Annotation of gforth/doc/vmgen.texi, revision 1.25
1.10 anton 1: \input texinfo @c -*-texinfo-*-
2: @comment %**start of header
3: @setfilename vmgen.info
1.1 anton 4: @include version.texi
1.10 anton 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:
1.25 ! anton 13: Copyright @copyright{} 2002, 03,2003 Free Software Foundation, Inc.
1.10 anton 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:
1.23 anton 30: @dircategory Software development
1.10 anton 31: @direntry
1.24 anton 32: * Vmgen: (vmgen). Virtual machine interpreter generator
1.10 anton 33: @end direntry
34:
35: @titlepage
36: @title Vmgen
37: @subtitle for Gforth version @value{VERSION}, @value{UPDATED}
1.11 anton 38: @author M. Anton Ertl (@email{anton@@mips.complang.tuwien.ac.at})
1.10 anton 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
1.11 anton 57: * Invoking Vmgen::
1.10 anton 58: * Example::
59: * Input File Format::
1.13 anton 60: * Error messages:: reported by Vmgen
1.10 anton 61: * Using the generated code::
1.13 anton 62: * Hints:: VM archictecture, efficiency
63: * The future::
1.10 anton 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::
1.18 anton 88: * Store Optimization::
1.11 anton 89: * Register Machines:: How to define register VM instructions
1.10 anton 90:
1.17 anton 91: Input File Grammar
92:
93: * Eval escapes:: what follows \E
94:
1.10 anton 95: Simple instructions
96:
97: * C Code Macros:: Macros recognized by Vmgen
98: * C Code restrictions:: Vmgen makes assumptions about C code
1.22 anton 99: * Stack growth direction:: is configurable per stack
1.10 anton 100:
101: Using the generated code
102:
103: * VM engine:: Executing VM code
104: * VM instruction table::
105: * VM code generation:: Creating VM code (in the front-end)
106: * Peephole optimization:: Creating VM superinstructions
107: * VM disassembler:: for debugging the front end
108: * VM profiler:: for finding worthwhile superinstructions
109:
1.13 anton 110: Hints
111:
112: * Floating point:: and stacks
113:
1.10 anton 114: Copying This Manual
115:
116: * GNU Free Documentation License:: License for copying this manual.
117:
118: @end detailmenu
119: @end menu
1.1 anton 120:
121: @c @ifnottex
1.11 anton 122: @c This file documents Vmgen (Gforth @value{VERSION}).
1.1 anton 123:
1.10 anton 124: @c ************************************************************
125: @node Introduction, Why interpreters?, Top, Top
1.2 anton 126: @chapter Introduction
1.1 anton 127:
128: Vmgen is a tool for writing efficient interpreters. It takes a simple
129: virtual machine description and generates efficient C code for dealing
130: with the virtual machine code in various ways (in particular, executing
131: it). The run-time efficiency of the resulting interpreters is usually
132: within a factor of 10 of machine code produced by an optimizing
133: compiler.
134:
1.11 anton 135: The interpreter design strategy supported by Vmgen is to divide the
1.1 anton 136: interpreter into two parts:
137:
138: @itemize @bullet
139:
140: @item The @emph{front end} takes the source code of the language to be
141: implemented, and translates it into virtual machine code. This is
142: similar to an ordinary compiler front end; typically an interpreter
143: front-end performs no optimization, so it is relatively simple to
144: implement and runs fast.
145:
146: @item The @emph{virtual machine interpreter} executes the virtual
147: machine code.
148:
149: @end itemize
150:
151: Such a division is usually used in interpreters, for modularity as well
1.6 anton 152: as for efficiency. The virtual machine code is typically passed between
153: front end and virtual machine interpreter in memory, like in a
1.1 anton 154: load-and-go compiler; this avoids the complexity and time cost of
155: writing the code to a file and reading it again.
156:
157: A @emph{virtual machine} (VM) represents the program as a sequence of
158: @emph{VM instructions}, following each other in memory, similar to real
159: machine code. Control flow occurs through VM branch instructions, like
160: in a real machine.
161:
1.12 anton 162: @cindex functionality features overview
1.11 anton 163: In this setup, Vmgen can generate most of the code dealing with virtual
1.1 anton 164: machine instructions from a simple description of the virtual machine
1.11 anton 165: instructions (@pxref{Input File Format}), in particular:
1.1 anton 166:
1.13 anton 167: @table @strong
1.1 anton 168:
169: @item VM instruction execution
170:
171: @item VM code generation
172: Useful in the front end.
173:
174: @item VM code decompiler
175: Useful for debugging the front end.
176:
177: @item VM code tracing
178: Useful for debugging the front end and the VM interpreter. You will
179: typically provide other means for debugging the user's programs at the
180: source level.
181:
182: @item VM code profiling
1.12 anton 183: Useful for optimizing the VM interpreter with superinstructions
1.11 anton 184: (@pxref{VM profiler}).
1.1 anton 185:
186: @end table
187:
1.13 anton 188: To create parts of the interpretive system that do not deal with VM
189: instructions, you have to use other tools (e.g., @command{bison}) and/or
190: hand-code them.
191:
1.12 anton 192: @cindex efficiency features overview
1.11 anton 193: @noindent
194: Vmgen supports efficient interpreters though various optimizations, in
1.1 anton 195: particular
196:
1.11 anton 197: @itemize @bullet
1.1 anton 198:
199: @item Threaded code
200:
201: @item Caching the top-of-stack in a register
202:
203: @item Combining VM instructions into superinstructions
204:
205: @item
206: Replicating VM (super)instructions for better BTB prediction accuracy
207: (not yet in vmgen-ex, but already in Gforth).
208:
209: @end itemize
210:
1.12 anton 211: @cindex speed for JVM
1.11 anton 212: As a result, Vmgen-based interpreters are only about an order of
213: magnitude slower than native code from an optimizing C compiler on small
1.1 anton 214: benchmarks; on large benchmarks, which spend more time in the run-time
1.2 anton 215: system, the slowdown is often less (e.g., the slowdown of a
216: Vmgen-generated JVM interpreter over the best JVM JIT compiler we
217: measured is only a factor of 2-3 for large benchmarks; some other JITs
218: and all other interpreters we looked at were slower than our
219: interpreter).
1.1 anton 220:
221: VMs are usually designed as stack machines (passing data between VM
1.11 anton 222: instructions on a stack), and Vmgen supports such designs especially
1.12 anton 223: well; however, you can also use Vmgen for implementing a register VM
224: (@pxref{Register Machines}) and still benefit from most of the advantages
225: offered by Vmgen.
1.1 anton 226:
1.2 anton 227: There are many potential uses of the instruction descriptions that are
228: not implemented at the moment, but we are open for feature requests, and
1.13 anton 229: we will consider new features if someone asks for them; so the feature
1.2 anton 230: list above is not exhaustive.
1.1 anton 231:
1.2 anton 232: @c *********************************************************************
1.10 anton 233: @node Why interpreters?, Concepts, Introduction, Top
1.2 anton 234: @chapter Why interpreters?
1.12 anton 235: @cindex interpreters, advantages
236: @cindex advantages of interpreters
237: @cindex advantages of vmgen
1.2 anton 238:
239: Interpreters are a popular language implementation technique because
240: they combine all three of the following advantages:
241:
1.11 anton 242: @itemize @bullet
1.2 anton 243:
244: @item Ease of implementation
245:
246: @item Portability
247:
248: @item Fast edit-compile-run cycle
249:
250: @end itemize
251:
1.12 anton 252: Vmgen makes it even easier to implement interpreters.
253:
254: @cindex speed of interpreters
1.2 anton 255: The main disadvantage of interpreters is their run-time speed. However,
256: there are huge differences between different interpreters in this area:
257: the slowdown over optimized C code on programs consisting of simple
258: operations is typically a factor of 10 for the more efficient
259: interpreters, and a factor of 1000 for the less efficient ones (the
260: slowdown for programs executing complex operations is less, because the
261: time spent in libraries for executing complex operations is the same in
262: all implementation strategies).
263:
1.12 anton 264: Vmgen supports techniques for building efficient interpreters.
1.2 anton 265:
266: @c ********************************************************************
1.11 anton 267: @node Concepts, Invoking Vmgen, Why interpreters?, Top
1.2 anton 268: @chapter Concepts
269:
1.10 anton 270: @menu
271: * Front end and VM interpreter:: Modularizing an interpretive system
272: * Data handling:: Stacks, registers, immediate arguments
273: * Dispatch:: From one VM instruction to the next
274: @end menu
275:
1.2 anton 276: @c --------------------------------------------------------------------
1.10 anton 277: @node Front end and VM interpreter, Data handling, Concepts, Concepts
278: @section Front end and VM interpreter
1.12 anton 279: @cindex modularization of interpreters
1.2 anton 280:
281: @cindex front-end
282: Interpretive systems are typically divided into a @emph{front end} that
283: parses the input language and produces an intermediate representation
284: for the program, and an interpreter that executes the intermediate
285: representation of the program.
286:
287: @cindex virtual machine
288: @cindex VM
1.12 anton 289: @cindex VM instruction
1.2 anton 290: @cindex instruction, VM
1.12 anton 291: @cindex VM branch instruction
292: @cindex branch instruction, VM
293: @cindex VM register
294: @cindex register, VM
295: @cindex opcode, VM instruction
296: @cindex immediate argument, VM instruction
1.2 anton 297: For efficient interpreters the intermediate representation of choice is
298: virtual machine code (rather than, e.g., an abstract syntax tree).
299: @emph{Virtual machine} (VM) code consists of VM instructions arranged
300: sequentially in memory; they are executed in sequence by the VM
1.12 anton 301: interpreter, but VM branch instructions can change the control flow and
302: are used for implementing control structures. The conceptual similarity
303: to real machine code results in the name @emph{virtual machine}.
304: Various terms similar to terms for real machines are used; e.g., there
305: are @emph{VM registers} (like the instruction pointer and stack
306: pointer(s)), and the VM instruction consists of an @emph{opcode} and
307: @emph{immediate arguments}.
1.2 anton 308:
1.11 anton 309: In this framework, Vmgen supports building the VM interpreter and any
1.2 anton 310: other component dealing with VM instructions. It does not have any
311: support for the front end, apart from VM code generation support. The
312: front end can be implemented with classical compiler front-end
1.3 anton 313: techniques, supported by tools like @command{flex} and @command{bison}.
1.2 anton 314:
315: The intermediate representation is usually just internal to the
316: interpreter, but some systems also support saving it to a file, either
317: as an image file, or in a full-blown linkable file format (e.g., JVM).
318: Vmgen currently has no special support for such features, but the
319: information in the instruction descriptions can be helpful, and we are
1.13 anton 320: open to feature requests and suggestions.
1.3 anton 321:
1.10 anton 322: @c --------------------------------------------------------------------
323: @node Data handling, Dispatch, Front end and VM interpreter, Concepts
1.3 anton 324: @section Data handling
325:
326: @cindex stack machine
327: @cindex register machine
328: Most VMs use one or more stacks for passing temporary data between VM
329: instructions. Another option is to use a register machine architecture
1.13 anton 330: for the virtual machine; we believe that using a stack architecture is
331: usually both simpler and faster.
332:
333: however, this option is slower or
1.3 anton 334: significantly more complex to implement than a stack machine architecture.
335:
336: Vmgen has special support and optimizations for stack VMs, making their
337: implementation easy and efficient.
338:
1.11 anton 339: You can also implement a register VM with Vmgen (@pxref{Register
340: Machines}), and you will still profit from most Vmgen features.
1.3 anton 341:
342: @cindex stack item size
343: @cindex size, stack items
344: Stack items all have the same size, so they typically will be as wide as
345: an integer, pointer, or floating-point value. Vmgen supports treating
346: two consecutive stack items as a single value, but anything larger is
347: best kept in some other memory area (e.g., the heap), with pointers to
348: the data on the stack.
349:
350: @cindex instruction stream
351: @cindex immediate arguments
352: Another source of data is immediate arguments VM instructions (in the VM
353: instruction stream). The VM instruction stream is handled similar to a
1.11 anton 354: stack in Vmgen.
1.3 anton 355:
356: @cindex garbage collection
357: @cindex reference counting
1.12 anton 358: Vmgen has no built-in support for, nor restrictions against
359: @emph{garbage collection}. If you need garbage collection, you need to
360: provide it in your run-time libraries. Using @emph{reference counting}
361: is probably harder, but might be possible (contact us if you are
362: interested).
1.3 anton 363: @c reference counting might be possible by including counting code in
364: @c the conversion macros.
365:
1.10 anton 366: @c --------------------------------------------------------------------
367: @node Dispatch, , Data handling, Concepts
1.6 anton 368: @section Dispatch
1.12 anton 369: @cindex Dispatch of VM instructions
370: @cindex main interpreter loop
1.6 anton 371:
1.11 anton 372: Understanding this section is probably not necessary for using Vmgen,
1.6 anton 373: but it may help. You may want to skip it now, and read it if you find statements about dispatch methods confusing.
374:
375: After executing one VM instruction, the VM interpreter has to dispatch
1.11 anton 376: the next VM instruction (Vmgen calls the dispatch routine @samp{NEXT}).
1.6 anton 377: Vmgen supports two methods of dispatch:
378:
1.13 anton 379: @table @strong
1.6 anton 380:
381: @item switch dispatch
1.12 anton 382: @cindex switch dispatch
1.6 anton 383: In this method the VM interpreter contains a giant @code{switch}
384: statement, with one @code{case} for each VM instruction. The VM
1.12 anton 385: instruction opcodes are represented by integers (e.g., produced by an
386: @code{enum}) in the VM code, and dispatch occurs by loading the next
387: opcode, @code{switch}ing on it, and continuing at the appropriate
388: @code{case}; after executing the VM instruction, the VM interpreter
389: jumps back to the dispatch code.
1.6 anton 390:
391: @item threaded code
1.12 anton 392: @cindex threaded code
393: This method represents a VM instruction opcode by the address of the
394: start of the machine code fragment for executing the VM instruction.
1.6 anton 395: Dispatch consists of loading this address, jumping to it, and
396: incrementing the VM instruction pointer. Typically the threaded-code
397: dispatch code is appended directly to the code for executing the VM
398: instruction. Threaded code cannot be implemented in ANSI C, but it can
1.11 anton 399: be implemented using GNU C's labels-as-values extension (@pxref{Labels
400: as Values, , Labels as Values, gcc.info, GNU C Manual}).
1.6 anton 401:
1.13 anton 402: @c call threading
1.6 anton 403: @end table
404:
1.12 anton 405: Threaded code can be twice as fast as switch dispatch, depending on the
406: interpreter, the benchmark, and the machine.
407:
1.3 anton 408: @c *************************************************************
1.11 anton 409: @node Invoking Vmgen, Example, Concepts, Top
410: @chapter Invoking Vmgen
1.12 anton 411: @cindex Invoking Vmgen
1.3 anton 412:
1.11 anton 413: The usual way to invoke Vmgen is as follows:
1.3 anton 414:
415: @example
1.13 anton 416: vmgen @var{inputfile}
1.3 anton 417: @end example
418:
1.13 anton 419: Here @var{inputfile} is the VM instruction description file, which
420: usually ends in @file{.vmg}. The output filenames are made by taking
421: the basename of @file{inputfile} (i.e., the output files will be created
422: in the current working directory) and replacing @file{.vmg} with
423: @file{-vm.i}, @file{-disasm.i}, @file{-gen.i}, @file{-labels.i},
424: @file{-profile.i}, and @file{-peephole.i}. E.g., @command{vmgen
425: hack/foo.vmg} will create @file{foo-vm.i}, @file{foo-disasm.i},
426: @file{foo-gen.i}, @file{foo-labels.i}, @file{foo-profile.i} and
427: @file{foo-peephole.i}.
1.3 anton 428:
1.11 anton 429: The command-line options supported by Vmgen are
1.3 anton 430:
431: @table @option
432:
433: @cindex -h, command-line option
434: @cindex --help, command-line option
435: @item --help
436: @itemx -h
437: Print a message about the command-line options
438:
439: @cindex -v, command-line option
440: @cindex --version, command-line option
441: @item --version
442: @itemx -v
443: Print version and exit
444: @end table
445:
446: @c env vars GFORTHDIR GFORTHDATADIR
447:
1.5 anton 448: @c ****************************************************************
1.11 anton 449: @node Example, Input File Format, Invoking Vmgen, Top
1.5 anton 450: @chapter Example
1.12 anton 451: @cindex example of a Vmgen-based interpreter
1.5 anton 452:
1.10 anton 453: @menu
454: * Example overview::
455: * Using profiling to create superinstructions::
456: @end menu
457:
458: @c --------------------------------------------------------------------
459: @node Example overview, Using profiling to create superinstructions, Example, Example
1.5 anton 460: @section Example overview
1.12 anton 461: @cindex example overview
462: @cindex @file{vmgen-ex}
463: @cindex @file{vmgen-ex2}
1.5 anton 464:
1.11 anton 465: There are two versions of the same example for using Vmgen:
1.5 anton 466: @file{vmgen-ex} and @file{vmgen-ex2} (you can also see Gforth as
467: example, but it uses additional (undocumented) features, and also
468: differs in some other respects). The example implements @emph{mini}, a
469: tiny Modula-2-like language with a small JavaVM-like virtual machine.
1.12 anton 470:
1.5 anton 471: The difference between the examples is that @file{vmgen-ex} uses many
472: casts, and @file{vmgen-ex2} tries to avoids most casts and uses unions
1.12 anton 473: instead. In the rest of this manual we usually mention just files in
474: @file{vmgen-ex}; if you want to use unions, use the equivalent file in
475: @file{vmgen-ex2}.
476: @cindex unions example
477: @cindex casts example
1.5 anton 478:
479: The files provided with each example are:
1.12 anton 480: @cindex example files
1.5 anton 481:
482: @example
483: Makefile
484: README
485: disasm.c wrapper file
486: engine.c wrapper file
487: peephole.c wrapper file
488: profile.c wrapper file
489: mini-inst.vmg simple VM instructions
490: mini-super.vmg superinstructions (empty at first)
491: mini.h common declarations
492: mini.l scanner
493: mini.y front end (parser, VM code generator)
494: support.c main() and other support functions
495: fib.mini example mini program
496: simple.mini example mini program
497: test.mini example mini program (tests everything)
498: test.out test.mini output
499: stat.awk script for aggregating profile information
500: peephole-blacklist list of instructions not allowed in superinstructions
501: seq2rule.awk script for creating superinstructions
502: @end example
503:
504: For your own interpreter, you would typically copy the following files
505: and change little, if anything:
1.12 anton 506: @cindex wrapper files
1.5 anton 507:
508: @example
509: disasm.c wrapper file
510: engine.c wrapper file
511: peephole.c wrapper file
512: profile.c wrapper file
513: stat.awk script for aggregating profile information
514: seq2rule.awk script for creating superinstructions
515: @end example
516:
1.11 anton 517: @noindent
1.5 anton 518: You would typically change much in or replace the following files:
519:
520: @example
521: Makefile
522: mini-inst.vmg simple VM instructions
523: mini.h common declarations
524: mini.l scanner
525: mini.y front end (parser, VM code generator)
526: support.c main() and other support functions
527: peephole-blacklist list of instructions not allowed in superinstructions
528: @end example
529:
530: You can build the example by @code{cd}ing into the example's directory,
1.12 anton 531: and then typing @code{make}; you can check that it works with @code{make
1.5 anton 532: check}. You can run run mini programs like this:
533:
534: @example
535: ./mini fib.mini
536: @end example
537:
1.12 anton 538: To learn about the options, type @code{./mini -h}.
1.5 anton 539:
1.10 anton 540: @c --------------------------------------------------------------------
541: @node Using profiling to create superinstructions, , Example overview, Example
1.5 anton 542: @section Using profiling to create superinstructions
1.12 anton 543: @cindex profiling example
544: @cindex superinstructions example
1.5 anton 545:
546: I have not added rules for this in the @file{Makefile} (there are many
547: options for selecting superinstructions, and I did not want to hardcode
548: one into the @file{Makefile}), but there are some supporting scripts, and
549: here's an example:
550:
551: Suppose you want to use @file{fib.mini} and @file{test.mini} as training
552: programs, you get the profiles like this:
553:
554: @example
555: make fib.prof test.prof #takes a few seconds
556: @end example
557:
558: You can aggregate these profiles with @file{stat.awk}:
559:
560: @example
561: awk -f stat.awk fib.prof test.prof
562: @end example
563:
564: The result contains lines like:
565:
566: @example
567: 2 16 36910041 loadlocal lit
568: @end example
569:
570: This means that the sequence @code{loadlocal lit} statically occurs a
571: total of 16 times in 2 profiles, with a dynamic execution count of
572: 36910041.
573:
574: The numbers can be used in various ways to select superinstructions.
575: E.g., if you just want to select all sequences with a dynamic
576: execution count exceeding 10000, you would use the following pipeline:
577:
578: @example
579: awk -f stat.awk fib.prof test.prof|
580: awk '$3>=10000'| #select sequences
581: fgrep -v -f peephole-blacklist| #eliminate wrong instructions
1.12 anton 582: awk -f seq2rule.awk| #transform sequences into superinstruction rules
1.5 anton 583: sort -k 3 >mini-super.vmg #sort sequences
584: @end example
585:
586: The file @file{peephole-blacklist} contains all instructions that
587: directly access a stack or stack pointer (for mini: @code{call},
588: @code{return}); the sort step is necessary to ensure that prefixes
1.13 anton 589: precede larger superinstructions.
1.5 anton 590:
591: Now you can create a version of mini with superinstructions by just
592: saying @samp{make}
593:
1.10 anton 594:
1.3 anton 595: @c ***************************************************************
1.13 anton 596: @node Input File Format, Error messages, Example, Top
1.3 anton 597: @chapter Input File Format
1.12 anton 598: @cindex input file format
599: @cindex format, input file
1.3 anton 600:
601: Vmgen takes as input a file containing specifications of virtual machine
602: instructions. This file usually has a name ending in @file{.vmg}.
603:
1.5 anton 604: Most examples are taken from the example in @file{vmgen-ex}.
1.3 anton 605:
1.10 anton 606: @menu
607: * Input File Grammar::
608: * Simple instructions::
609: * Superinstructions::
1.18 anton 610: * Store Optimization::
1.11 anton 611: * Register Machines:: How to define register VM instructions
1.10 anton 612: @end menu
613:
614: @c --------------------------------------------------------------------
615: @node Input File Grammar, Simple instructions, Input File Format, Input File Format
1.3 anton 616: @section Input File Grammar
1.12 anton 617: @cindex grammar, input file
618: @cindex input file grammar
1.3 anton 619:
620: The grammar is in EBNF format, with @code{@var{a}|@var{b}} meaning
621: ``@var{a} or @var{b}'', @code{@{@var{c}@}} meaning 0 or more repetitions
622: of @var{c} and @code{[@var{d}]} meaning 0 or 1 repetitions of @var{d}.
623:
1.12 anton 624: @cindex free-format, not
1.15 anton 625: @cindex newlines, significance in syntax
1.3 anton 626: Vmgen input is not free-format, so you have to take care where you put
1.15 anton 627: newlines (and, in a few cases, white space).
1.3 anton 628:
629: @example
1.15 anton 630: description: @{instruction|comment|eval-escape|c-escape@}
1.3 anton 631:
632: instruction: simple-inst|superinst
633:
1.15 anton 634: simple-inst: ident '(' stack-effect ')' newline c-code newline newline
1.3 anton 635:
1.15 anton 636: stack-effect: @{ident@} '--' @{ident@}
1.3 anton 637:
1.15 anton 638: super-inst: ident '=' ident @{ident@}
1.3 anton 639:
1.12 anton 640: comment: '\ ' text newline
1.3 anton 641:
1.13 anton 642: eval-escape: '\E ' text newline
1.15 anton 643:
644: c-escape: '\C ' text newline
1.3 anton 645: @end example
646: @c \+ \- \g \f \c
647:
648: Note that the @code{\}s in this grammar are meant literally, not as
1.5 anton 649: C-style encodings for non-printable characters.
1.3 anton 650:
1.15 anton 651: There are two ways to delimit the C code in @code{simple-inst}:
652:
653: @itemize @bullet
654:
655: @item
656: If you start it with a @samp{@{} at the start of a line (i.e., not even
657: white space before it), you have to end it with a @samp{@}} at the start
658: of a line (followed by a newline). In this case you may have empty
659: lines within the C code (typically used between variable definitions and
660: statements).
661:
662: @item
663: You do not start it with @samp{@{}. Then the C code ends at the first
664: empty line, so you cannot have empty lines within this code.
665:
666: @end itemize
667:
668: The text in @code{comment}, @code{eval-escape} and @code{c-escape} must
669: not contain a newline. @code{Ident} must conform to the usual
670: conventions of C identifiers (otherwise the C compiler would choke on
671: the Vmgen output), except that idents in @code{stack-effect} may have a
672: stack prefix (for stack prefix syntax, @pxref{Eval escapes}).
673:
674: @cindex C escape
675: @cindex @code{\C}
676: @cindex conditional compilation of Vmgen output
677: The @code{c-escape} passes the text through to each output file (without
678: the @samp{\C}). This is useful mainly for conditional compilation
679: (i.e., you write @samp{\C #if ...} etc.).
680:
681: @cindex sync lines
682: @cindex @code{#line}
683: In addition to the syntax given in the grammer, Vmgen also processes
684: sync lines (lines starting with @samp{#line}), as produced by @samp{m4
685: -s} (@pxref{Invoking m4, , Invoking m4, m4.info, GNU m4}) and similar
686: tools. This allows associating C compiler error messages with the
687: original source of the C code.
1.3 anton 688:
689: Vmgen understands a few extensions beyond the grammar given here, but
690: these extensions are only useful for building Gforth. You can find a
691: description of the format used for Gforth in @file{prim}.
692:
1.17 anton 693: @menu
694: * Eval escapes:: what follows \E
695: @end menu
696:
697: @node Eval escapes, , Input File Grammar, Input File Grammar
1.10 anton 698: @subsection Eval escapes
1.12 anton 699: @cindex escape to Forth
700: @cindex eval escape
1.15 anton 701: @cindex @code{\E}
1.13 anton 702:
1.3 anton 703: @c woanders?
704: The text in @code{eval-escape} is Forth code that is evaluated when
1.13 anton 705: Vmgen reads the line. You will normally use this feature to define
706: stacks and types.
707:
708: If you do not know (and do not want to learn) Forth, you can build the
709: text according to the following grammar; these rules are normally all
710: Forth you need for using Vmgen:
1.3 anton 711:
712: @example
1.18 anton 713: text: stack-decl|type-prefix-decl|stack-prefix-decl|set-flag
1.3 anton 714:
1.12 anton 715: stack-decl: 'stack ' ident ident ident
1.3 anton 716: type-prefix-decl:
1.12 anton 717: 's" ' string '" ' ('single'|'double') ident 'type-prefix' ident
718: stack-prefix-decl: ident 'stack-prefix' string
1.22 anton 719: set-flag: ('store-optimization'|'include-skipped-insts') ('on'|'off')
1.3 anton 720: @end example
721:
722: Note that the syntax of this code is not checked thoroughly (there are
1.13 anton 723: many other Forth program fragments that could be written in an
724: eval-escape).
1.3 anton 725:
1.14 anton 726: A stack prefix can contain letters, digits, or @samp{:}, and may start
727: with an @samp{#}; e.g., in Gforth the return stack has the stack prefix
728: @samp{R:}. This restriction is not checked during the stack prefix
729: definition, but it is enforced by the parsing rules for stack items
730: later.
731:
1.3 anton 732: If you know Forth, the stack effects of the non-standard words involved
733: are:
1.12 anton 734: @findex stack
735: @findex type-prefix
736: @findex single
737: @findex double
738: @findex stack-prefix
1.18 anton 739: @findex store-optimization
1.3 anton 740: @example
1.22 anton 741: stack ( "name" "pointer" "type" -- )
742: ( name execution: -- stack )
743: type-prefix ( addr u item-size stack "prefix" -- )
744: single ( -- item-size )
745: double ( -- item-size )
746: stack-prefix ( stack "prefix" -- )
747: store-optimization ( -- addr )
748: include-skipped-insts ( -- addr )
1.3 anton 749: @end example
750:
1.14 anton 751: An @var{item-size} takes three cells on the stack.
1.5 anton 752:
1.10 anton 753: @c --------------------------------------------------------------------
754: @node Simple instructions, Superinstructions, Input File Grammar, Input File Format
1.3 anton 755: @section Simple instructions
1.12 anton 756: @cindex simple VM instruction
757: @cindex instruction, simple VM
1.3 anton 758:
759: We will use the following simple VM instruction description as example:
760:
761: @example
762: sub ( i1 i2 -- i )
763: i = i1-i2;
764: @end example
765:
766: The first line specifies the name of the VM instruction (@code{sub}) and
767: its stack effect (@code{i1 i2 -- i}). The rest of the description is
768: just plain C code.
769:
770: @cindex stack effect
1.12 anton 771: @cindex effect, stack
1.3 anton 772: The stack effect specifies that @code{sub} pulls two integers from the
1.12 anton 773: data stack and puts them in the C variables @code{i1} and @code{i2}
774: (with the rightmost item (@code{i2}) taken from the top of stack;
775: intuition: if you push @code{i1}, then @code{i2} on the stack, the
776: resulting stack picture is @code{i1 i2}) and later pushes one integer
777: (@code{i}) on the data stack (the rightmost item is on the top
778: afterwards).
779:
780: @cindex prefix, type
781: @cindex type prefix
782: @cindex default stack of a type prefix
1.3 anton 783: How do we know the type and stack of the stack items? Vmgen uses
784: prefixes, similar to Fortran; in contrast to Fortran, you have to
785: define the prefix first:
786:
787: @example
788: \E s" Cell" single data-stack type-prefix i
789: @end example
790:
791: This defines the prefix @code{i} to refer to the type @code{Cell}
792: (defined as @code{long} in @file{mini.h}) and, by default, to the
793: @code{data-stack}. It also specifies that this type takes one stack
794: item (@code{single}). The type prefix is part of the variable name.
795:
1.12 anton 796: @cindex stack definition
797: @cindex defining a stack
1.3 anton 798: Before we can use @code{data-stack} in this way, we have to define it:
799:
800: @example
801: \E stack data-stack sp Cell
802: @end example
803: @c !! use something other than Cell
804:
1.12 anton 805: @cindex stack basic type
806: @cindex basic type of a stack
807: @cindex type of a stack, basic
1.3 anton 808: This line defines the stack @code{data-stack}, which uses the stack
809: pointer @code{sp}, and each item has the basic type @code{Cell}; other
810: types have to fit into one or two @code{Cell}s (depending on whether the
1.12 anton 811: type is @code{single} or @code{double} wide), and are cast from and to
812: Cells on accessing the @code{data-stack} with type cast macros
1.22 anton 813: (@pxref{VM engine}). By default, stacks grow towards lower addresses in
814: Vmgen-erated interpreters (@pxref{Stack growth direction}).
1.3 anton 815:
1.12 anton 816: @cindex stack prefix
817: @cindex prefix, stack
1.3 anton 818: We can override the default stack of a stack item by using a stack
819: prefix. E.g., consider the following instruction:
820:
821: @example
822: lit ( #i -- i )
823: @end example
824:
825: The VM instruction @code{lit} takes the item @code{i} from the
1.5 anton 826: instruction stream (indicated by the prefix @code{#}), and pushes it on
1.3 anton 827: the (default) data stack. The stack prefix is not part of the variable
828: name. Stack prefixes are defined like this:
829:
830: @example
831: \E inst-stream stack-prefix #
832: @end example
833:
1.5 anton 834: This definition defines that the stack prefix @code{#} specifies the
1.3 anton 835: ``stack'' @code{inst-stream}. Since the instruction stream behaves a
836: little differently than an ordinary stack, it is predefined, and you do
837: not need to define it.
838:
1.12 anton 839: @cindex instruction stream
1.3 anton 840: The instruction stream contains instructions and their immediate
841: arguments, so specifying that an argument comes from the instruction
842: stream indicates an immediate argument. Of course, instruction stream
843: arguments can only appear to the left of @code{--} in the stack effect.
844: If there are multiple instruction stream arguments, the leftmost is the
845: first one (just as the intuition suggests).
846:
1.10 anton 847: @menu
848: * C Code Macros:: Macros recognized by Vmgen
849: * C Code restrictions:: Vmgen makes assumptions about C code
1.22 anton 850: * Stack growth direction:: is configurable per stack
1.10 anton 851: @end menu
852:
853: @c --------------------------------------------------------------------
854: @node C Code Macros, C Code restrictions, Simple instructions, Simple instructions
855: @subsection C Code Macros
1.12 anton 856: @cindex macros recognized by Vmgen
857: @cindex basic block, VM level
1.5 anton 858:
859: Vmgen recognizes the following strings in the C code part of simple
860: instructions:
861:
1.12 anton 862: @table @code
1.5 anton 863:
864: @item SET_IP
1.12 anton 865: @findex SET_IP
1.11 anton 866: As far as Vmgen is concerned, a VM instruction containing this ends a VM
1.5 anton 867: basic block (used in profiling to delimit profiled sequences). On the C
868: level, this also sets the instruction pointer.
869:
870: @item SUPER_END
1.12 anton 871: @findex SUPER_END
872: This ends a basic block (for profiling), even if the instruction
873: contains no @code{SET_IP}.
1.5 anton 874:
1.13 anton 875: @item INST_TAIL;
876: @findex INST_TAIL;
877: Vmgen replaces @samp{INST_TAIL;} with code for ending a VM instruction and
878: dispatching the next VM instruction. Even without a @samp{INST_TAIL;} this
1.12 anton 879: happens automatically when control reaches the end of the C code. If
880: you want to have this in the middle of the C code, you need to use
1.13 anton 881: @samp{INST_TAIL;}. A typical example is a conditional VM branch:
1.5 anton 882:
883: @example
1.11 anton 884: if (branch_condition) @{
1.13 anton 885: SET_IP(target); INST_TAIL;
1.11 anton 886: @}
1.5 anton 887: /* implicit tail follows here */
888: @end example
889:
1.13 anton 890: In this example, @samp{INST_TAIL;} is not strictly necessary, because there
1.5 anton 891: is another one implicitly after the if-statement, but using it improves
892: branch prediction accuracy slightly and allows other optimizations.
893:
894: @item SUPER_CONTINUE
1.12 anton 895: @findex SUPER_CONTINUE
1.5 anton 896: This indicates that the implicit tail at the end of the VM instruction
897: dispatches the sequentially next VM instruction even if there is a
898: @code{SET_IP} in the VM instruction. This enables an optimization that
899: is not yet implemented in the vmgen-ex code (but in Gforth). The
900: typical application is in conditional VM branches:
901:
902: @example
1.11 anton 903: if (branch_condition) @{
1.13 anton 904: SET_IP(target); INST_TAIL; /* now this INST_TAIL is necessary */
1.11 anton 905: @}
1.5 anton 906: SUPER_CONTINUE;
907: @end example
908:
909: @end table
910:
1.11 anton 911: Note that Vmgen is not smart about C-level tokenization, comments,
1.5 anton 912: strings, or conditional compilation, so it will interpret even a
913: commented-out SUPER_END as ending a basic block (or, e.g.,
1.13 anton 914: @samp{RESET_IP;} as @samp{SET_IP;}). Conversely, Vmgen requires the literal
1.11 anton 915: presence of these strings; Vmgen will not see them if they are hiding in
1.5 anton 916: a C preprocessor macro.
917:
918:
1.10 anton 919: @c --------------------------------------------------------------------
1.22 anton 920: @node C Code restrictions, Stack growth direction, C Code Macros, Simple instructions
1.10 anton 921: @subsection C Code restrictions
1.12 anton 922: @cindex C code restrictions
923: @cindex restrictions on C code
924: @cindex assumptions about C code
925:
926: @cindex accessing stack (pointer)
927: @cindex stack pointer, access
928: @cindex instruction pointer, access
1.5 anton 929: Vmgen generates code and performs some optimizations under the
930: assumption that the user-supplied C code does not access the stack
931: pointers or stack items, and that accesses to the instruction pointer
932: only occur through special macros. In general you should heed these
933: restrictions. However, if you need to break these restrictions, read
934: the following.
935:
936: Accessing a stack or stack pointer directly can be a problem for several
937: reasons:
1.12 anton 938: @cindex stack caching, restriction on C code
939: @cindex superinstructions, restrictions on components
1.5 anton 940:
1.11 anton 941: @itemize @bullet
1.5 anton 942:
943: @item
1.12 anton 944: Vmgen optionally supports caching the top-of-stack item in a local
945: variable (that is allocated to a register). This is the most frequent
946: source of trouble. You can deal with it either by not using
947: top-of-stack caching (slowdown factor 1-1.4, depending on machine), or
948: by inserting flushing code (e.g., @samp{IF_spTOS(sp[...] = spTOS);}) at
949: the start and reloading code (e.g., @samp{IF_spTOS(spTOS = sp[0])}) at
950: the end of problematic C code. Vmgen inserts a stack pointer update
951: before the start of the user-supplied C code, so the flushing code has
952: to use an index that corrects for that. In the future, this flushing
953: may be done automatically by mentioning a special string in the C code.
1.5 anton 954: @c sometimes flushing and/or reloading unnecessary
955:
956: @item
1.11 anton 957: The Vmgen-erated code loads the stack items from stack-pointer-indexed
1.5 anton 958: memory into variables before the user-supplied C code, and stores them
959: from variables to stack-pointer-indexed memory afterwards. If you do
960: any writes to the stack through its stack pointer in your C code, it
1.13 anton 961: will not affect the variables, and your write may be overwritten by the
1.5 anton 962: stores after the C code. Similarly, a read from a stack using a stack
963: pointer will not reflect computations of stack items in the same VM
964: instruction.
965:
966: @item
967: Superinstructions keep stack items in variables across the whole
968: superinstruction. So you should not include VM instructions, that
1.12 anton 969: access a stack or stack pointer, as components of superinstructions
970: (@pxref{VM profiler}).
1.5 anton 971:
972: @end itemize
973:
974: You should access the instruction pointer only through its special
975: macros (@samp{IP}, @samp{SET_IP}, @samp{IPTOS}); this ensure that these
976: macros can be implemented in several ways for best performance.
977: @samp{IP} points to the next instruction, and @samp{IPTOS} is its
978: contents.
979:
1.22 anton 980: @c --------------------------------------------------------------------
981: @node Stack growth direction, , C Code restrictions, Simple instructions
982: @subsection Stack growth direction
983: @cindex stack growth direction
984:
985: @cindex @code{stack-access-transform}
986: By default, the stacks grow towards lower addresses. You can change
987: this for a stack by setting the @code{stack-access-transform} field of
988: the stack to an xt @code{( itemnum -- index )} that performs the
989: appropriate index transformation.
990:
991: E.g., if you want to let @code{data-stack} grow towards higher
992: addresses, with the stack pointer always pointing just beyond the
993: top-of-stack, use this right after defining @code{data-stack}:
994:
995: @example
996: \E : sp-access-transform ( itemnum -- index ) negate 1- ;
997: \E ' sp-access-transform ' data-stack >body stack-access-transform !
998: @end example
999:
1000: This means that @code{sp-access-transform} will be used to generate
1001: indexes for accessing @code{data-stack}. The definition of
1002: @code{sp-access-transform} above transforms n into -n-1, e.g, 1 into -2.
1003: This will access the 0th data-stack element (top-of-stack) at sp[-1],
1004: the 1st at sp[-2], etc., which is the typical way upward-growing
1005: stacks are used. If you need a different transform and do not know
1006: enough Forth to program it, let me know.
1.5 anton 1007:
1.10 anton 1008: @c --------------------------------------------------------------------
1.18 anton 1009: @node Superinstructions, Store Optimization, Simple instructions, Input File Format
1.3 anton 1010: @section Superinstructions
1.12 anton 1011: @cindex superinstructions, defining
1012: @cindex defining superinstructions
1.5 anton 1013:
1.8 anton 1014: Note: don't invest too much work in (static) superinstructions; a future
1.11 anton 1015: version of Vmgen will support dynamic superinstructions (see Ian
1.8 anton 1016: Piumarta and Fabio Riccardi, @cite{Optimizing Direct Threaded Code by
1017: Selective Inlining}, PLDI'98), and static superinstructions have much
1.12 anton 1018: less benefit in that context (preliminary results indicate only a factor
1019: 1.1 speedup).
1.8 anton 1020:
1.5 anton 1021: Here is an example of a superinstruction definition:
1022:
1023: @example
1024: lit_sub = lit sub
1025: @end example
1026:
1027: @code{lit_sub} is the name of the superinstruction, and @code{lit} and
1028: @code{sub} are its components. This superinstruction performs the same
1029: action as the sequence @code{lit} and @code{sub}. It is generated
1030: automatically by the VM code generation functions whenever that sequence
1.11 anton 1031: occurs, so if you want to use this superinstruction, you just need to
1032: add this definition (and even that can be partially automatized,
1033: @pxref{VM profiler}).
1.5 anton 1034:
1.12 anton 1035: @cindex prefixes of superinstructions
1.5 anton 1036: Vmgen requires that the component instructions are simple instructions
1.11 anton 1037: defined before superinstructions using the components. Currently, Vmgen
1.5 anton 1038: also requires that all the subsequences at the start of a
1039: superinstruction (prefixes) must be defined as superinstruction before
1040: the superinstruction. I.e., if you want to define a superinstruction
1041:
1042: @example
1.12 anton 1043: foo4 = load add sub mul
1.5 anton 1044: @end example
1045:
1.12 anton 1046: you first have to define @code{load}, @code{add}, @code{sub} and
1047: @code{mul}, plus
1.5 anton 1048:
1049: @example
1.12 anton 1050: foo2 = load add
1051: foo3 = load add sub
1.5 anton 1052: @end example
1053:
1054: Here, @code{sumof4} is the longest prefix of @code{sumof5}, and @code{sumof3}
1055: is the longest prefix of @code{sumof4}.
1056:
1.11 anton 1057: Note that Vmgen assumes that only the code it generates accesses stack
1.5 anton 1058: pointers, the instruction pointer, and various stack items, and it
1059: performs optimizations based on this assumption. Therefore, VM
1.12 anton 1060: instructions where your C code changes the instruction pointer should
1061: only be used as last component; a VM instruction where your C code
1062: accesses a stack pointer should not be used as component at all. Vmgen
1063: does not check these restrictions, they just result in bugs in your
1064: interpreter.
1.5 anton 1065:
1.22 anton 1066: @cindex include-skipped-insts
1067: The Vmgen flag @code{include-skipped-insts} influences superinstruction
1068: code generation. Currently there is no support in the peephole
1069: optimizer for both variations, so leave this flag alone for now.
1070:
1.12 anton 1071: @c -------------------------------------------------------------------
1.18 anton 1072: @node Store Optimization, Register Machines, Superinstructions, Input File Format
1073: @section Store Optimization
1074: @cindex store optimization
1075: @cindex optimization, stack stores
1076: @cindex stack stores, optimization
1077: @cindex eliminating stack stores
1078:
1079: This minor optimization (0.6\%--0.8\% reduction in executed instructions
1080: for Gforth) puts additional requirements on the instruction descriptions
1081: and is therefore disabled by default.
1082:
1083: What does it do? Consider an instruction like
1084:
1085: @example
1086: dup ( n -- n n )
1087: @end example
1088:
1089: For simplicity, also assume that we are not caching the top-of-stack in
1090: a register. Now, the C code for dup first loads @code{n} from the
1091: stack, and then stores it twice to the stack, one time to the address
1092: where it came from; that time is unnecessary, but gcc does not optimize
1093: it away, so vmgen can do it instead (if you turn on the store
1094: optimization).
1095:
1096: Vmgen uses the stack item's name to determine if the stack item contains
1097: the same value as it did at the start. Therefore, if you use the store
1098: optimization, you have to ensure that stack items that have the same
1099: name on input and output also have the same value, and are not changed
1100: in the C code you supply. I.e., the following code could fail if you
1101: turn on the store optimization:
1102:
1103: @example
1104: add1 ( n -- n )
1105: n++;
1106: @end example
1107:
1108: Instead, you have to use different names, i.e.:
1109:
1110: @example
1111: add1 ( n1 -- n1 )
1112: n2=n1+1;
1113: @end example
1114:
1.22 anton 1115: Similarly, the store optimization assumes that the stack pointer is only
1116: changed by Vmgen-erated code. If your C code changes the stack pointer,
1117: use different names in input and output stack items to avoid a (probably
1118: wrong) store optimization, or turn the store optimization off for this
1119: VM instruction.
1120:
1.18 anton 1121: To turn on the store optimization, write
1122:
1123: @example
1124: \E store-optimization on
1125: @end example
1126:
1127: at the start of the file. You can turn this optimization on or off
1128: between any two VM instruction descriptions. For turning it off again,
1129: you can use
1130:
1131: @example
1132: \E store-optimization off
1133: @end example
1134:
1135: @c -------------------------------------------------------------------
1136: @node Register Machines, , Store Optimization, Input File Format
1.11 anton 1137: @section Register Machines
1.12 anton 1138: @cindex Register VM
1139: @cindex Superinstructions for register VMs
1140: @cindex tracing of register VMs
1.11 anton 1141:
1142: If you want to implement a register VM rather than a stack VM with
1143: Vmgen, there are two ways to do it: Directly and through
1144: superinstructions.
1145:
1146: If you use the direct way, you define instructions that take the
1147: register numbers as immediate arguments, like this:
1148:
1149: @example
1150: add3 ( #src1 #src2 #dest -- )
1151: reg[dest] = reg[src1]+reg[src2];
1152: @end example
1153:
1.12 anton 1154: A disadvantage of this method is that during tracing you only see the
1155: register numbers, but not the register contents. Actually, with an
1156: appropriate definition of @code{printarg_src} (@pxref{VM engine}), you
1157: can print the values of the source registers on entry, but you cannot
1158: print the value of the destination register on exit.
1159:
1.11 anton 1160: If you use superinstructions to define a register VM, you define simple
1161: instructions that use a stack, and then define superinstructions that
1162: have no overall stack effect, like this:
1163:
1164: @example
1165: loadreg ( #src -- n )
1166: n = reg[src];
1167:
1168: storereg ( n #dest -- )
1169: reg[dest] = n;
1170:
1171: adds ( n1 n2 -- n )
1172: n = n1+n2;
1173:
1174: add3 = loadreg loadreg adds storereg
1175: @end example
1176:
1177: An advantage of this method is that you see the values and not just the
1.12 anton 1178: register numbers in tracing. A disadvantage of this method is that
1.11 anton 1179: currently you cannot generate superinstructions directly, but only
1180: through generating a sequence of simple instructions (we might change
1181: this in the future if there is demand).
1182:
1183: Could the register VM support be improved, apart from the issues
1184: mentioned above? It is hard to see how to do it in a general way,
1185: because there are a number of different designs that different people
1186: mean when they use the term @emph{register machine} in connection with
1187: VM interpreters. However, if you have ideas or requests in that
1188: direction, please let me know (@pxref{Contact}).
1189:
1.5 anton 1190: @c ********************************************************************
1.13 anton 1191: @node Error messages, Using the generated code, Input File Format, Top
1192: @chapter Error messages
1193: @cindex error messages
1194:
1195: These error messages are created by Vmgen:
1196:
1197: @table @code
1198:
1199: @cindex @code{# can only be on the input side} error
1200: @item # can only be on the input side
1201: You have used an instruction-stream prefix (usually @samp{#}) after the
1202: @samp{--} (the output side); you can only use it before (the input
1203: side).
1204:
1205: @cindex @code{prefix for this combination must be defined earlier} error
1.20 anton 1206: @item the prefix for this superinstruction must be defined earlier
1.13 anton 1207: You have defined a superinstruction (e.g. @code{abc = a b c}) without
1208: defining its direct prefix (e.g., @code{ab = a b}),
1209: @xref{Superinstructions}.
1210:
1211: @cindex @code{sync line syntax} error
1212: @item sync line syntax
1213: If you are using a preprocessor (e.g., @command{m4}) to generate Vmgen
1214: input code, you may want to create @code{#line} directives (aka sync
1215: lines). This error indicates that such a line is not in th syntax
1.16 anton 1216: expected by Vmgen (this should not happen; please report the offending
1217: line in a bug report).
1.13 anton 1218:
1219: @cindex @code{syntax error, wrong char} error
1.22 anton 1220: @item syntax error, wrong char
1.16 anton 1221: A syntax error. If you do not see right away where the error is, it may
1222: be helpful to check the following: Did you put an empty line in a VM
1223: instruction where the C code is not delimited by braces (then the empty
1224: line ends the VM instruction)? If you used brace-delimited C code, did
1225: you put the delimiting braces (and only those) at the start of the line,
1226: without preceding white space? Did you forget a delimiting brace?
1.13 anton 1227:
1228: @cindex @code{too many stacks} error
1229: @item too many stacks
1.16 anton 1230: Vmgen currently supports 3 stacks (plus the instruction stream); if you
1231: need more, let us know.
1.13 anton 1232:
1233: @cindex @code{unknown prefix} error
1234: @item unknown prefix
1235: The stack item does not match any defined type prefix (after stripping
1236: away any stack prefix). You should either declare the type prefix you
1237: want for that stack item, or use a different type prefix
1238:
1.22 anton 1239: @cindex @code{unknown primitive} error
1.13 anton 1240: @item unknown primitive
1241: You have used the name of a simple VM instruction in a superinstruction
1242: definition without defining the simple VM instruction first.
1243:
1244: @end table
1245:
1246: In addition, the C compiler can produce errors due to code produced by
1247: Vmgen; e.g., you need to define type cast functions.
1248:
1249: @c ********************************************************************
1250: @node Using the generated code, Hints, Error messages, Top
1.5 anton 1251: @chapter Using the generated code
1.12 anton 1252: @cindex generated code, usage
1253: @cindex Using vmgen-erated code
1.5 anton 1254:
1.11 anton 1255: The easiest way to create a working VM interpreter with Vmgen is
1.12 anton 1256: probably to start with @file{vmgen-ex}, and modify it for your purposes.
1.13 anton 1257: This chapter explains what the various wrapper and generated files do.
1258: It also contains reference-manual style descriptions of the macros,
1259: variables etc. used by the generated code, and you can skip that on
1260: first reading.
1.5 anton 1261:
1.10 anton 1262: @menu
1263: * VM engine:: Executing VM code
1264: * VM instruction table::
1265: * VM code generation:: Creating VM code (in the front-end)
1266: * Peephole optimization:: Creating VM superinstructions
1267: * VM disassembler:: for debugging the front end
1268: * VM profiler:: for finding worthwhile superinstructions
1269: @end menu
1.6 anton 1270:
1.10 anton 1271: @c --------------------------------------------------------------------
1272: @node VM engine, VM instruction table, Using the generated code, Using the generated code
1.5 anton 1273: @section VM engine
1.12 anton 1274: @cindex VM instruction execution
1275: @cindex engine
1276: @cindex executing VM code
1277: @cindex @file{engine.c}
1278: @cindex @file{-vm.i} output file
1.5 anton 1279:
1280: The VM engine is the VM interpreter that executes the VM code. It is
1281: essential for an interpretive system.
1282:
1.6 anton 1283: Vmgen supports two methods of VM instruction dispatch: @emph{threaded
1284: code} (fast, but gcc-specific), and @emph{switch dispatch} (slow, but
1285: portable across C compilers); you can use conditional compilation
1286: (@samp{defined(__GNUC__)}) to choose between these methods, and our
1287: example does so.
1288:
1289: For both methods, the VM engine is contained in a C-level function.
1290: Vmgen generates most of the contents of the function for you
1291: (@file{@var{name}-vm.i}), but you have to define this function, and
1292: macros and variables used in the engine, and initialize the variables.
1293: In our example the engine function also includes
1294: @file{@var{name}-labels.i} (@pxref{VM instruction table}).
1295:
1.12 anton 1296: @cindex tracing VM code
1.13 anton 1297: @cindex superinstructions and tracing
1.12 anton 1298: In addition to executing the code, the VM engine can optionally also
1299: print out a trace of the executed instructions, their arguments and
1300: results. For superinstructions it prints the trace as if only component
1301: instructions were executed; this allows to introduce new
1302: superinstructions while keeping the traces comparable to old ones
1303: (important for regression tests).
1304:
1305: It costs significant performance to check in each instruction whether to
1306: print tracing code, so we recommend producing two copies of the engine:
1307: one for fast execution, and one for tracing. See the rules for
1308: @file{engine.o} and @file{engine-debug.o} in @file{vmgen-ex/Makefile}
1309: for an example.
1310:
1.6 anton 1311: The following macros and variables are used in @file{@var{name}-vm.i}:
1.5 anton 1312:
1313: @table @code
1314:
1.12 anton 1315: @findex LABEL
1.5 anton 1316: @item LABEL(@var{inst_name})
1317: This is used just before each VM instruction to provide a jump or
1.11 anton 1318: @code{switch} label (the @samp{:} is provided by Vmgen). For switch
1.13 anton 1319: dispatch this should expand to @samp{case @var{label}:}; for
1320: threaded-code dispatch this should just expand to @samp{@var{label}:}.
1.12 anton 1321: In either case @var{label} is usually the @var{inst_name} with some
1322: prefix or suffix to avoid naming conflicts.
1.5 anton 1323:
1.12 anton 1324: @findex LABEL2
1.9 anton 1325: @item LABEL2(@var{inst_name})
1326: This will be used for dynamic superinstructions; at the moment, this
1327: should expand to nothing.
1328:
1.12 anton 1329: @findex NAME
1.5 anton 1330: @item NAME(@var{inst_name_string})
1331: Called on entering a VM instruction with a string containing the name of
1.13 anton 1332: the VM instruction as parameter. In normal execution this should be
1333: expand to nothing, but for tracing this usually prints the name, and
1334: possibly other information (several VM registers in our example).
1.5 anton 1335:
1.12 anton 1336: @findex DEF_CA
1.5 anton 1337: @item DEF_CA
1338: Usually empty. Called just inside a new scope at the start of a VM
1339: instruction. Can be used to define variables that should be visible
1340: during every VM instruction. If you define this macro as non-empty, you
1341: have to provide the finishing @samp{;} in the macro.
1342:
1.12 anton 1343: @findex NEXT_P0
1344: @findex NEXT_P1
1345: @findex NEXT_P2
1.5 anton 1346: @item NEXT_P0 NEXT_P1 NEXT_P2
1347: The three parts of instruction dispatch. They can be defined in
1348: different ways for best performance on various processors (see
1349: @file{engine.c} in the example or @file{engine/threaded.h} in Gforth).
1.12 anton 1350: @samp{NEXT_P0} is invoked right at the start of the VM instruction (but
1.5 anton 1351: after @samp{DEF_CA}), @samp{NEXT_P1} right after the user-supplied C
1352: code, and @samp{NEXT_P2} at the end. The actual jump has to be
1.13 anton 1353: performed by @samp{NEXT_P2} (if you would do it earlier, important parts
1354: of the VM instruction would not be executed).
1.5 anton 1355:
1356: The simplest variant is if @samp{NEXT_P2} does everything and the other
1357: macros do nothing. Then also related macros like @samp{IP},
1358: @samp{SET_IP}, @samp{IP}, @samp{INC_IP} and @samp{IPTOS} are very
1359: straightforward to define. For switch dispatch this code consists just
1.12 anton 1360: of a jump to the dispatch code (@samp{goto next_inst;} in our example);
1.5 anton 1361: for direct threaded code it consists of something like
1.11 anton 1362: @samp{(@{cfa=*ip++; goto *cfa;@})}.
1.5 anton 1363:
1.12 anton 1364: Pulling code (usually the @samp{cfa=*ip++;}) up into @samp{NEXT_P1}
1.5 anton 1365: usually does not cause problems, but pulling things up into
1366: @samp{NEXT_P0} usually requires changing the other macros (and, at least
1367: for Gforth on Alpha, it does not buy much, because the compiler often
1368: manages to schedule the relevant stuff up by itself). An even more
1369: extreme variant is to pull code up even further, into, e.g., NEXT_P1 of
1370: the previous VM instruction (prefetching, useful on PowerPCs).
1371:
1.12 anton 1372: @findex INC_IP
1.5 anton 1373: @item INC_IP(@var{n})
1.8 anton 1374: This increments @code{IP} by @var{n}.
1375:
1.12 anton 1376: @findex SET_IP
1.8 anton 1377: @item SET_IP(@var{target})
1378: This sets @code{IP} to @var{target}.
1.5 anton 1379:
1.12 anton 1380: @cindex type cast macro
1381: @findex vm_@var{A}2@var{B}
1.5 anton 1382: @item vm_@var{A}2@var{B}(a,b)
1383: Type casting macro that assigns @samp{a} (of type @var{A}) to @samp{b}
1384: (of type @var{B}). This is mainly used for getting stack items into
1385: variables and back. So you need to define macros for every combination
1386: of stack basic type (@code{Cell} in our example) and type-prefix types
1387: used with that stack (in both directions). For the type-prefix type,
1388: you use the type-prefix (not the C type string) as type name (e.g.,
1389: @samp{vm_Cell2i}, not @samp{vm_Cell2Cell}). In addition, you have to
1.12 anton 1390: define a vm_@var{X}2@var{X} macro for the stack's basic type @var{X}
1391: (used in superinstructions).
1.5 anton 1392:
1.12 anton 1393: @cindex instruction stream, basic type
1.5 anton 1394: The stack basic type for the predefined @samp{inst-stream} is
1395: @samp{Cell}. If you want a stack with the same item size, making its
1396: basic type @samp{Cell} usually reduces the number of macros you have to
1397: define.
1398:
1.12 anton 1399: @cindex unions in type cast macros
1400: @cindex casts in type cast macros
1401: @cindex type casting between floats and integers
1.5 anton 1402: Here our examples differ a lot: @file{vmgen-ex} uses casts in these
1403: macros, whereas @file{vmgen-ex2} uses union-field selection (or
1.12 anton 1404: assignment to union fields). Note that casting floats into integers and
1405: vice versa changes the bit pattern (and you do not want that). In this
1406: case your options are to use a (temporary) union, or to take the address
1407: of the value, cast the pointer, and dereference that (not always
1408: possible, and sometimes expensive).
1.5 anton 1409:
1.12 anton 1410: @findex vm_two@var{A}2@var{B}
1411: @findex vm_@var{B}2two@var{A}
1.5 anton 1412: @item vm_two@var{A}2@var{B}(a1,a2,b)
1413: @item vm_@var{B}2two@var{A}(b,a1,a2)
1.12 anton 1414: Type casting between two stack items (@code{a1}, @code{a2}) and a
1.5 anton 1415: variable @code{b} of a type that takes two stack items. This does not
1.12 anton 1416: occur in our small examples, but you can look at Gforth for examples
1417: (see @code{vm_twoCell2d} in @file{engine/forth.h}).
1.5 anton 1418:
1.12 anton 1419: @cindex stack pointer definition
1420: @cindex instruction pointer definition
1.5 anton 1421: @item @var{stackpointer}
1422: For each stack used, the stackpointer name given in the stack
1423: declaration is used. For a regular stack this must be an l-expression;
1424: typically it is a variable declared as a pointer to the stack's basic
1425: type. For @samp{inst-stream}, the name is @samp{IP}, and it can be a
1426: plain r-value; typically it is a macro that abstracts away the
1.12 anton 1427: differences between the various implementations of @code{NEXT_P*}.
1.22 anton 1428:
1429: @cindex IMM_ARG
1430: @findex IMM_ARG
1431: @item IMM_ARG(access,value)
1432: Define this to expland to ``(access)''. This is just a placeholder for
1433: future extensions.
1.5 anton 1434:
1.12 anton 1435: @cindex top of stack caching
1436: @cindex stack caching
1437: @cindex TOS
1438: @findex IPTOS
1.5 anton 1439: @item @var{stackpointer}TOS
1440: The top-of-stack for the stack pointed to by @var{stackpointer}. If you
1441: are using top-of-stack caching for that stack, this should be defined as
1442: variable; if you are not using top-of-stack caching for that stack, this
1443: should be a macro expanding to @samp{@var{stackpointer}[0]}. The stack
1444: pointer for the predefined @samp{inst-stream} is called @samp{IP}, so
1445: the top-of-stack is called @samp{IPTOS}.
1446:
1.12 anton 1447: @findex IF_@var{stackpointer}TOS
1.5 anton 1448: @item IF_@var{stackpointer}TOS(@var{expr})
1449: Macro for executing @var{expr}, if top-of-stack caching is used for the
1450: @var{stackpointer} stack. I.e., this should do @var{expr} if there is
1451: top-of-stack caching for @var{stackpointer}; otherwise it should do
1452: nothing.
1453:
1.12 anton 1454: @findex SUPER_END
1.8 anton 1455: @item SUPER_END
1456: This is used by the VM profiler (@pxref{VM profiler}); it should not do
1457: anything in normal operation, and call @code{vm_count_block(IP)} for
1458: profiling.
1459:
1.12 anton 1460: @findex SUPER_CONTINUE
1.8 anton 1461: @item SUPER_CONTINUE
1.11 anton 1462: This is just a hint to Vmgen and does nothing at the C level.
1.8 anton 1463:
1.12 anton 1464: @findex VM_DEBUG
1.5 anton 1465: @item VM_DEBUG
1466: If this is defined, the tracing code will be compiled in (slower
1467: interpretation, but better debugging). Our example compiles two
1468: versions of the engine, a fast-running one that cannot trace, and one
1469: with potential tracing and profiling.
1470:
1.12 anton 1471: @findex vm_debug
1.5 anton 1472: @item vm_debug
1473: Needed only if @samp{VM_DEBUG} is defined. If this variable contains
1474: true, the VM instructions produce trace output. It can be turned on or
1475: off at any time.
1476:
1.12 anton 1477: @findex vm_out
1.5 anton 1478: @item vm_out
1479: Needed only if @samp{VM_DEBUG} is defined. Specifies the file on which
1480: to print the trace output (type @samp{FILE *}).
1481:
1.12 anton 1482: @findex printarg_@var{type}
1.5 anton 1483: @item printarg_@var{type}(@var{value})
1484: Needed only if @samp{VM_DEBUG} is defined. Macro or function for
1485: printing @var{value} in a way appropriate for the @var{type}. This is
1486: used for printing the values of stack items during tracing. @var{Type}
1487: is normally the type prefix specified in a @code{type-prefix} definition
1488: (e.g., @samp{printarg_i}); in superinstructions it is currently the
1489: basic type of the stack.
1490:
1491: @end table
1492:
1.6 anton 1493:
1.10 anton 1494: @c --------------------------------------------------------------------
1495: @node VM instruction table, VM code generation, VM engine, Using the generated code
1496: @section VM instruction table
1.12 anton 1497: @cindex instruction table
1498: @cindex opcode definition
1499: @cindex labels for threaded code
1500: @cindex @code{vm_prim}, definition
1501: @cindex @file{-labels.i} output file
1.6 anton 1502:
1503: For threaded code we also need to produce a table containing the labels
1504: of all VM instructions. This is needed for VM code generation
1505: (@pxref{VM code generation}), and it has to be done in the engine
1506: function, because the labels are not visible outside. It then has to be
1507: passed outside the function (and assigned to @samp{vm_prim}), to be used
1508: by the VM code generation functions.
1509:
1510: This means that the engine function has to be called first to produce
1511: the VM instruction table, and later, after generating VM code, it has to
1512: be called again to execute the generated VM code (yes, this is ugly).
1513: In our example program, these two modes of calling the engine function
1514: are differentiated by the value of the parameter ip0 (if it equals 0,
1515: then the table is passed out, otherwise the VM code is executed); in our
1516: example, we pass the table out by assigning it to @samp{vm_prim} and
1517: returning from @samp{engine}.
1518:
1.12 anton 1519: In our example (@file{vmgen-ex/engine.c}), we also build such a table for
1520: switch dispatch; this is mainly done for uniformity.
1.6 anton 1521:
1522: For switch dispatch, we also need to define the VM instruction opcodes
1523: used as case labels in an @code{enum}.
1524:
1525: For both purposes (VM instruction table, and enum), the file
1.11 anton 1526: @file{@var{name}-labels.i} is generated by Vmgen. You have to define
1.6 anton 1527: the following macro used in this file:
1.5 anton 1528:
1.12 anton 1529: @table @code
1.5 anton 1530:
1.12 anton 1531: @findex INST_ADDR
1.5 anton 1532: @item INST_ADDR(@var{inst_name})
1533: For switch dispatch, this is just the name of the switch label (the same
1.6 anton 1534: name as used in @samp{LABEL(@var{inst_name})}), for both uses of
1535: @file{@var{name}-labels.i}. For threaded-code dispatch, this is the
1536: address of the label defined in @samp{LABEL(@var{inst_name})}); the
1.11 anton 1537: address is taken with @samp{&&} (@pxref{Labels as Values, , Labels as
1538: Values, gcc.info, GNU C Manual}).
1.5 anton 1539:
1540: @end table
1541:
1542:
1.10 anton 1543: @c --------------------------------------------------------------------
1544: @node VM code generation, Peephole optimization, VM instruction table, Using the generated code
1.6 anton 1545: @section VM code generation
1.12 anton 1546: @cindex VM code generation
1547: @cindex code generation, VM
1548: @cindex @file{-gen.i} output file
1.6 anton 1549:
1550: Vmgen generates VM code generation functions in @file{@var{name}-gen.i}
1551: that the front end can call to generate VM code. This is essential for
1552: an interpretive system.
1553:
1.12 anton 1554: @findex gen_@var{inst}
1.11 anton 1555: For a VM instruction @samp{x ( #a b #c -- d )}, Vmgen generates a
1.6 anton 1556: function with the prototype
1557:
1558: @example
1559: void gen_x(Inst **ctp, a_type a, c_type c)
1560: @end example
1561:
1562: The @code{ctp} argument points to a pointer to the next instruction.
1563: @code{*ctp} is increased by the generation functions; i.e., you should
1564: allocate memory for the code to be generated beforehand, and start with
1565: *ctp set at the start of this memory area. Before running out of
1566: memory, allocate a new area, and generate a VM-level jump to the new
1.12 anton 1567: area (this overflow handling is not implemented in our examples).
1.6 anton 1568:
1.12 anton 1569: @cindex immediate arguments, VM code generation
1.6 anton 1570: The other arguments correspond to the immediate arguments of the VM
1571: instruction (with their appropriate types as defined in the
1572: @code{type_prefix} declaration.
1573:
1574: The following types, variables, and functions are used in
1575: @file{@var{name}-gen.i}:
1576:
1.12 anton 1577: @table @code
1.6 anton 1578:
1.12 anton 1579: @findex Inst
1.6 anton 1580: @item Inst
1581: The type of the VM instruction; if you use threaded code, this is
1582: @code{void *}; for switch dispatch this is an integer type.
1583:
1.12 anton 1584: @cindex @code{vm_prim}, use
1.6 anton 1585: @item vm_prim
1586: The VM instruction table (type: @code{Inst *}, @pxref{VM instruction table}).
1587:
1.12 anton 1588: @findex gen_inst
1.6 anton 1589: @item gen_inst(Inst **ctp, Inst i)
1590: This function compiles the instruction @code{i}. Take a look at it in
1591: @file{vmgen-ex/peephole.c}. It is trivial when you don't want to use
1592: superinstructions (just the last two lines of the example function), and
1593: slightly more complicated in the example due to its ability to use
1594: superinstructions (@pxref{Peephole optimization}).
1595:
1.12 anton 1596: @findex genarg_@var{type_prefix}
1.6 anton 1597: @item genarg_@var{type_prefix}(Inst **ctp, @var{type} @var{type_prefix})
1598: This compiles an immediate argument of @var{type} (as defined in a
1599: @code{type-prefix} definition). These functions are trivial to define
1600: (see @file{vmgen-ex/support.c}). You need one of these functions for
1601: every type that you use as immediate argument.
1602:
1603: @end table
1604:
1.12 anton 1605: @findex BB_BOUNDARY
1.6 anton 1606: In addition to using these functions to generate code, you should call
1607: @code{BB_BOUNDARY} at every basic block entry point if you ever want to
1608: use superinstructions (or if you want to use the profiling supported by
1.12 anton 1609: Vmgen; but this support is also useful mainly for selecting
1610: superinstructions). If you use @code{BB_BOUNDARY}, you should also
1611: define it (take a look at its definition in @file{vmgen-ex/mini.y}).
1.6 anton 1612:
1613: You do not need to call @code{BB_BOUNDARY} after branches, because you
1614: will not define superinstructions that contain branches in the middle
1615: (and if you did, and it would work, there would be no reason to end the
1616: superinstruction at the branch), and because the branches announce
1617: themselves to the profiler.
1618:
1619:
1.10 anton 1620: @c --------------------------------------------------------------------
1621: @node Peephole optimization, VM disassembler, VM code generation, Using the generated code
1.6 anton 1622: @section Peephole optimization
1.12 anton 1623: @cindex peephole optimization
1624: @cindex superinstructions, generating
1625: @cindex @file{peephole.c}
1626: @cindex @file{-peephole.i} output file
1.6 anton 1627:
1628: You need peephole optimization only if you want to use
1629: superinstructions. But having the code for it does not hurt much if you
1630: do not use superinstructions.
1631:
1632: A simple greedy peephole optimization algorithm is used for
1633: superinstruction selection: every time @code{gen_inst} compiles a VM
1.12 anton 1634: instruction, it checks if it can combine it with the last VM instruction
1.6 anton 1635: (which may also be a superinstruction resulting from a previous peephole
1636: optimization); if so, it changes the last instruction to the combined
1637: instruction instead of laying down @code{i} at the current @samp{*ctp}.
1638:
1639: The code for peephole optimization is in @file{vmgen-ex/peephole.c}.
1640: You can use this file almost verbatim. Vmgen generates
1641: @file{@var{file}-peephole.i} which contains data for the peephoile
1642: optimizer.
1643:
1.12 anton 1644: @findex init_peeptable
1.6 anton 1645: You have to call @samp{init_peeptable()} after initializing
1646: @samp{vm_prim}, and before compiling any VM code to initialize data
1647: structures for peephole optimization. After that, compiling with the VM
1648: code generation functions will automatically combine VM instructions
1649: into superinstructions. Since you do not want to combine instructions
1650: across VM branch targets (otherwise there will not be a proper VM
1651: instruction to branch to), you have to call @code{BB_BOUNDARY}
1652: (@pxref{VM code generation}) at branch targets.
1653:
1654:
1.10 anton 1655: @c --------------------------------------------------------------------
1656: @node VM disassembler, VM profiler, Peephole optimization, Using the generated code
1.6 anton 1657: @section VM disassembler
1.12 anton 1658: @cindex VM disassembler
1659: @cindex disassembler, VM code
1660: @cindex @file{disasm.c}
1661: @cindex @file{-disasm.i} output file
1.6 anton 1662:
1663: A VM code disassembler is optional for an interpretive system, but
1664: highly recommended during its development and maintenance, because it is
1665: very useful for detecting bugs in the front end (and for distinguishing
1666: them from VM interpreter bugs).
1667:
1668: Vmgen supports VM code disassembling by generating
1669: @file{@var{file}-disasm.i}. This code has to be wrapped into a
1.12 anton 1670: function, as is done in @file{vmgen-ex/disasm.c}. You can use this file
1.6 anton 1671: almost verbatim. In addition to @samp{vm_@var{A}2@var{B}(a,b)},
1672: @samp{vm_out}, @samp{printarg_@var{type}(@var{value})}, which are
1673: explained above, the following macros and variables are used in
1674: @file{@var{file}-disasm.i} (and you have to define them):
1675:
1.12 anton 1676: @table @code
1.6 anton 1677:
1678: @item ip
1679: This variable points to the opcode of the current VM instruction.
1680:
1.12 anton 1681: @cindex @code{IP}, @code{IPTOS} in disassmbler
1.6 anton 1682: @item IP IPTOS
1683: @samp{IPTOS} is the first argument of the current VM instruction, and
1684: @samp{IP} points to it; this is just as in the engine, but here
1685: @samp{ip} points to the opcode of the VM instruction (in contrast to the
1686: engine, where @samp{ip} points to the next cell, or even one further).
1687:
1.12 anton 1688: @findex VM_IS_INST
1.6 anton 1689: @item VM_IS_INST(Inst i, int n)
1690: Tests if the opcode @samp{i} is the same as the @samp{n}th entry in the
1691: VM instruction table.
1692:
1693: @end table
1694:
1695:
1.10 anton 1696: @c --------------------------------------------------------------------
1697: @node VM profiler, , VM disassembler, Using the generated code
1.7 anton 1698: @section VM profiler
1.12 anton 1699: @cindex VM profiler
1700: @cindex profiling for selecting superinstructions
1701: @cindex superinstructions and profiling
1702: @cindex @file{profile.c}
1703: @cindex @file{-profile.i} output file
1.7 anton 1704:
1705: The VM profiler is designed for getting execution and occurence counts
1706: for VM instruction sequences, and these counts can then be used for
1707: selecting sequences as superinstructions. The VM profiler is probably
1.8 anton 1708: not useful as profiling tool for the interpretive system. I.e., the VM
1.7 anton 1709: profiler is useful for the developers, but not the users of the
1.8 anton 1710: interpretive system.
1.7 anton 1711:
1.8 anton 1712: The output of the profiler is: for each basic block (executed at least
1713: once), it produces the dynamic execution count of that basic block and
1714: all its subsequences; e.g.,
1.7 anton 1715:
1.8 anton 1716: @example
1717: 9227465 lit storelocal
1718: 9227465 storelocal branch
1719: 9227465 lit storelocal branch
1720: @end example
1.7 anton 1721:
1.8 anton 1722: I.e., a basic block consisting of @samp{lit storelocal branch} is
1723: executed 9227465 times.
1.6 anton 1724:
1.12 anton 1725: @cindex @file{stat.awk}
1726: @cindex @file{seq2rule.awk}
1.8 anton 1727: This output can be combined in various ways. E.g.,
1.12 anton 1728: @file{vmgen-ex/stat.awk} adds up the occurences of a given sequence wrt
1.8 anton 1729: dynamic execution, static occurence, and per-program occurence. E.g.,
1.3 anton 1730:
1.8 anton 1731: @example
1732: 2 16 36910041 loadlocal lit
1733: @end example
1.2 anton 1734:
1.12 anton 1735: @noindent
1.8 anton 1736: indicates that the sequence @samp{loadlocal lit} occurs in 2 programs,
1737: in 16 places, and has been executed 36910041 times. Now you can select
1738: superinstructions in any way you like (note that compile time and space
1739: typically limit the number of superinstructions to 100--1000). After
1740: you have done that, @file{vmgen/seq2rule.awk} turns lines of the form
1.11 anton 1741: above into rules for inclusion in a Vmgen input file. Note that this
1.8 anton 1742: script does not ensure that all prefixes are defined, so you have to do
1743: that in other ways. So, an overall script for turning profiles into
1744: superinstructions can look like this:
1.2 anton 1745:
1.8 anton 1746: @example
1747: awk -f stat.awk fib.prof test.prof|
1748: awk '$3>=10000'| #select sequences
1749: fgrep -v -f peephole-blacklist| #eliminate wrong instructions
1750: awk -f seq2rule.awk| #turn into superinstructions
1751: sort -k 3 >mini-super.vmg #sort sequences
1752: @end example
1.2 anton 1753:
1.8 anton 1754: Here the dynamic count is used for selecting sequences (preliminary
1755: results indicate that the static count gives better results, though);
1.12 anton 1756: the third line eliminates sequences containing instructions that must not
1.8 anton 1757: occur in a superinstruction, because they access a stack directly. The
1758: dynamic count selection ensures that all subsequences (including
1759: prefixes) of longer sequences occur (because subsequences have at least
1760: the same count as the longer sequences); the sort in the last line
1761: ensures that longer superinstructions occur after their prefixes.
1762:
1.12 anton 1763: But before using this, you have to have the profiler. Vmgen supports its
1.8 anton 1764: creation by generating @file{@var{file}-profile.i}; you also need the
1765: wrapper file @file{vmgen-ex/profile.c} that you can use almost verbatim.
1766:
1.12 anton 1767: @cindex @code{SUPER_END} in profiling
1768: @cindex @code{BB_BOUNDARY} in profiling
1.8 anton 1769: The profiler works by recording the targets of all VM control flow
1770: changes (through @code{SUPER_END} during execution, and through
1771: @code{BB_BOUNDARY} in the front end), and counting (through
1772: @code{SUPER_END}) how often they were targeted. After the program run,
1773: the numbers are corrected such that each VM basic block has the correct
1.12 anton 1774: count (entering a block without executing a branch does not increase the
1775: count, and the correction fixes that), then the subsequences of all
1776: basic blocks are printed. To get all this, you just have to define
1777: @code{SUPER_END} (and @code{BB_BOUNDARY}) appropriately, and call
1778: @code{vm_print_profile(FILE *file)} when you want to output the profile
1779: on @code{file}.
1.8 anton 1780:
1.12 anton 1781: @cindex @code{VM_IS_INST} in profiling
1782: The @file{@var{file}-profile.i} is similar to the disassembler file, and
1.8 anton 1783: it uses variables and functions defined in @file{vmgen-ex/profile.c},
1784: plus @code{VM_IS_INST} already defined for the VM disassembler
1785: (@pxref{VM disassembler}).
1786:
1.13 anton 1787: @c **********************************************************
1788: @node Hints, The future, Using the generated code, Top
1789: @chapter Hints
1790: @cindex hints
1791:
1792: @menu
1793: * Floating point:: and stacks
1794: @end menu
1795:
1796: @c --------------------------------------------------------------------
1797: @node Floating point, , Hints, Hints
1798: @section Floating point
1799:
1800: How should you deal with floating point values? Should you use the same
1801: stack as for integers/pointers, or a different one? This section
1802: discusses this issue with a view on execution speed.
1803:
1804: The simpler approach is to use a separate floating-point stack. This
1805: allows you to choose FP value size without considering the size of the
1806: integers/pointers, and you avoid a number of performance problems. The
1807: main downside is that this needs an FP stack pointer (and that may not
1808: fit in the register file on the 386 arhitecture, costing some
1809: performance, but comparatively little if you take the other option into
1810: account). If you use a separate FP stack (with stack pointer @code{fp}),
1811: using an fpTOS is helpful on most machines, but some spill the fpTOS
1812: register into memory, and fpTOS should not be used there.
1813:
1814: The other approach is to share one stack (pointed to by, say, @code{sp})
1815: between integer/pointer and floating-point values. This is ok if you do
1816: not use @code{spTOS}. If you do use @code{spTOS}, the compiler has to
1817: decide whether to put that variable into an integer or a floating point
1818: register, and the other type of operation becomes quite expensive on
1819: most machines (because moving values between integer and FP registers is
1820: quite expensive). If a value of one type has to be synthesized out of
1821: two values of the other type (@code{double} types), things are even more
1822: interesting.
1823:
1824: One way around this problem would be to not use the @code{spTOS}
1825: supported by Vmgen, but to use explicit top-of-stack variables (one for
1826: integers, one for FP values), and having a kind of accumulator+stack
1827: architecture (e.g., Ocaml bytecode uses this approach); however, this is
1828: a major change, and it's ramifications are not completely clear.
1.10 anton 1829:
1830: @c **********************************************************
1.13 anton 1831: @node The future, Changes, Hints, Top
1832: @chapter The future
1833: @cindex future ideas
1834:
1.21 anton 1835: We have a number of ideas for future versions of Vmgen. However, there
1.13 anton 1836: are so many possible things to do that we would like some feedback from
1837: you. What are you doing with Vmgen, what features are you missing, and
1838: why?
1839:
1840: One idea we are thinking about is to generate just one @file{.c} file
1841: instead of letting you copy and adapt all the wrapper files (you would
1842: still have to define stuff like the type-specific macros, and stack
1843: pointers etc. somewhere). The advantage would be that, if we change the
1844: wrapper files between versions, you would not need to integrate your
1845: changes and our changes to them; Vmgen would also be easier to use for
1846: beginners. The main disadvantage of that is that it would reduce the
1847: flexibility of Vmgen a little (well, those who like flexibility could
1848: still patch the resulting @file{.c} file, like they are now doing for
1849: the wrapper files). In any case, if you are doing things to the wrapper
1850: files that would cause problems in a generated-@file{.c}-file approach,
1851: please let us know.
1852:
1853: @c **********************************************************
1854: @node Changes, Contact, The future, Top
1.8 anton 1855: @chapter Changes
1.12 anton 1856: @cindex Changes from old versions
1.8 anton 1857:
1.19 anton 1858: User-visible changes between 0.5.9-20020822 and 0.5.9-20020901:
1859:
1860: The store optimization is now disabled by default, but can be enabled by
1861: the user (@pxref{Store Optimization}). Documentation for this
1862: optimization is also new.
1863:
1864: User-visible changes between 0.5.9-20010501 and 0.5.9-20020822:
1.17 anton 1865:
1866: There is now a manual (in info, HTML, Postscript, or plain text format).
1867:
1868: There is the vmgen-ex2 variant of the vmgen-ex example; the new
1869: variant uses a union type instead of lots of casting.
1870:
1871: Both variants of the example can now be compiled with an ANSI C compiler
1872: (using switch dispatch and losing quite a bit of performance); tested
1873: with @command{lcc}.
1874:
1.11 anton 1875: Users of the gforth-0.5.9-20010501 version of Vmgen need to change
1.8 anton 1876: several things in their source code to use the current version. I
1877: recommend keeping the gforth-0.5.9-20010501 version until you have
1878: completed the change (note that you can have several versions of Gforth
1879: installed at the same time). I hope to avoid such incompatible changes
1880: in the future.
1.2 anton 1881:
1.8 anton 1882: The required changes are:
1883:
1884: @table @code
1.13 anton 1885:
1886: @cindex @code{TAIL;}, changes
1887: @item TAIL;
1888: has been renamed into @code{INST_TAIL;} (less chance of an accidental
1889: match).
1.2 anton 1890:
1.12 anton 1891: @cindex @code{vm_@var{A}2@var{B}}, changes
1.8 anton 1892: @item vm_@var{A}2@var{B}
1893: now takes two arguments.
1894:
1.12 anton 1895: @cindex @code{vm_two@var{A}2@var{B}}, changes
1.8 anton 1896: @item vm_two@var{A}2@var{B}(b,a1,a2);
1897: changed to vm_two@var{A}2@var{B}(a1,a2,b) (note the absence of the @samp{;}).
1898:
1899: @end table
1.2 anton 1900:
1.8 anton 1901: Also some new macros have to be defined, e.g., @code{INST_ADDR}, and
1902: @code{LABEL}; some macros have to be defined in new contexts, e.g.,
1903: @code{VM_IS_INST} is now also needed in the disassembler.
1.4 anton 1904:
1.12 anton 1905: @c *********************************************************
1.10 anton 1906: @node Contact, Copying This Manual, Changes, Top
1.8 anton 1907: @chapter Contact
1.17 anton 1908:
1909: To report a bug, use
1910: @url{https://savannah.gnu.org/bugs/?func=addbug&group_id=2672}.
1911:
1912: For discussion on Vmgen (e.g., how to use it), use the mailing list
1913: @email{bug-vmgen@@mail.freesoftware.fsf.org} (use
1914: @url{http://mail.gnu.org/mailman/listinfo/help-vmgen} to subscribe).
1915:
1916: You can find vmgen information at
1917: @url{http://www.complang.tuwien.ac.at/anton/vmgen/}.
1.4 anton 1918:
1.12 anton 1919: @c ***********************************************************
1.10 anton 1920: @node Copying This Manual, Index, Contact, Top
1921: @appendix Copying This Manual
1922:
1923: @menu
1924: * GNU Free Documentation License:: License for copying this manual.
1925: @end menu
1926:
1927: @include fdl.texi
1928:
1929:
1930: @node Index, , Copying This Manual, Top
1931: @unnumbered Index
1932:
1933: @printindex cp
1934:
1935: @bye
FreeBSD-CVSweb <freebsd-cvsweb@FreeBSD.org>
![[BACK]](/cvsweb/icons/back.gif){kind=icon}
Return to vmgen.texi CVS log ![[TXT]](/cvsweb/icons/text.gif){kind=icon}
![[DIR]](/cvsweb/icons/dir.gif){kind=icon}
Up to [gforth] / gforth / doc