvantage of that is that it would reduce the
flexibility of Vmgen a little (well, those who like flexibility could
still patch the resulting `.c' file, like they are now doing for the
wrapper files).  In any case, if you are doing things to the wrapper
files that would cause problems in a generated-`.c'-file approach,
please let us know.


File: vmgen.info,  Node: Changes,  Next: Contact,  Prev: The future,  Up: Top

Changes
*******

   User-visible changes between 0.5.9-20020822 and 0.5.9-20020901:

   The store optimization is now disabled by default, but can be
enabled by the user (*note Store Optimization::).  Documentation for
this optimization is also new.

   User-visible changes between 0.5.9-20010501 and 0.5.9-20020822:

   There is now a manual (in info, HTML, Postscript, or plain text
format).

   There is the vmgen-ex2 variant of the vmgen-ex example; the new
variant uses a union type instead of lots of casting.

   Both variants of the example can now be compiled with an ANSI C
compiler (using switch dispatch and losing quite a bit of performance);
tested with `lcc'.

   Users of the gforth-0.5.9-20010501 version of Vmgen need to change
several things in their source code to use the current version.  I
recommend keeping the gforth-0.5.9-20010501 version until you have
completed the change (note that you can have several versions of Gforth
installed at the same time).  I hope to avoid such incompatible changes
in the future.

   The required changes are:

`TAIL;'
     has been renamed into `INST_TAIL;' (less chance of an accidental
     match).

`vm_A2B'
     now takes two arguments.

`vm_twoA2B(b,a1,a2);'
     changed to vm_twoA2B(a1,a2,b) (note the absence of the `;').

   Also some new macros have to be defined, e.g., `INST_ADDR', and
`LABEL'; some macros have to be defined in new contexts, e.g.,
`VM_IS_INST' is now also needed in the disassembler.


File: vmgen.info,  Node: Contact,  Next: Copying This Manual,  Prev: Changes,  Up: Top

Contact
*******

   To report a bug, use
<https://savannah.gnu.org/bugs/?func=addbug&group_id=2672>.

   For discussion on Vmgen (e.g., how to use it), use the mailing list
<bug-vmgen@mail.freesoftware.fsf.org> (use
<http://mail.gnu.org/mailman/listinfo/help-vmgen> to subscribe).

   You can find vmgen information at
<http://www.complang.tuwien.ac.at/anton/vmgen/>.


File: vmgen.info,  Node: Copying This Manual,  Next: Index,  Prev: Contact,  Up: Top

Copying This Manual
*******************

* Menu:

* GNU Free Documentation License::  License for copying this manual.


File: vmgen.info,  Node: GNU Free Documentation License,  Up: Copying This Manual

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File: vmgen.info,  Node: Index,  Prev: Copying This Manual,  Up: Top

Index
*****

* Menu:

* # can only be on the input side error: Error messages.
* #line:                                 Input File Grammar.
* -disasm.i output file:                 VM disassembler.
* -gen.i output file:                    VM code generation.
* -h, command-line option:               Invoking Vmgen.
* -help, command-line option:            Invoking Vmgen.
* -labels.i output file:                 VM instruction table.
* -peephole.i output file:               Peephole optimization.
* -profile.i output file:                VM profiler.
* -v, command-line option:               Invoking Vmgen.
* -version, command-line option:         Invoking Vmgen.
* -vm.i output file:                     VM engine.
* \C:                                    Input File Grammar.
* \E:                                    Eval escapes.
* accessing stack (pointer):             C Code restrictions.
* advantages of interpreters:            Why interpreters?.
* advantages of vmgen:                   Why interpreters?.
* assumptions about C code:              C Code restrictions.
* basic block, VM level:                 C Code Macros.
* basic type of a stack:                 Simple instructions.
* BB_BOUNDARY in profiling:              VM profiler.
* branch instruction, VM:                Front end and VM interpreter.
* C code restrictions:                   C Code restrictions.
* C escape:                              Input File Grammar.
* casts example:                         Example overview.
* casts in type cast macros:             VM engine.
* Changes from old versions:             Changes.
* code generation, VM:                   VM code generation.
* conditional compilation of Vmgen output: Input File Grammar.
* default stack of a type prefix:        Simple instructions.
* defining a stack:                      Simple instructions.
* defining superinstructions:            Superinstructions.
* disasm.c:                              VM disassembler.
* disassembler, VM code:                 VM disassembler.
* Dispatch of VM instructions:           Dispatch.
* effect, stack:                         Simple instructions.
* efficiency features overview:          Introduction.
* eliminating stack stores:              Store Optimization.
* engine:                                VM engine.
* engine.c:                              VM engine.
* error messages:                        Error messages.
* escape to Forth:                       Eval escapes.
* eval escape:                           Eval escapes.
* example files:                         Example overview.
* example of a Vmgen-based interpreter:  Example.
* example overview:                      Example overview.
* executing VM code:                     VM engine.
* FDL, GNU Free Documentation License:   GNU Free Documentation License.
* format, input file:                    Input File Format.
* free-format, not:                      Input File Grammar.
* front-end:                             Front end and VM interpreter.
* functionality features overview:       Introduction.
* future ideas:                          The future.
* garbage collection:                    Data handling.
* generated code, usage:                 Using the generated code.
* grammar, input file:                   Input File Grammar.
* hints:                                 Hints.
* IMM_ARG:                               VM engine.
* immediate argument, VM instruction:    Front end and VM interpreter.
* immediate arguments:                   Data handling.
* immediate arguments, VM code generation: VM code generation.
* include-skipped-insts:                 Superinstructions.
* input file format:                     Input File Format.
* input file grammar:                    Input File Grammar.
* instruction pointer definition:        VM engine.
* instruction pointer, access:           C Code restrictions.
* instruction stream <1>:                Simple instructions.
* instruction stream:                    Data handling.
* instruction stream, basic type:        VM engine.
* instruction table:                     VM instruction table.
* instruction, simple VM:                Simple instructions.
* instruction, VM:                       Front end and VM interpreter.
* interpreters, advantages:              Why interpreters?.
* Invoking Vmgen:                        Invoking Vmgen.
* IP, IPTOS in disassmbler:              VM disassembler.
* labels for threaded code:              VM instruction table.
* macros recognized by Vmgen:            C Code Macros.
* main interpreter loop:                 Dispatch.
* modularization of interpreters:        Front end and VM interpreter.
* newlines, significance in syntax:      Input File Grammar.
* opcode definition:                     VM instruction table.
* opcode, VM instruction:                Front end and VM interpreter.
* optimization, stack stores:            Store Optimization.
* peephole optimization:                 Peephole optimization.
* peephole.c:                            Peephole optimization.
* prefix for this combination must be defined earlier error: Error messages.
* prefix, stack:                         Simple instructions.
* prefix, type:                          Simple instructions.
* prefixes of superinstructions:         Superinstructions.
* profile.c:                             VM profiler.
* profiling example:                     Using profiling to create superinstructions.
* profiling for selecting superinstructions: VM profiler.
* reference counting:                    Data handling.
* register machine:                      Data handling.
* Register VM:                           Register Machines.
* register, VM:                          Front end and VM interpreter.
* restrictions on C code:                C Code restrictions.
* seq2rule.awk:                          VM profiler.
* simple VM instruction:                 Simple instructions.
* size, stack items:                     Data handling.
* speed for JVM:                         Introduction.
* speed of interpreters:                 Why interpreters?.
* stack basic type:                      Simple instructions.
* stack caching:                         VM engine.
* stack caching, restriction on C code:  C Code restrictions.
* stack definition:                      Simple instructions.
* stack effect:                          Simple instructions.
* stack growth direction:                Stack growth direction.
* stack item size:                       Data handling.
* stack machine:                         Data handling.
* stack pointer definition:              VM engine.
* stack pointer, access:                 C Code restrictions.
* stack prefix:                          Simple instructions.
* stack stores, optimization:            Store Optimization.
* stack-access-transform:                Stack growth direction.
* stat.awk:                              VM profiler.
* store optimization:                    Store Optimization.
* SUPER_END in profiling:                VM profiler.
* superinstructions and profiling:       VM profiler.
* superinstructions and tracing:         VM engine.
* superinstructions example:             Using profiling to create superinstructions.
* Superinstructions for register VMs:    Register Machines.
* superinstructions, defining:           Superinstructions.
* superinstructions, generating:         Peephole optimization.
* superinstructions, restrictions on components: C Code restrictions.
* switch dispatch:                       Dispatch.
* sync line syntax error:                Error messages.
* sync lines:                            Input File Grammar.
* syntax error, wrong char error:        Error messages.
* TAIL;, changes:                        Changes.
* threaded code:                         Dispatch.
* too many stacks error:                 Error messages.
* top of stack caching:                  VM engine.
* TOS:                                   VM engine.
* tracing of register VMs:               Register Machines.
* tracing VM code:                       VM engine.
* type cast macro:                       VM engine.
* type casting between floats and integers: VM engine.
* type of a stack, basic:                Simple instructions.
* type prefix:                           Simple instructions.
* unions example:                        Example overview.
* unions in type cast macros:            VM engine.
* unknown prefix error:                  Error messages.
* unknown primitive error:               Error messages.
* Using vmgen-erated code:               Using the generated code.
* virtual machine:                       Front end and VM interpreter.
* VM:                                    Front end and VM interpreter.
* VM branch instruction:                 Front end and VM interpreter.
* VM code generation:                    VM code generation.
* VM disassembler:                       VM disassembler.
* VM instruction:                        Front end and VM interpreter.
* VM instruction execution:              VM engine.
* VM profiler:                           VM profiler.
* VM register:                           Front end and VM interpreter.
* vm_A2B, changes:                       Changes.
* VM_IS_INST in profiling:               VM profiler.
* vm_prim, definition:                   VM instruction table.
* vm_prim, use:                          VM code generation.
* vm_twoA2B, changes:                    Changes.
* vmgen-ex:                              Example overview.
* vmgen-ex2:                             Example overview.
* wrapper files:                         Example overview.


