Annotation of gforth/doc/vmgen.texi, revision 1.30
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:
1.27 anton 13: Copyright @copyright{} 2002,2003,2005 Free Software Foundation, Inc.
1.10 anton 14:
15: @quotation
16: Permission is granted to copy, distribute and/or modify this document
1.30 ! anton 17: under the terms of the GNU Free Documentation License, Version 1.2 or
1.10 anton 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:
1.29 anton 97: * Explicit stack access:: If the C code accesses a stack pointer
1.10 anton 98: * C Code Macros:: Macros recognized by Vmgen
99: * C Code restrictions:: Vmgen makes assumptions about C code
1.22 anton 100: * Stack growth direction:: is configurable per stack
1.10 anton 101:
102: Using the generated code
103:
104: * VM engine:: Executing VM code
105: * VM instruction table::
106: * VM code generation:: Creating VM code (in the front-end)
107: * Peephole optimization:: Creating VM superinstructions
108: * VM disassembler:: for debugging the front end
109: * VM profiler:: for finding worthwhile superinstructions
110:
1.13 anton 111: Hints
112:
113: * Floating point:: and stacks
114:
1.10 anton 115: Copying This Manual
116:
117: * GNU Free Documentation License:: License for copying this manual.
118:
119: @end detailmenu
120: @end menu
1.1 anton 121:
122: @c @ifnottex
1.11 anton 123: @c This file documents Vmgen (Gforth @value{VERSION}).
1.1 anton 124:
1.10 anton 125: @c ************************************************************
126: @node Introduction, Why interpreters?, Top, Top
1.2 anton 127: @chapter Introduction
1.1 anton 128:
129: Vmgen is a tool for writing efficient interpreters. It takes a simple
130: virtual machine description and generates efficient C code for dealing
131: with the virtual machine code in various ways (in particular, executing
132: it). The run-time efficiency of the resulting interpreters is usually
133: within a factor of 10 of machine code produced by an optimizing
134: compiler.
135:
1.11 anton 136: The interpreter design strategy supported by Vmgen is to divide the
1.1 anton 137: interpreter into two parts:
138:
139: @itemize @bullet
140:
141: @item The @emph{front end} takes the source code of the language to be
142: implemented, and translates it into virtual machine code. This is
143: similar to an ordinary compiler front end; typically an interpreter
144: front-end performs no optimization, so it is relatively simple to
145: implement and runs fast.
146:
147: @item The @emph{virtual machine interpreter} executes the virtual
148: machine code.
149:
150: @end itemize
151:
152: Such a division is usually used in interpreters, for modularity as well
1.6 anton 153: as for efficiency. The virtual machine code is typically passed between
154: front end and virtual machine interpreter in memory, like in a
1.1 anton 155: load-and-go compiler; this avoids the complexity and time cost of
156: writing the code to a file and reading it again.
157:
158: A @emph{virtual machine} (VM) represents the program as a sequence of
159: @emph{VM instructions}, following each other in memory, similar to real
160: machine code. Control flow occurs through VM branch instructions, like
161: in a real machine.
162:
1.12 anton 163: @cindex functionality features overview
1.11 anton 164: In this setup, Vmgen can generate most of the code dealing with virtual
1.1 anton 165: machine instructions from a simple description of the virtual machine
1.11 anton 166: instructions (@pxref{Input File Format}), in particular:
1.1 anton 167:
1.13 anton 168: @table @strong
1.1 anton 169:
170: @item VM instruction execution
171:
172: @item VM code generation
173: Useful in the front end.
174:
175: @item VM code decompiler
176: Useful for debugging the front end.
177:
178: @item VM code tracing
179: Useful for debugging the front end and the VM interpreter. You will
180: typically provide other means for debugging the user's programs at the
181: source level.
182:
183: @item VM code profiling
1.12 anton 184: Useful for optimizing the VM interpreter with superinstructions
1.11 anton 185: (@pxref{VM profiler}).
1.1 anton 186:
187: @end table
188:
1.13 anton 189: To create parts of the interpretive system that do not deal with VM
190: instructions, you have to use other tools (e.g., @command{bison}) and/or
191: hand-code them.
192:
1.12 anton 193: @cindex efficiency features overview
1.11 anton 194: @noindent
195: Vmgen supports efficient interpreters though various optimizations, in
1.1 anton 196: particular
197:
1.11 anton 198: @itemize @bullet
1.1 anton 199:
200: @item Threaded code
201:
202: @item Caching the top-of-stack in a register
203:
204: @item Combining VM instructions into superinstructions
205:
206: @item
207: Replicating VM (super)instructions for better BTB prediction accuracy
208: (not yet in vmgen-ex, but already in Gforth).
209:
210: @end itemize
211:
1.12 anton 212: @cindex speed for JVM
1.11 anton 213: As a result, Vmgen-based interpreters are only about an order of
214: magnitude slower than native code from an optimizing C compiler on small
1.1 anton 215: benchmarks; on large benchmarks, which spend more time in the run-time
1.2 anton 216: system, the slowdown is often less (e.g., the slowdown of a
217: Vmgen-generated JVM interpreter over the best JVM JIT compiler we
218: measured is only a factor of 2-3 for large benchmarks; some other JITs
219: and all other interpreters we looked at were slower than our
220: interpreter).
1.1 anton 221:
222: VMs are usually designed as stack machines (passing data between VM
1.11 anton 223: instructions on a stack), and Vmgen supports such designs especially
1.12 anton 224: well; however, you can also use Vmgen for implementing a register VM
225: (@pxref{Register Machines}) and still benefit from most of the advantages
226: offered by Vmgen.
1.1 anton 227:
1.2 anton 228: There are many potential uses of the instruction descriptions that are
229: not implemented at the moment, but we are open for feature requests, and
1.13 anton 230: we will consider new features if someone asks for them; so the feature
1.2 anton 231: list above is not exhaustive.
1.1 anton 232:
1.2 anton 233: @c *********************************************************************
1.10 anton 234: @node Why interpreters?, Concepts, Introduction, Top
1.2 anton 235: @chapter Why interpreters?
1.12 anton 236: @cindex interpreters, advantages
237: @cindex advantages of interpreters
238: @cindex advantages of vmgen
1.2 anton 239:
240: Interpreters are a popular language implementation technique because
241: they combine all three of the following advantages:
242:
1.11 anton 243: @itemize @bullet
1.2 anton 244:
245: @item Ease of implementation
246:
247: @item Portability
248:
249: @item Fast edit-compile-run cycle
250:
251: @end itemize
252:
1.12 anton 253: Vmgen makes it even easier to implement interpreters.
254:
255: @cindex speed of interpreters
1.2 anton 256: The main disadvantage of interpreters is their run-time speed. However,
257: there are huge differences between different interpreters in this area:
258: the slowdown over optimized C code on programs consisting of simple
259: operations is typically a factor of 10 for the more efficient
260: interpreters, and a factor of 1000 for the less efficient ones (the
261: slowdown for programs executing complex operations is less, because the
262: time spent in libraries for executing complex operations is the same in
263: all implementation strategies).
264:
1.12 anton 265: Vmgen supports techniques for building efficient interpreters.
1.2 anton 266:
267: @c ********************************************************************
1.11 anton 268: @node Concepts, Invoking Vmgen, Why interpreters?, Top
1.2 anton 269: @chapter Concepts
270:
1.10 anton 271: @menu
272: * Front end and VM interpreter:: Modularizing an interpretive system
273: * Data handling:: Stacks, registers, immediate arguments
274: * Dispatch:: From one VM instruction to the next
275: @end menu
276:
1.2 anton 277: @c --------------------------------------------------------------------
1.10 anton 278: @node Front end and VM interpreter, Data handling, Concepts, Concepts
279: @section Front end and VM interpreter
1.12 anton 280: @cindex modularization of interpreters
1.2 anton 281:
282: @cindex front-end
283: Interpretive systems are typically divided into a @emph{front end} that
284: parses the input language and produces an intermediate representation
285: for the program, and an interpreter that executes the intermediate
286: representation of the program.
287:
288: @cindex virtual machine
289: @cindex VM
1.12 anton 290: @cindex VM instruction
1.2 anton 291: @cindex instruction, VM
1.12 anton 292: @cindex VM branch instruction
293: @cindex branch instruction, VM
294: @cindex VM register
295: @cindex register, VM
296: @cindex opcode, VM instruction
297: @cindex immediate argument, VM instruction
1.2 anton 298: For efficient interpreters the intermediate representation of choice is
299: virtual machine code (rather than, e.g., an abstract syntax tree).
300: @emph{Virtual machine} (VM) code consists of VM instructions arranged
301: sequentially in memory; they are executed in sequence by the VM
1.12 anton 302: interpreter, but VM branch instructions can change the control flow and
303: are used for implementing control structures. The conceptual similarity
304: to real machine code results in the name @emph{virtual machine}.
305: Various terms similar to terms for real machines are used; e.g., there
306: are @emph{VM registers} (like the instruction pointer and stack
307: pointer(s)), and the VM instruction consists of an @emph{opcode} and
308: @emph{immediate arguments}.
1.2 anton 309:
1.11 anton 310: In this framework, Vmgen supports building the VM interpreter and any
1.2 anton 311: other component dealing with VM instructions. It does not have any
312: support for the front end, apart from VM code generation support. The
313: front end can be implemented with classical compiler front-end
1.3 anton 314: techniques, supported by tools like @command{flex} and @command{bison}.
1.2 anton 315:
316: The intermediate representation is usually just internal to the
317: interpreter, but some systems also support saving it to a file, either
318: as an image file, or in a full-blown linkable file format (e.g., JVM).
319: Vmgen currently has no special support for such features, but the
320: information in the instruction descriptions can be helpful, and we are
1.13 anton 321: open to feature requests and suggestions.
1.3 anton 322:
1.10 anton 323: @c --------------------------------------------------------------------
324: @node Data handling, Dispatch, Front end and VM interpreter, Concepts
1.3 anton 325: @section Data handling
326:
327: @cindex stack machine
328: @cindex register machine
329: Most VMs use one or more stacks for passing temporary data between VM
330: instructions. Another option is to use a register machine architecture
1.13 anton 331: for the virtual machine; we believe that using a stack architecture is
332: usually both simpler and faster.
333:
334: however, this option is slower or
1.3 anton 335: significantly more complex to implement than a stack machine architecture.
336:
337: Vmgen has special support and optimizations for stack VMs, making their
338: implementation easy and efficient.
339:
1.11 anton 340: You can also implement a register VM with Vmgen (@pxref{Register
341: Machines}), and you will still profit from most Vmgen features.
1.3 anton 342:
343: @cindex stack item size
344: @cindex size, stack items
345: Stack items all have the same size, so they typically will be as wide as
346: an integer, pointer, or floating-point value. Vmgen supports treating
347: two consecutive stack items as a single value, but anything larger is
348: best kept in some other memory area (e.g., the heap), with pointers to
349: the data on the stack.
350:
351: @cindex instruction stream
352: @cindex immediate arguments
353: Another source of data is immediate arguments VM instructions (in the VM
354: instruction stream). The VM instruction stream is handled similar to a
1.11 anton 355: stack in Vmgen.
1.3 anton 356:
357: @cindex garbage collection
358: @cindex reference counting
1.12 anton 359: Vmgen has no built-in support for, nor restrictions against
360: @emph{garbage collection}. If you need garbage collection, you need to
361: provide it in your run-time libraries. Using @emph{reference counting}
362: is probably harder, but might be possible (contact us if you are
363: interested).
1.3 anton 364: @c reference counting might be possible by including counting code in
365: @c the conversion macros.
366:
1.10 anton 367: @c --------------------------------------------------------------------
368: @node Dispatch, , Data handling, Concepts
1.6 anton 369: @section Dispatch
1.12 anton 370: @cindex Dispatch of VM instructions
371: @cindex main interpreter loop
1.6 anton 372:
1.11 anton 373: Understanding this section is probably not necessary for using Vmgen,
1.6 anton 374: but it may help. You may want to skip it now, and read it if you find statements about dispatch methods confusing.
375:
376: After executing one VM instruction, the VM interpreter has to dispatch
1.11 anton 377: the next VM instruction (Vmgen calls the dispatch routine @samp{NEXT}).
1.6 anton 378: Vmgen supports two methods of dispatch:
379:
1.13 anton 380: @table @strong
1.6 anton 381:
382: @item switch dispatch
1.12 anton 383: @cindex switch dispatch
1.6 anton 384: In this method the VM interpreter contains a giant @code{switch}
385: statement, with one @code{case} for each VM instruction. The VM
1.12 anton 386: instruction opcodes are represented by integers (e.g., produced by an
387: @code{enum}) in the VM code, and dispatch occurs by loading the next
388: opcode, @code{switch}ing on it, and continuing at the appropriate
389: @code{case}; after executing the VM instruction, the VM interpreter
390: jumps back to the dispatch code.
1.6 anton 391:
392: @item threaded code
1.12 anton 393: @cindex threaded code
394: This method represents a VM instruction opcode by the address of the
395: start of the machine code fragment for executing the VM instruction.
1.6 anton 396: Dispatch consists of loading this address, jumping to it, and
397: incrementing the VM instruction pointer. Typically the threaded-code
398: dispatch code is appended directly to the code for executing the VM
399: instruction. Threaded code cannot be implemented in ANSI C, but it can
1.11 anton 400: be implemented using GNU C's labels-as-values extension (@pxref{Labels
401: as Values, , Labels as Values, gcc.info, GNU C Manual}).
1.6 anton 402:
1.13 anton 403: @c call threading
1.6 anton 404: @end table
405:
1.12 anton 406: Threaded code can be twice as fast as switch dispatch, depending on the
407: interpreter, the benchmark, and the machine.
408:
1.3 anton 409: @c *************************************************************
1.11 anton 410: @node Invoking Vmgen, Example, Concepts, Top
411: @chapter Invoking Vmgen
1.12 anton 412: @cindex Invoking Vmgen
1.3 anton 413:
1.11 anton 414: The usual way to invoke Vmgen is as follows:
1.3 anton 415:
416: @example
1.13 anton 417: vmgen @var{inputfile}
1.3 anton 418: @end example
419:
1.13 anton 420: Here @var{inputfile} is the VM instruction description file, which
421: usually ends in @file{.vmg}. The output filenames are made by taking
422: the basename of @file{inputfile} (i.e., the output files will be created
423: in the current working directory) and replacing @file{.vmg} with
424: @file{-vm.i}, @file{-disasm.i}, @file{-gen.i}, @file{-labels.i},
425: @file{-profile.i}, and @file{-peephole.i}. E.g., @command{vmgen
426: hack/foo.vmg} will create @file{foo-vm.i}, @file{foo-disasm.i},
427: @file{foo-gen.i}, @file{foo-labels.i}, @file{foo-profile.i} and
428: @file{foo-peephole.i}.
1.3 anton 429:
1.11 anton 430: The command-line options supported by Vmgen are
1.3 anton 431:
432: @table @option
433:
434: @cindex -h, command-line option
435: @cindex --help, command-line option
436: @item --help
437: @itemx -h
438: Print a message about the command-line options
439:
440: @cindex -v, command-line option
441: @cindex --version, command-line option
442: @item --version
443: @itemx -v
444: Print version and exit
445: @end table
446:
447: @c env vars GFORTHDIR GFORTHDATADIR
448:
1.5 anton 449: @c ****************************************************************
1.11 anton 450: @node Example, Input File Format, Invoking Vmgen, Top
1.5 anton 451: @chapter Example
1.12 anton 452: @cindex example of a Vmgen-based interpreter
1.5 anton 453:
1.10 anton 454: @menu
455: * Example overview::
456: * Using profiling to create superinstructions::
457: @end menu
458:
459: @c --------------------------------------------------------------------
460: @node Example overview, Using profiling to create superinstructions, Example, Example
1.5 anton 461: @section Example overview
1.12 anton 462: @cindex example overview
463: @cindex @file{vmgen-ex}
464: @cindex @file{vmgen-ex2}
1.5 anton 465:
1.11 anton 466: There are two versions of the same example for using Vmgen:
1.5 anton 467: @file{vmgen-ex} and @file{vmgen-ex2} (you can also see Gforth as
468: example, but it uses additional (undocumented) features, and also
469: differs in some other respects). The example implements @emph{mini}, a
470: tiny Modula-2-like language with a small JavaVM-like virtual machine.
1.12 anton 471:
1.5 anton 472: The difference between the examples is that @file{vmgen-ex} uses many
473: casts, and @file{vmgen-ex2} tries to avoids most casts and uses unions
1.12 anton 474: instead. In the rest of this manual we usually mention just files in
475: @file{vmgen-ex}; if you want to use unions, use the equivalent file in
476: @file{vmgen-ex2}.
477: @cindex unions example
478: @cindex casts example
1.5 anton 479:
480: The files provided with each example are:
1.12 anton 481: @cindex example files
1.5 anton 482:
483: @example
484: Makefile
485: README
486: disasm.c wrapper file
487: engine.c wrapper file
488: peephole.c wrapper file
489: profile.c wrapper file
490: mini-inst.vmg simple VM instructions
491: mini-super.vmg superinstructions (empty at first)
492: mini.h common declarations
493: mini.l scanner
494: mini.y front end (parser, VM code generator)
495: support.c main() and other support functions
496: fib.mini example mini program
497: simple.mini example mini program
498: test.mini example mini program (tests everything)
499: test.out test.mini output
500: stat.awk script for aggregating profile information
501: peephole-blacklist list of instructions not allowed in superinstructions
502: seq2rule.awk script for creating superinstructions
503: @end example
504:
505: For your own interpreter, you would typically copy the following files
506: and change little, if anything:
1.12 anton 507: @cindex wrapper files
1.5 anton 508:
509: @example
510: disasm.c wrapper file
511: engine.c wrapper file
512: peephole.c wrapper file
513: profile.c wrapper file
514: stat.awk script for aggregating profile information
515: seq2rule.awk script for creating superinstructions
516: @end example
517:
1.11 anton 518: @noindent
1.5 anton 519: You would typically change much in or replace the following files:
520:
521: @example
522: Makefile
523: mini-inst.vmg simple VM instructions
524: mini.h common declarations
525: mini.l scanner
526: mini.y front end (parser, VM code generator)
527: support.c main() and other support functions
528: peephole-blacklist list of instructions not allowed in superinstructions
529: @end example
530:
531: You can build the example by @code{cd}ing into the example's directory,
1.12 anton 532: and then typing @code{make}; you can check that it works with @code{make
1.5 anton 533: check}. You can run run mini programs like this:
534:
535: @example
536: ./mini fib.mini
537: @end example
538:
1.12 anton 539: To learn about the options, type @code{./mini -h}.
1.5 anton 540:
1.10 anton 541: @c --------------------------------------------------------------------
542: @node Using profiling to create superinstructions, , Example overview, Example
1.5 anton 543: @section Using profiling to create superinstructions
1.12 anton 544: @cindex profiling example
545: @cindex superinstructions example
1.5 anton 546:
547: I have not added rules for this in the @file{Makefile} (there are many
548: options for selecting superinstructions, and I did not want to hardcode
549: one into the @file{Makefile}), but there are some supporting scripts, and
550: here's an example:
551:
552: Suppose you want to use @file{fib.mini} and @file{test.mini} as training
553: programs, you get the profiles like this:
554:
555: @example
556: make fib.prof test.prof #takes a few seconds
557: @end example
558:
559: You can aggregate these profiles with @file{stat.awk}:
560:
561: @example
562: awk -f stat.awk fib.prof test.prof
563: @end example
564:
565: The result contains lines like:
566:
567: @example
568: 2 16 36910041 loadlocal lit
569: @end example
570:
571: This means that the sequence @code{loadlocal lit} statically occurs a
572: total of 16 times in 2 profiles, with a dynamic execution count of
573: 36910041.
574:
575: The numbers can be used in various ways to select superinstructions.
576: E.g., if you just want to select all sequences with a dynamic
577: execution count exceeding 10000, you would use the following pipeline:
578:
579: @example
580: awk -f stat.awk fib.prof test.prof|
581: awk '$3>=10000'| #select sequences
582: fgrep -v -f peephole-blacklist| #eliminate wrong instructions
1.12 anton 583: awk -f seq2rule.awk| #transform sequences into superinstruction rules
1.5 anton 584: sort -k 3 >mini-super.vmg #sort sequences
585: @end example
586:
587: The file @file{peephole-blacklist} contains all instructions that
588: directly access a stack or stack pointer (for mini: @code{call},
589: @code{return}); the sort step is necessary to ensure that prefixes
1.13 anton 590: precede larger superinstructions.
1.5 anton 591:
592: Now you can create a version of mini with superinstructions by just
593: saying @samp{make}
594:
1.10 anton 595:
1.3 anton 596: @c ***************************************************************
1.13 anton 597: @node Input File Format, Error messages, Example, Top
1.3 anton 598: @chapter Input File Format
1.12 anton 599: @cindex input file format
600: @cindex format, input file
1.3 anton 601:
602: Vmgen takes as input a file containing specifications of virtual machine
603: instructions. This file usually has a name ending in @file{.vmg}.
604:
1.5 anton 605: Most examples are taken from the example in @file{vmgen-ex}.
1.3 anton 606:
1.10 anton 607: @menu
608: * Input File Grammar::
609: * Simple instructions::
610: * Superinstructions::
1.18 anton 611: * Store Optimization::
1.11 anton 612: * Register Machines:: How to define register VM instructions
1.10 anton 613: @end menu
614:
615: @c --------------------------------------------------------------------
616: @node Input File Grammar, Simple instructions, Input File Format, Input File Format
1.3 anton 617: @section Input File Grammar
1.12 anton 618: @cindex grammar, input file
619: @cindex input file grammar
1.3 anton 620:
621: The grammar is in EBNF format, with @code{@var{a}|@var{b}} meaning
622: ``@var{a} or @var{b}'', @code{@{@var{c}@}} meaning 0 or more repetitions
623: of @var{c} and @code{[@var{d}]} meaning 0 or 1 repetitions of @var{d}.
624:
1.12 anton 625: @cindex free-format, not
1.15 anton 626: @cindex newlines, significance in syntax
1.3 anton 627: Vmgen input is not free-format, so you have to take care where you put
1.15 anton 628: newlines (and, in a few cases, white space).
1.3 anton 629:
630: @example
1.15 anton 631: description: @{instruction|comment|eval-escape|c-escape@}
1.3 anton 632:
633: instruction: simple-inst|superinst
634:
1.15 anton 635: simple-inst: ident '(' stack-effect ')' newline c-code newline newline
1.3 anton 636:
1.15 anton 637: stack-effect: @{ident@} '--' @{ident@}
1.3 anton 638:
1.15 anton 639: super-inst: ident '=' ident @{ident@}
1.3 anton 640:
1.12 anton 641: comment: '\ ' text newline
1.3 anton 642:
1.13 anton 643: eval-escape: '\E ' text newline
1.15 anton 644:
645: c-escape: '\C ' text newline
1.3 anton 646: @end example
647: @c \+ \- \g \f \c
648:
649: Note that the @code{\}s in this grammar are meant literally, not as
1.5 anton 650: C-style encodings for non-printable characters.
1.3 anton 651:
1.15 anton 652: There are two ways to delimit the C code in @code{simple-inst}:
653:
654: @itemize @bullet
655:
656: @item
657: If you start it with a @samp{@{} at the start of a line (i.e., not even
658: white space before it), you have to end it with a @samp{@}} at the start
659: of a line (followed by a newline). In this case you may have empty
660: lines within the C code (typically used between variable definitions and
661: statements).
662:
663: @item
664: You do not start it with @samp{@{}. Then the C code ends at the first
665: empty line, so you cannot have empty lines within this code.
666:
667: @end itemize
668:
669: The text in @code{comment}, @code{eval-escape} and @code{c-escape} must
670: not contain a newline. @code{Ident} must conform to the usual
671: conventions of C identifiers (otherwise the C compiler would choke on
672: the Vmgen output), except that idents in @code{stack-effect} may have a
673: stack prefix (for stack prefix syntax, @pxref{Eval escapes}).
674:
675: @cindex C escape
676: @cindex @code{\C}
677: @cindex conditional compilation of Vmgen output
678: The @code{c-escape} passes the text through to each output file (without
679: the @samp{\C}). This is useful mainly for conditional compilation
680: (i.e., you write @samp{\C #if ...} etc.).
681:
682: @cindex sync lines
683: @cindex @code{#line}
684: In addition to the syntax given in the grammer, Vmgen also processes
685: sync lines (lines starting with @samp{#line}), as produced by @samp{m4
686: -s} (@pxref{Invoking m4, , Invoking m4, m4.info, GNU m4}) and similar
687: tools. This allows associating C compiler error messages with the
688: original source of the C code.
1.3 anton 689:
690: Vmgen understands a few extensions beyond the grammar given here, but
691: these extensions are only useful for building Gforth. You can find a
692: description of the format used for Gforth in @file{prim}.
693:
1.17 anton 694: @menu
695: * Eval escapes:: what follows \E
696: @end menu
697:
698: @node Eval escapes, , Input File Grammar, Input File Grammar
1.10 anton 699: @subsection Eval escapes
1.12 anton 700: @cindex escape to Forth
701: @cindex eval escape
1.15 anton 702: @cindex @code{\E}
1.13 anton 703:
1.3 anton 704: @c woanders?
705: The text in @code{eval-escape} is Forth code that is evaluated when
1.13 anton 706: Vmgen reads the line. You will normally use this feature to define
707: stacks and types.
708:
709: If you do not know (and do not want to learn) Forth, you can build the
710: text according to the following grammar; these rules are normally all
711: Forth you need for using Vmgen:
1.3 anton 712:
713: @example
1.18 anton 714: text: stack-decl|type-prefix-decl|stack-prefix-decl|set-flag
1.3 anton 715:
1.12 anton 716: stack-decl: 'stack ' ident ident ident
1.3 anton 717: type-prefix-decl:
1.12 anton 718: 's" ' string '" ' ('single'|'double') ident 'type-prefix' ident
719: stack-prefix-decl: ident 'stack-prefix' string
1.22 anton 720: set-flag: ('store-optimization'|'include-skipped-insts') ('on'|'off')
1.3 anton 721: @end example
722:
723: Note that the syntax of this code is not checked thoroughly (there are
1.13 anton 724: many other Forth program fragments that could be written in an
725: eval-escape).
1.3 anton 726:
1.14 anton 727: A stack prefix can contain letters, digits, or @samp{:}, and may start
728: with an @samp{#}; e.g., in Gforth the return stack has the stack prefix
729: @samp{R:}. This restriction is not checked during the stack prefix
730: definition, but it is enforced by the parsing rules for stack items
731: later.
732:
1.3 anton 733: If you know Forth, the stack effects of the non-standard words involved
734: are:
1.12 anton 735: @findex stack
736: @findex type-prefix
737: @findex single
738: @findex double
739: @findex stack-prefix
1.18 anton 740: @findex store-optimization
1.3 anton 741: @example
1.22 anton 742: stack ( "name" "pointer" "type" -- )
743: ( name execution: -- stack )
744: type-prefix ( addr u item-size stack "prefix" -- )
745: single ( -- item-size )
746: double ( -- item-size )
747: stack-prefix ( stack "prefix" -- )
748: store-optimization ( -- addr )
749: include-skipped-insts ( -- addr )
1.3 anton 750: @end example
751:
1.14 anton 752: An @var{item-size} takes three cells on the stack.
1.5 anton 753:
1.10 anton 754: @c --------------------------------------------------------------------
755: @node Simple instructions, Superinstructions, Input File Grammar, Input File Format
1.3 anton 756: @section Simple instructions
1.12 anton 757: @cindex simple VM instruction
758: @cindex instruction, simple VM
1.3 anton 759:
760: We will use the following simple VM instruction description as example:
761:
762: @example
763: sub ( i1 i2 -- i )
764: i = i1-i2;
765: @end example
766:
767: The first line specifies the name of the VM instruction (@code{sub}) and
768: its stack effect (@code{i1 i2 -- i}). The rest of the description is
769: just plain C code.
770:
771: @cindex stack effect
1.12 anton 772: @cindex effect, stack
1.3 anton 773: The stack effect specifies that @code{sub} pulls two integers from the
1.12 anton 774: data stack and puts them in the C variables @code{i1} and @code{i2}
775: (with the rightmost item (@code{i2}) taken from the top of stack;
776: intuition: if you push @code{i1}, then @code{i2} on the stack, the
777: resulting stack picture is @code{i1 i2}) and later pushes one integer
778: (@code{i}) on the data stack (the rightmost item is on the top
779: afterwards).
780:
781: @cindex prefix, type
782: @cindex type prefix
783: @cindex default stack of a type prefix
1.3 anton 784: How do we know the type and stack of the stack items? Vmgen uses
785: prefixes, similar to Fortran; in contrast to Fortran, you have to
786: define the prefix first:
787:
788: @example
789: \E s" Cell" single data-stack type-prefix i
790: @end example
791:
792: This defines the prefix @code{i} to refer to the type @code{Cell}
793: (defined as @code{long} in @file{mini.h}) and, by default, to the
794: @code{data-stack}. It also specifies that this type takes one stack
795: item (@code{single}). The type prefix is part of the variable name.
796:
1.12 anton 797: @cindex stack definition
798: @cindex defining a stack
1.3 anton 799: Before we can use @code{data-stack} in this way, we have to define it:
800:
801: @example
802: \E stack data-stack sp Cell
803: @end example
804: @c !! use something other than Cell
805:
1.12 anton 806: @cindex stack basic type
807: @cindex basic type of a stack
808: @cindex type of a stack, basic
1.3 anton 809: This line defines the stack @code{data-stack}, which uses the stack
810: pointer @code{sp}, and each item has the basic type @code{Cell}; other
811: types have to fit into one or two @code{Cell}s (depending on whether the
1.12 anton 812: type is @code{single} or @code{double} wide), and are cast from and to
813: Cells on accessing the @code{data-stack} with type cast macros
1.22 anton 814: (@pxref{VM engine}). By default, stacks grow towards lower addresses in
815: Vmgen-erated interpreters (@pxref{Stack growth direction}).
1.3 anton 816:
1.12 anton 817: @cindex stack prefix
818: @cindex prefix, stack
1.3 anton 819: We can override the default stack of a stack item by using a stack
820: prefix. E.g., consider the following instruction:
821:
822: @example
823: lit ( #i -- i )
824: @end example
825:
826: The VM instruction @code{lit} takes the item @code{i} from the
1.5 anton 827: instruction stream (indicated by the prefix @code{#}), and pushes it on
1.3 anton 828: the (default) data stack. The stack prefix is not part of the variable
829: name. Stack prefixes are defined like this:
830:
831: @example
832: \E inst-stream stack-prefix #
1.29 anton 833: \E data-stack stack-prefix S:
1.3 anton 834: @end example
835:
1.5 anton 836: This definition defines that the stack prefix @code{#} specifies the
1.3 anton 837: ``stack'' @code{inst-stream}. Since the instruction stream behaves a
838: little differently than an ordinary stack, it is predefined, and you do
839: not need to define it.
840:
1.12 anton 841: @cindex instruction stream
1.3 anton 842: The instruction stream contains instructions and their immediate
843: arguments, so specifying that an argument comes from the instruction
844: stream indicates an immediate argument. Of course, instruction stream
845: arguments can only appear to the left of @code{--} in the stack effect.
846: If there are multiple instruction stream arguments, the leftmost is the
847: first one (just as the intuition suggests).
848:
1.10 anton 849: @menu
1.29 anton 850: * Explicit stack access:: If the C code accesses a stack pointer
1.10 anton 851: * C Code Macros:: Macros recognized by Vmgen
852: * C Code restrictions:: Vmgen makes assumptions about C code
1.22 anton 853: * Stack growth direction:: is configurable per stack
1.10 anton 854: @end menu
855:
856: @c --------------------------------------------------------------------
1.29 anton 857: @node Explicit stack access, C Code Macros, Simple instructions, Simple instructions
858: @subsection Explicit stack access
859: @cindex stack access, explicit
860: @cindex Stack pointer access
861: @cindex explicit stack access
862:
863: Not all stack effects can be specified using the stack effect
864: specifications above. For VM instructions that have other stack
865: effects, you can specify them explicitly by accessing the stack
866: pointer in the C code; however, you have to notify Vmgen of such
867: explicit stack accesses, otherwise Vmgens optimizations could conflict
868: with your explicit stack accesses.
869:
870: You notify Vmgen by putting @code{...} with the appropriate stack
871: prefix into the stack comment. Then the VM instruction will first
872: take the other stack items specified in the stack effect into C
873: variables, then make sure that all other stack items for that stack
874: are in memory, and that the stack pointer for the stack points to the
875: top-of-stack (by default, unless you change the stack access
876: transformation: @pxref{Stack growth direction}).
877:
878: The general rule is: If you mention a stack pointer in the C code of a
879: VM instruction, you should put a @code{...} for that stack in the stack
880: effect.
881:
882: Consider this example:
883:
884: @example
885: return ( #iadjust S:... target afp i1 -- i2 )
886: SET_IP(target);
887: sp = (Cell *)(((char *)sp)+iadjust);
888: fp = afp;
889: i2=i1;
890: @end example
891:
892: First the variables @code{target afp i1} are popped off the stack,
893: then the stack pointer @code{sp} is set correctly for the new stack
894: depth, then the C code changes the stack depth and does other things,
895: and finally @code{i2} is pushed on the stack with the new depth.
896:
897: The position of the @code{...} within the stack effect does not
898: matter. You can use several @code{...}s, for different stacks, and
899: also several for the same stack (that has no additional effect). If
900: you use @code{...} without a stack prefix, this specifies all the
901: stacks except the instruction stream.
902:
903: You cannot use @code{...} for the instruction stream, but that is not
904: necessary: At the start of the C code, @code{IP} points to the start
905: of the next VM instruction (i.e., right beyond the end of the current
906: VM instruction), and you can change the instruction pointer with
907: @code{SET_IP} (@pxref{VM engine}).
908:
909:
910: @c --------------------------------------------------------------------
911: @node C Code Macros, C Code restrictions, Explicit stack access, Simple instructions
1.10 anton 912: @subsection C Code Macros
1.12 anton 913: @cindex macros recognized by Vmgen
914: @cindex basic block, VM level
1.5 anton 915:
916: Vmgen recognizes the following strings in the C code part of simple
917: instructions:
918:
1.12 anton 919: @table @code
1.5 anton 920:
921: @item SET_IP
1.12 anton 922: @findex SET_IP
1.11 anton 923: As far as Vmgen is concerned, a VM instruction containing this ends a VM
1.5 anton 924: basic block (used in profiling to delimit profiled sequences). On the C
925: level, this also sets the instruction pointer.
926:
927: @item SUPER_END
1.12 anton 928: @findex SUPER_END
929: This ends a basic block (for profiling), even if the instruction
930: contains no @code{SET_IP}.
1.5 anton 931:
1.13 anton 932: @item INST_TAIL;
933: @findex INST_TAIL;
934: Vmgen replaces @samp{INST_TAIL;} with code for ending a VM instruction and
935: dispatching the next VM instruction. Even without a @samp{INST_TAIL;} this
1.12 anton 936: happens automatically when control reaches the end of the C code. If
937: you want to have this in the middle of the C code, you need to use
1.13 anton 938: @samp{INST_TAIL;}. A typical example is a conditional VM branch:
1.5 anton 939:
940: @example
1.11 anton 941: if (branch_condition) @{
1.13 anton 942: SET_IP(target); INST_TAIL;
1.11 anton 943: @}
1.5 anton 944: /* implicit tail follows here */
945: @end example
946:
1.13 anton 947: In this example, @samp{INST_TAIL;} is not strictly necessary, because there
1.5 anton 948: is another one implicitly after the if-statement, but using it improves
949: branch prediction accuracy slightly and allows other optimizations.
950:
951: @item SUPER_CONTINUE
1.12 anton 952: @findex SUPER_CONTINUE
1.5 anton 953: This indicates that the implicit tail at the end of the VM instruction
954: dispatches the sequentially next VM instruction even if there is a
955: @code{SET_IP} in the VM instruction. This enables an optimization that
956: is not yet implemented in the vmgen-ex code (but in Gforth). The
957: typical application is in conditional VM branches:
958:
959: @example
1.11 anton 960: if (branch_condition) @{
1.13 anton 961: SET_IP(target); INST_TAIL; /* now this INST_TAIL is necessary */
1.11 anton 962: @}
1.5 anton 963: SUPER_CONTINUE;
964: @end example
965:
1.27 anton 966: @item VM_JUMP
967: @findex VM_JUMP
968: @code{VM_JUMP(target)} is equivalent to @code{goto *(target)}, but
969: allows Vmgen to do dynamic superinstructions and replication. You
1.28 anton 970: still need to say @code{SUPER_END}. Also, the goto only happens at
971: the end (wherever the VM_JUMP is). Essentially, this just suppresses
972: much of the ordinary dispatch mechanism.
1.27 anton 973:
1.5 anton 974: @end table
975:
1.11 anton 976: Note that Vmgen is not smart about C-level tokenization, comments,
1.5 anton 977: strings, or conditional compilation, so it will interpret even a
978: commented-out SUPER_END as ending a basic block (or, e.g.,
1.13 anton 979: @samp{RESET_IP;} as @samp{SET_IP;}). Conversely, Vmgen requires the literal
1.11 anton 980: presence of these strings; Vmgen will not see them if they are hiding in
1.5 anton 981: a C preprocessor macro.
982:
983:
1.10 anton 984: @c --------------------------------------------------------------------
1.22 anton 985: @node C Code restrictions, Stack growth direction, C Code Macros, Simple instructions
1.10 anton 986: @subsection C Code restrictions
1.12 anton 987: @cindex C code restrictions
988: @cindex restrictions on C code
989: @cindex assumptions about C code
990:
991: @cindex accessing stack (pointer)
992: @cindex stack pointer, access
993: @cindex instruction pointer, access
1.5 anton 994: Vmgen generates code and performs some optimizations under the
995: assumption that the user-supplied C code does not access the stack
996: pointers or stack items, and that accesses to the instruction pointer
997: only occur through special macros. In general you should heed these
998: restrictions. However, if you need to break these restrictions, read
999: the following.
1000:
1001: Accessing a stack or stack pointer directly can be a problem for several
1002: reasons:
1.12 anton 1003: @cindex stack caching, restriction on C code
1004: @cindex superinstructions, restrictions on components
1.5 anton 1005:
1.11 anton 1006: @itemize @bullet
1.5 anton 1007:
1008: @item
1.12 anton 1009: Vmgen optionally supports caching the top-of-stack item in a local
1010: variable (that is allocated to a register). This is the most frequent
1011: source of trouble. You can deal with it either by not using
1012: top-of-stack caching (slowdown factor 1-1.4, depending on machine), or
1013: by inserting flushing code (e.g., @samp{IF_spTOS(sp[...] = spTOS);}) at
1014: the start and reloading code (e.g., @samp{IF_spTOS(spTOS = sp[0])}) at
1015: the end of problematic C code. Vmgen inserts a stack pointer update
1016: before the start of the user-supplied C code, so the flushing code has
1017: to use an index that corrects for that. In the future, this flushing
1018: may be done automatically by mentioning a special string in the C code.
1.5 anton 1019: @c sometimes flushing and/or reloading unnecessary
1020:
1021: @item
1.11 anton 1022: The Vmgen-erated code loads the stack items from stack-pointer-indexed
1.5 anton 1023: memory into variables before the user-supplied C code, and stores them
1024: from variables to stack-pointer-indexed memory afterwards. If you do
1025: any writes to the stack through its stack pointer in your C code, it
1.13 anton 1026: will not affect the variables, and your write may be overwritten by the
1.5 anton 1027: stores after the C code. Similarly, a read from a stack using a stack
1028: pointer will not reflect computations of stack items in the same VM
1029: instruction.
1030:
1031: @item
1032: Superinstructions keep stack items in variables across the whole
1033: superinstruction. So you should not include VM instructions, that
1.12 anton 1034: access a stack or stack pointer, as components of superinstructions
1035: (@pxref{VM profiler}).
1.5 anton 1036:
1037: @end itemize
1038:
1039: You should access the instruction pointer only through its special
1040: macros (@samp{IP}, @samp{SET_IP}, @samp{IPTOS}); this ensure that these
1041: macros can be implemented in several ways for best performance.
1042: @samp{IP} points to the next instruction, and @samp{IPTOS} is its
1043: contents.
1044:
1.22 anton 1045: @c --------------------------------------------------------------------
1046: @node Stack growth direction, , C Code restrictions, Simple instructions
1047: @subsection Stack growth direction
1048: @cindex stack growth direction
1049:
1050: @cindex @code{stack-access-transform}
1051: By default, the stacks grow towards lower addresses. You can change
1052: this for a stack by setting the @code{stack-access-transform} field of
1053: the stack to an xt @code{( itemnum -- index )} that performs the
1054: appropriate index transformation.
1055:
1056: E.g., if you want to let @code{data-stack} grow towards higher
1057: addresses, with the stack pointer always pointing just beyond the
1058: top-of-stack, use this right after defining @code{data-stack}:
1059:
1060: @example
1061: \E : sp-access-transform ( itemnum -- index ) negate 1- ;
1062: \E ' sp-access-transform ' data-stack >body stack-access-transform !
1063: @end example
1064:
1065: This means that @code{sp-access-transform} will be used to generate
1066: indexes for accessing @code{data-stack}. The definition of
1067: @code{sp-access-transform} above transforms n into -n-1, e.g, 1 into -2.
1068: This will access the 0th data-stack element (top-of-stack) at sp[-1],
1069: the 1st at sp[-2], etc., which is the typical way upward-growing
1070: stacks are used. If you need a different transform and do not know
1071: enough Forth to program it, let me know.
1.5 anton 1072:
1.10 anton 1073: @c --------------------------------------------------------------------
1.18 anton 1074: @node Superinstructions, Store Optimization, Simple instructions, Input File Format
1.3 anton 1075: @section Superinstructions
1.12 anton 1076: @cindex superinstructions, defining
1077: @cindex defining superinstructions
1.5 anton 1078:
1.8 anton 1079: Note: don't invest too much work in (static) superinstructions; a future
1.11 anton 1080: version of Vmgen will support dynamic superinstructions (see Ian
1.8 anton 1081: Piumarta and Fabio Riccardi, @cite{Optimizing Direct Threaded Code by
1082: Selective Inlining}, PLDI'98), and static superinstructions have much
1.12 anton 1083: less benefit in that context (preliminary results indicate only a factor
1084: 1.1 speedup).
1.8 anton 1085:
1.5 anton 1086: Here is an example of a superinstruction definition:
1087:
1088: @example
1089: lit_sub = lit sub
1090: @end example
1091:
1092: @code{lit_sub} is the name of the superinstruction, and @code{lit} and
1093: @code{sub} are its components. This superinstruction performs the same
1094: action as the sequence @code{lit} and @code{sub}. It is generated
1095: automatically by the VM code generation functions whenever that sequence
1.11 anton 1096: occurs, so if you want to use this superinstruction, you just need to
1097: add this definition (and even that can be partially automatized,
1098: @pxref{VM profiler}).
1.5 anton 1099:
1.12 anton 1100: @cindex prefixes of superinstructions
1.5 anton 1101: Vmgen requires that the component instructions are simple instructions
1.11 anton 1102: defined before superinstructions using the components. Currently, Vmgen
1.5 anton 1103: also requires that all the subsequences at the start of a
1104: superinstruction (prefixes) must be defined as superinstruction before
1105: the superinstruction. I.e., if you want to define a superinstruction
1106:
1107: @example
1.12 anton 1108: foo4 = load add sub mul
1.5 anton 1109: @end example
1110:
1.12 anton 1111: you first have to define @code{load}, @code{add}, @code{sub} and
1112: @code{mul}, plus
1.5 anton 1113:
1114: @example
1.12 anton 1115: foo2 = load add
1116: foo3 = load add sub
1.5 anton 1117: @end example
1118:
1119: Here, @code{sumof4} is the longest prefix of @code{sumof5}, and @code{sumof3}
1120: is the longest prefix of @code{sumof4}.
1121:
1.11 anton 1122: Note that Vmgen assumes that only the code it generates accesses stack
1.5 anton 1123: pointers, the instruction pointer, and various stack items, and it
1124: performs optimizations based on this assumption. Therefore, VM
1.12 anton 1125: instructions where your C code changes the instruction pointer should
1126: only be used as last component; a VM instruction where your C code
1127: accesses a stack pointer should not be used as component at all. Vmgen
1128: does not check these restrictions, they just result in bugs in your
1129: interpreter.
1.5 anton 1130:
1.22 anton 1131: @cindex include-skipped-insts
1132: The Vmgen flag @code{include-skipped-insts} influences superinstruction
1133: code generation. Currently there is no support in the peephole
1134: optimizer for both variations, so leave this flag alone for now.
1135:
1.12 anton 1136: @c -------------------------------------------------------------------
1.18 anton 1137: @node Store Optimization, Register Machines, Superinstructions, Input File Format
1138: @section Store Optimization
1139: @cindex store optimization
1140: @cindex optimization, stack stores
1141: @cindex stack stores, optimization
1142: @cindex eliminating stack stores
1143:
1144: This minor optimization (0.6\%--0.8\% reduction in executed instructions
1145: for Gforth) puts additional requirements on the instruction descriptions
1146: and is therefore disabled by default.
1147:
1148: What does it do? Consider an instruction like
1149:
1150: @example
1151: dup ( n -- n n )
1152: @end example
1153:
1154: For simplicity, also assume that we are not caching the top-of-stack in
1155: a register. Now, the C code for dup first loads @code{n} from the
1156: stack, and then stores it twice to the stack, one time to the address
1157: where it came from; that time is unnecessary, but gcc does not optimize
1158: it away, so vmgen can do it instead (if you turn on the store
1159: optimization).
1160:
1161: Vmgen uses the stack item's name to determine if the stack item contains
1162: the same value as it did at the start. Therefore, if you use the store
1163: optimization, you have to ensure that stack items that have the same
1164: name on input and output also have the same value, and are not changed
1165: in the C code you supply. I.e., the following code could fail if you
1166: turn on the store optimization:
1167:
1168: @example
1169: add1 ( n -- n )
1170: n++;
1171: @end example
1172:
1173: Instead, you have to use different names, i.e.:
1174:
1175: @example
1.27 anton 1176: add1 ( n1 -- n2 )
1.18 anton 1177: n2=n1+1;
1178: @end example
1179:
1.22 anton 1180: Similarly, the store optimization assumes that the stack pointer is only
1181: changed by Vmgen-erated code. If your C code changes the stack pointer,
1182: use different names in input and output stack items to avoid a (probably
1183: wrong) store optimization, or turn the store optimization off for this
1184: VM instruction.
1185:
1.18 anton 1186: To turn on the store optimization, write
1187:
1188: @example
1189: \E store-optimization on
1190: @end example
1191:
1192: at the start of the file. You can turn this optimization on or off
1193: between any two VM instruction descriptions. For turning it off again,
1194: you can use
1195:
1196: @example
1197: \E store-optimization off
1198: @end example
1199:
1200: @c -------------------------------------------------------------------
1201: @node Register Machines, , Store Optimization, Input File Format
1.11 anton 1202: @section Register Machines
1.12 anton 1203: @cindex Register VM
1204: @cindex Superinstructions for register VMs
1205: @cindex tracing of register VMs
1.11 anton 1206:
1207: If you want to implement a register VM rather than a stack VM with
1208: Vmgen, there are two ways to do it: Directly and through
1209: superinstructions.
1210:
1211: If you use the direct way, you define instructions that take the
1212: register numbers as immediate arguments, like this:
1213:
1214: @example
1215: add3 ( #src1 #src2 #dest -- )
1216: reg[dest] = reg[src1]+reg[src2];
1217: @end example
1218:
1.12 anton 1219: A disadvantage of this method is that during tracing you only see the
1220: register numbers, but not the register contents. Actually, with an
1221: appropriate definition of @code{printarg_src} (@pxref{VM engine}), you
1222: can print the values of the source registers on entry, but you cannot
1223: print the value of the destination register on exit.
1224:
1.11 anton 1225: If you use superinstructions to define a register VM, you define simple
1226: instructions that use a stack, and then define superinstructions that
1227: have no overall stack effect, like this:
1228:
1229: @example
1230: loadreg ( #src -- n )
1231: n = reg[src];
1232:
1233: storereg ( n #dest -- )
1234: reg[dest] = n;
1235:
1236: adds ( n1 n2 -- n )
1237: n = n1+n2;
1238:
1239: add3 = loadreg loadreg adds storereg
1240: @end example
1241:
1242: An advantage of this method is that you see the values and not just the
1.12 anton 1243: register numbers in tracing. A disadvantage of this method is that
1.11 anton 1244: currently you cannot generate superinstructions directly, but only
1245: through generating a sequence of simple instructions (we might change
1246: this in the future if there is demand).
1247:
1248: Could the register VM support be improved, apart from the issues
1249: mentioned above? It is hard to see how to do it in a general way,
1250: because there are a number of different designs that different people
1251: mean when they use the term @emph{register machine} in connection with
1252: VM interpreters. However, if you have ideas or requests in that
1253: direction, please let me know (@pxref{Contact}).
1254:
1.5 anton 1255: @c ********************************************************************
1.13 anton 1256: @node Error messages, Using the generated code, Input File Format, Top
1257: @chapter Error messages
1258: @cindex error messages
1259:
1260: These error messages are created by Vmgen:
1261:
1262: @table @code
1263:
1264: @cindex @code{# can only be on the input side} error
1265: @item # can only be on the input side
1266: You have used an instruction-stream prefix (usually @samp{#}) after the
1267: @samp{--} (the output side); you can only use it before (the input
1268: side).
1269:
1270: @cindex @code{prefix for this combination must be defined earlier} error
1.20 anton 1271: @item the prefix for this superinstruction must be defined earlier
1.13 anton 1272: You have defined a superinstruction (e.g. @code{abc = a b c}) without
1273: defining its direct prefix (e.g., @code{ab = a b}),
1274: @xref{Superinstructions}.
1275:
1276: @cindex @code{sync line syntax} error
1277: @item sync line syntax
1278: If you are using a preprocessor (e.g., @command{m4}) to generate Vmgen
1279: input code, you may want to create @code{#line} directives (aka sync
1280: lines). This error indicates that such a line is not in th syntax
1.16 anton 1281: expected by Vmgen (this should not happen; please report the offending
1282: line in a bug report).
1.13 anton 1283:
1284: @cindex @code{syntax error, wrong char} error
1.22 anton 1285: @item syntax error, wrong char
1.16 anton 1286: A syntax error. If you do not see right away where the error is, it may
1287: be helpful to check the following: Did you put an empty line in a VM
1288: instruction where the C code is not delimited by braces (then the empty
1289: line ends the VM instruction)? If you used brace-delimited C code, did
1290: you put the delimiting braces (and only those) at the start of the line,
1291: without preceding white space? Did you forget a delimiting brace?
1.13 anton 1292:
1293: @cindex @code{too many stacks} error
1294: @item too many stacks
1.16 anton 1295: Vmgen currently supports 3 stacks (plus the instruction stream); if you
1296: need more, let us know.
1.13 anton 1297:
1298: @cindex @code{unknown prefix} error
1299: @item unknown prefix
1300: The stack item does not match any defined type prefix (after stripping
1301: away any stack prefix). You should either declare the type prefix you
1302: want for that stack item, or use a different type prefix
1303:
1.22 anton 1304: @cindex @code{unknown primitive} error
1.13 anton 1305: @item unknown primitive
1306: You have used the name of a simple VM instruction in a superinstruction
1307: definition without defining the simple VM instruction first.
1308:
1309: @end table
1310:
1311: In addition, the C compiler can produce errors due to code produced by
1312: Vmgen; e.g., you need to define type cast functions.
1313:
1314: @c ********************************************************************
1315: @node Using the generated code, Hints, Error messages, Top
1.5 anton 1316: @chapter Using the generated code
1.12 anton 1317: @cindex generated code, usage
1318: @cindex Using vmgen-erated code
1.5 anton 1319:
1.11 anton 1320: The easiest way to create a working VM interpreter with Vmgen is
1.12 anton 1321: probably to start with @file{vmgen-ex}, and modify it for your purposes.
1.13 anton 1322: This chapter explains what the various wrapper and generated files do.
1323: It also contains reference-manual style descriptions of the macros,
1324: variables etc. used by the generated code, and you can skip that on
1325: first reading.
1.5 anton 1326:
1.10 anton 1327: @menu
1328: * VM engine:: Executing VM code
1329: * VM instruction table::
1330: * VM code generation:: Creating VM code (in the front-end)
1331: * Peephole optimization:: Creating VM superinstructions
1332: * VM disassembler:: for debugging the front end
1333: * VM profiler:: for finding worthwhile superinstructions
1334: @end menu
1.6 anton 1335:
1.10 anton 1336: @c --------------------------------------------------------------------
1337: @node VM engine, VM instruction table, Using the generated code, Using the generated code
1.5 anton 1338: @section VM engine
1.12 anton 1339: @cindex VM instruction execution
1340: @cindex engine
1341: @cindex executing VM code
1342: @cindex @file{engine.c}
1343: @cindex @file{-vm.i} output file
1.5 anton 1344:
1345: The VM engine is the VM interpreter that executes the VM code. It is
1346: essential for an interpretive system.
1347:
1.6 anton 1348: Vmgen supports two methods of VM instruction dispatch: @emph{threaded
1349: code} (fast, but gcc-specific), and @emph{switch dispatch} (slow, but
1350: portable across C compilers); you can use conditional compilation
1351: (@samp{defined(__GNUC__)}) to choose between these methods, and our
1352: example does so.
1353:
1354: For both methods, the VM engine is contained in a C-level function.
1355: Vmgen generates most of the contents of the function for you
1356: (@file{@var{name}-vm.i}), but you have to define this function, and
1357: macros and variables used in the engine, and initialize the variables.
1358: In our example the engine function also includes
1359: @file{@var{name}-labels.i} (@pxref{VM instruction table}).
1360:
1.12 anton 1361: @cindex tracing VM code
1.13 anton 1362: @cindex superinstructions and tracing
1.12 anton 1363: In addition to executing the code, the VM engine can optionally also
1364: print out a trace of the executed instructions, their arguments and
1365: results. For superinstructions it prints the trace as if only component
1366: instructions were executed; this allows to introduce new
1367: superinstructions while keeping the traces comparable to old ones
1368: (important for regression tests).
1369:
1370: It costs significant performance to check in each instruction whether to
1371: print tracing code, so we recommend producing two copies of the engine:
1372: one for fast execution, and one for tracing. See the rules for
1373: @file{engine.o} and @file{engine-debug.o} in @file{vmgen-ex/Makefile}
1374: for an example.
1375:
1.6 anton 1376: The following macros and variables are used in @file{@var{name}-vm.i}:
1.5 anton 1377:
1378: @table @code
1379:
1.12 anton 1380: @findex LABEL
1.5 anton 1381: @item LABEL(@var{inst_name})
1382: This is used just before each VM instruction to provide a jump or
1.11 anton 1383: @code{switch} label (the @samp{:} is provided by Vmgen). For switch
1.13 anton 1384: dispatch this should expand to @samp{case @var{label}:}; for
1385: threaded-code dispatch this should just expand to @samp{@var{label}:}.
1.12 anton 1386: In either case @var{label} is usually the @var{inst_name} with some
1387: prefix or suffix to avoid naming conflicts.
1.5 anton 1388:
1.12 anton 1389: @findex LABEL2
1.9 anton 1390: @item LABEL2(@var{inst_name})
1391: This will be used for dynamic superinstructions; at the moment, this
1392: should expand to nothing.
1393:
1.12 anton 1394: @findex NAME
1.5 anton 1395: @item NAME(@var{inst_name_string})
1396: Called on entering a VM instruction with a string containing the name of
1.13 anton 1397: the VM instruction as parameter. In normal execution this should be
1398: expand to nothing, but for tracing this usually prints the name, and
1399: possibly other information (several VM registers in our example).
1.5 anton 1400:
1.12 anton 1401: @findex DEF_CA
1.5 anton 1402: @item DEF_CA
1403: Usually empty. Called just inside a new scope at the start of a VM
1404: instruction. Can be used to define variables that should be visible
1405: during every VM instruction. If you define this macro as non-empty, you
1406: have to provide the finishing @samp{;} in the macro.
1407:
1.12 anton 1408: @findex NEXT_P0
1409: @findex NEXT_P1
1410: @findex NEXT_P2
1.5 anton 1411: @item NEXT_P0 NEXT_P1 NEXT_P2
1412: The three parts of instruction dispatch. They can be defined in
1413: different ways for best performance on various processors (see
1414: @file{engine.c} in the example or @file{engine/threaded.h} in Gforth).
1.12 anton 1415: @samp{NEXT_P0} is invoked right at the start of the VM instruction (but
1.5 anton 1416: after @samp{DEF_CA}), @samp{NEXT_P1} right after the user-supplied C
1417: code, and @samp{NEXT_P2} at the end. The actual jump has to be
1.13 anton 1418: performed by @samp{NEXT_P2} (if you would do it earlier, important parts
1419: of the VM instruction would not be executed).
1.5 anton 1420:
1421: The simplest variant is if @samp{NEXT_P2} does everything and the other
1422: macros do nothing. Then also related macros like @samp{IP},
1423: @samp{SET_IP}, @samp{IP}, @samp{INC_IP} and @samp{IPTOS} are very
1424: straightforward to define. For switch dispatch this code consists just
1.12 anton 1425: of a jump to the dispatch code (@samp{goto next_inst;} in our example);
1.5 anton 1426: for direct threaded code it consists of something like
1.11 anton 1427: @samp{(@{cfa=*ip++; goto *cfa;@})}.
1.5 anton 1428:
1.12 anton 1429: Pulling code (usually the @samp{cfa=*ip++;}) up into @samp{NEXT_P1}
1.5 anton 1430: usually does not cause problems, but pulling things up into
1431: @samp{NEXT_P0} usually requires changing the other macros (and, at least
1432: for Gforth on Alpha, it does not buy much, because the compiler often
1433: manages to schedule the relevant stuff up by itself). An even more
1434: extreme variant is to pull code up even further, into, e.g., NEXT_P1 of
1435: the previous VM instruction (prefetching, useful on PowerPCs).
1436:
1.12 anton 1437: @findex INC_IP
1.5 anton 1438: @item INC_IP(@var{n})
1.8 anton 1439: This increments @code{IP} by @var{n}.
1440:
1.12 anton 1441: @findex SET_IP
1.8 anton 1442: @item SET_IP(@var{target})
1443: This sets @code{IP} to @var{target}.
1.5 anton 1444:
1.12 anton 1445: @cindex type cast macro
1446: @findex vm_@var{A}2@var{B}
1.5 anton 1447: @item vm_@var{A}2@var{B}(a,b)
1448: Type casting macro that assigns @samp{a} (of type @var{A}) to @samp{b}
1449: (of type @var{B}). This is mainly used for getting stack items into
1450: variables and back. So you need to define macros for every combination
1451: of stack basic type (@code{Cell} in our example) and type-prefix types
1452: used with that stack (in both directions). For the type-prefix type,
1453: you use the type-prefix (not the C type string) as type name (e.g.,
1454: @samp{vm_Cell2i}, not @samp{vm_Cell2Cell}). In addition, you have to
1.12 anton 1455: define a vm_@var{X}2@var{X} macro for the stack's basic type @var{X}
1456: (used in superinstructions).
1.5 anton 1457:
1.12 anton 1458: @cindex instruction stream, basic type
1.5 anton 1459: The stack basic type for the predefined @samp{inst-stream} is
1460: @samp{Cell}. If you want a stack with the same item size, making its
1461: basic type @samp{Cell} usually reduces the number of macros you have to
1462: define.
1463:
1.12 anton 1464: @cindex unions in type cast macros
1465: @cindex casts in type cast macros
1466: @cindex type casting between floats and integers
1.5 anton 1467: Here our examples differ a lot: @file{vmgen-ex} uses casts in these
1468: macros, whereas @file{vmgen-ex2} uses union-field selection (or
1.12 anton 1469: assignment to union fields). Note that casting floats into integers and
1470: vice versa changes the bit pattern (and you do not want that). In this
1471: case your options are to use a (temporary) union, or to take the address
1472: of the value, cast the pointer, and dereference that (not always
1473: possible, and sometimes expensive).
1.5 anton 1474:
1.12 anton 1475: @findex vm_two@var{A}2@var{B}
1476: @findex vm_@var{B}2two@var{A}
1.5 anton 1477: @item vm_two@var{A}2@var{B}(a1,a2,b)
1478: @item vm_@var{B}2two@var{A}(b,a1,a2)
1.12 anton 1479: Type casting between two stack items (@code{a1}, @code{a2}) and a
1.5 anton 1480: variable @code{b} of a type that takes two stack items. This does not
1.12 anton 1481: occur in our small examples, but you can look at Gforth for examples
1482: (see @code{vm_twoCell2d} in @file{engine/forth.h}).
1.5 anton 1483:
1.12 anton 1484: @cindex stack pointer definition
1485: @cindex instruction pointer definition
1.5 anton 1486: @item @var{stackpointer}
1487: For each stack used, the stackpointer name given in the stack
1488: declaration is used. For a regular stack this must be an l-expression;
1489: typically it is a variable declared as a pointer to the stack's basic
1490: type. For @samp{inst-stream}, the name is @samp{IP}, and it can be a
1491: plain r-value; typically it is a macro that abstracts away the
1.12 anton 1492: differences between the various implementations of @code{NEXT_P*}.
1.22 anton 1493:
1494: @cindex IMM_ARG
1495: @findex IMM_ARG
1496: @item IMM_ARG(access,value)
1497: Define this to expland to ``(access)''. This is just a placeholder for
1498: future extensions.
1.5 anton 1499:
1.12 anton 1500: @cindex top of stack caching
1501: @cindex stack caching
1502: @cindex TOS
1503: @findex IPTOS
1.5 anton 1504: @item @var{stackpointer}TOS
1505: The top-of-stack for the stack pointed to by @var{stackpointer}. If you
1506: are using top-of-stack caching for that stack, this should be defined as
1507: variable; if you are not using top-of-stack caching for that stack, this
1508: should be a macro expanding to @samp{@var{stackpointer}[0]}. The stack
1509: pointer for the predefined @samp{inst-stream} is called @samp{IP}, so
1510: the top-of-stack is called @samp{IPTOS}.
1511:
1.12 anton 1512: @findex IF_@var{stackpointer}TOS
1.5 anton 1513: @item IF_@var{stackpointer}TOS(@var{expr})
1514: Macro for executing @var{expr}, if top-of-stack caching is used for the
1515: @var{stackpointer} stack. I.e., this should do @var{expr} if there is
1516: top-of-stack caching for @var{stackpointer}; otherwise it should do
1517: nothing.
1518:
1.12 anton 1519: @findex SUPER_END
1.8 anton 1520: @item SUPER_END
1521: This is used by the VM profiler (@pxref{VM profiler}); it should not do
1522: anything in normal operation, and call @code{vm_count_block(IP)} for
1523: profiling.
1524:
1.12 anton 1525: @findex SUPER_CONTINUE
1.8 anton 1526: @item SUPER_CONTINUE
1.11 anton 1527: This is just a hint to Vmgen and does nothing at the C level.
1.8 anton 1528:
1.26 anton 1529: @findex MAYBE_UNUSED
1530: @item MAYBE_UNUSED
1531: This should be defined as @code{__attribute__((unused))} for gcc-2.7 and
1532: higher. It suppresses the warnings about unused variables in the code
1533: for superinstructions. You need to define this only if you are using
1534: superinstructions.
1535:
1.12 anton 1536: @findex VM_DEBUG
1.5 anton 1537: @item VM_DEBUG
1538: If this is defined, the tracing code will be compiled in (slower
1539: interpretation, but better debugging). Our example compiles two
1540: versions of the engine, a fast-running one that cannot trace, and one
1541: with potential tracing and profiling.
1542:
1.12 anton 1543: @findex vm_debug
1.5 anton 1544: @item vm_debug
1545: Needed only if @samp{VM_DEBUG} is defined. If this variable contains
1546: true, the VM instructions produce trace output. It can be turned on or
1547: off at any time.
1548:
1.12 anton 1549: @findex vm_out
1.5 anton 1550: @item vm_out
1551: Needed only if @samp{VM_DEBUG} is defined. Specifies the file on which
1552: to print the trace output (type @samp{FILE *}).
1553:
1.12 anton 1554: @findex printarg_@var{type}
1.5 anton 1555: @item printarg_@var{type}(@var{value})
1556: Needed only if @samp{VM_DEBUG} is defined. Macro or function for
1557: printing @var{value} in a way appropriate for the @var{type}. This is
1558: used for printing the values of stack items during tracing. @var{Type}
1559: is normally the type prefix specified in a @code{type-prefix} definition
1560: (e.g., @samp{printarg_i}); in superinstructions it is currently the
1561: basic type of the stack.
1562:
1563: @end table
1564:
1.6 anton 1565:
1.10 anton 1566: @c --------------------------------------------------------------------
1567: @node VM instruction table, VM code generation, VM engine, Using the generated code
1568: @section VM instruction table
1.12 anton 1569: @cindex instruction table
1570: @cindex opcode definition
1571: @cindex labels for threaded code
1572: @cindex @code{vm_prim}, definition
1573: @cindex @file{-labels.i} output file
1.6 anton 1574:
1575: For threaded code we also need to produce a table containing the labels
1576: of all VM instructions. This is needed for VM code generation
1577: (@pxref{VM code generation}), and it has to be done in the engine
1578: function, because the labels are not visible outside. It then has to be
1579: passed outside the function (and assigned to @samp{vm_prim}), to be used
1580: by the VM code generation functions.
1581:
1582: This means that the engine function has to be called first to produce
1583: the VM instruction table, and later, after generating VM code, it has to
1584: be called again to execute the generated VM code (yes, this is ugly).
1585: In our example program, these two modes of calling the engine function
1586: are differentiated by the value of the parameter ip0 (if it equals 0,
1587: then the table is passed out, otherwise the VM code is executed); in our
1588: example, we pass the table out by assigning it to @samp{vm_prim} and
1589: returning from @samp{engine}.
1590:
1.12 anton 1591: In our example (@file{vmgen-ex/engine.c}), we also build such a table for
1592: switch dispatch; this is mainly done for uniformity.
1.6 anton 1593:
1594: For switch dispatch, we also need to define the VM instruction opcodes
1595: used as case labels in an @code{enum}.
1596:
1597: For both purposes (VM instruction table, and enum), the file
1.11 anton 1598: @file{@var{name}-labels.i} is generated by Vmgen. You have to define
1.6 anton 1599: the following macro used in this file:
1.5 anton 1600:
1.12 anton 1601: @table @code
1.5 anton 1602:
1.12 anton 1603: @findex INST_ADDR
1.5 anton 1604: @item INST_ADDR(@var{inst_name})
1605: For switch dispatch, this is just the name of the switch label (the same
1.6 anton 1606: name as used in @samp{LABEL(@var{inst_name})}), for both uses of
1607: @file{@var{name}-labels.i}. For threaded-code dispatch, this is the
1608: address of the label defined in @samp{LABEL(@var{inst_name})}); the
1.11 anton 1609: address is taken with @samp{&&} (@pxref{Labels as Values, , Labels as
1610: Values, gcc.info, GNU C Manual}).
1.5 anton 1611:
1612: @end table
1613:
1614:
1.10 anton 1615: @c --------------------------------------------------------------------
1616: @node VM code generation, Peephole optimization, VM instruction table, Using the generated code
1.6 anton 1617: @section VM code generation
1.12 anton 1618: @cindex VM code generation
1619: @cindex code generation, VM
1620: @cindex @file{-gen.i} output file
1.6 anton 1621:
1622: Vmgen generates VM code generation functions in @file{@var{name}-gen.i}
1623: that the front end can call to generate VM code. This is essential for
1624: an interpretive system.
1625:
1.12 anton 1626: @findex gen_@var{inst}
1.11 anton 1627: For a VM instruction @samp{x ( #a b #c -- d )}, Vmgen generates a
1.6 anton 1628: function with the prototype
1629:
1630: @example
1631: void gen_x(Inst **ctp, a_type a, c_type c)
1632: @end example
1633:
1634: The @code{ctp} argument points to a pointer to the next instruction.
1635: @code{*ctp} is increased by the generation functions; i.e., you should
1636: allocate memory for the code to be generated beforehand, and start with
1637: *ctp set at the start of this memory area. Before running out of
1638: memory, allocate a new area, and generate a VM-level jump to the new
1.12 anton 1639: area (this overflow handling is not implemented in our examples).
1.6 anton 1640:
1.12 anton 1641: @cindex immediate arguments, VM code generation
1.6 anton 1642: The other arguments correspond to the immediate arguments of the VM
1643: instruction (with their appropriate types as defined in the
1644: @code{type_prefix} declaration.
1645:
1646: The following types, variables, and functions are used in
1647: @file{@var{name}-gen.i}:
1648:
1.12 anton 1649: @table @code
1.6 anton 1650:
1.12 anton 1651: @findex Inst
1.6 anton 1652: @item Inst
1653: The type of the VM instruction; if you use threaded code, this is
1654: @code{void *}; for switch dispatch this is an integer type.
1655:
1.12 anton 1656: @cindex @code{vm_prim}, use
1.6 anton 1657: @item vm_prim
1658: The VM instruction table (type: @code{Inst *}, @pxref{VM instruction table}).
1659:
1.12 anton 1660: @findex gen_inst
1.6 anton 1661: @item gen_inst(Inst **ctp, Inst i)
1662: This function compiles the instruction @code{i}. Take a look at it in
1663: @file{vmgen-ex/peephole.c}. It is trivial when you don't want to use
1664: superinstructions (just the last two lines of the example function), and
1665: slightly more complicated in the example due to its ability to use
1666: superinstructions (@pxref{Peephole optimization}).
1667:
1.12 anton 1668: @findex genarg_@var{type_prefix}
1.6 anton 1669: @item genarg_@var{type_prefix}(Inst **ctp, @var{type} @var{type_prefix})
1670: This compiles an immediate argument of @var{type} (as defined in a
1671: @code{type-prefix} definition). These functions are trivial to define
1672: (see @file{vmgen-ex/support.c}). You need one of these functions for
1673: every type that you use as immediate argument.
1674:
1675: @end table
1676:
1.12 anton 1677: @findex BB_BOUNDARY
1.6 anton 1678: In addition to using these functions to generate code, you should call
1679: @code{BB_BOUNDARY} at every basic block entry point if you ever want to
1680: use superinstructions (or if you want to use the profiling supported by
1.12 anton 1681: Vmgen; but this support is also useful mainly for selecting
1682: superinstructions). If you use @code{BB_BOUNDARY}, you should also
1683: define it (take a look at its definition in @file{vmgen-ex/mini.y}).
1.6 anton 1684:
1685: You do not need to call @code{BB_BOUNDARY} after branches, because you
1686: will not define superinstructions that contain branches in the middle
1687: (and if you did, and it would work, there would be no reason to end the
1688: superinstruction at the branch), and because the branches announce
1689: themselves to the profiler.
1690:
1691:
1.10 anton 1692: @c --------------------------------------------------------------------
1693: @node Peephole optimization, VM disassembler, VM code generation, Using the generated code
1.6 anton 1694: @section Peephole optimization
1.12 anton 1695: @cindex peephole optimization
1696: @cindex superinstructions, generating
1697: @cindex @file{peephole.c}
1698: @cindex @file{-peephole.i} output file
1.6 anton 1699:
1700: You need peephole optimization only if you want to use
1701: superinstructions. But having the code for it does not hurt much if you
1702: do not use superinstructions.
1703:
1704: A simple greedy peephole optimization algorithm is used for
1705: superinstruction selection: every time @code{gen_inst} compiles a VM
1.12 anton 1706: instruction, it checks if it can combine it with the last VM instruction
1.6 anton 1707: (which may also be a superinstruction resulting from a previous peephole
1708: optimization); if so, it changes the last instruction to the combined
1709: instruction instead of laying down @code{i} at the current @samp{*ctp}.
1710:
1711: The code for peephole optimization is in @file{vmgen-ex/peephole.c}.
1712: You can use this file almost verbatim. Vmgen generates
1.26 anton 1713: @file{@var{file}-peephole.i} which contains data for the peephole
1.6 anton 1714: optimizer.
1715:
1.12 anton 1716: @findex init_peeptable
1.6 anton 1717: You have to call @samp{init_peeptable()} after initializing
1718: @samp{vm_prim}, and before compiling any VM code to initialize data
1719: structures for peephole optimization. After that, compiling with the VM
1720: code generation functions will automatically combine VM instructions
1721: into superinstructions. Since you do not want to combine instructions
1722: across VM branch targets (otherwise there will not be a proper VM
1723: instruction to branch to), you have to call @code{BB_BOUNDARY}
1724: (@pxref{VM code generation}) at branch targets.
1725:
1726:
1.10 anton 1727: @c --------------------------------------------------------------------
1728: @node VM disassembler, VM profiler, Peephole optimization, Using the generated code
1.6 anton 1729: @section VM disassembler
1.12 anton 1730: @cindex VM disassembler
1731: @cindex disassembler, VM code
1732: @cindex @file{disasm.c}
1733: @cindex @file{-disasm.i} output file
1.6 anton 1734:
1735: A VM code disassembler is optional for an interpretive system, but
1736: highly recommended during its development and maintenance, because it is
1737: very useful for detecting bugs in the front end (and for distinguishing
1738: them from VM interpreter bugs).
1739:
1740: Vmgen supports VM code disassembling by generating
1741: @file{@var{file}-disasm.i}. This code has to be wrapped into a
1.12 anton 1742: function, as is done in @file{vmgen-ex/disasm.c}. You can use this file
1.6 anton 1743: almost verbatim. In addition to @samp{vm_@var{A}2@var{B}(a,b)},
1744: @samp{vm_out}, @samp{printarg_@var{type}(@var{value})}, which are
1745: explained above, the following macros and variables are used in
1746: @file{@var{file}-disasm.i} (and you have to define them):
1747:
1.12 anton 1748: @table @code
1.6 anton 1749:
1750: @item ip
1751: This variable points to the opcode of the current VM instruction.
1752:
1.12 anton 1753: @cindex @code{IP}, @code{IPTOS} in disassmbler
1.6 anton 1754: @item IP IPTOS
1755: @samp{IPTOS} is the first argument of the current VM instruction, and
1756: @samp{IP} points to it; this is just as in the engine, but here
1757: @samp{ip} points to the opcode of the VM instruction (in contrast to the
1758: engine, where @samp{ip} points to the next cell, or even one further).
1759:
1.12 anton 1760: @findex VM_IS_INST
1.6 anton 1761: @item VM_IS_INST(Inst i, int n)
1762: Tests if the opcode @samp{i} is the same as the @samp{n}th entry in the
1763: VM instruction table.
1764:
1765: @end table
1766:
1767:
1.10 anton 1768: @c --------------------------------------------------------------------
1769: @node VM profiler, , VM disassembler, Using the generated code
1.7 anton 1770: @section VM profiler
1.12 anton 1771: @cindex VM profiler
1772: @cindex profiling for selecting superinstructions
1773: @cindex superinstructions and profiling
1774: @cindex @file{profile.c}
1775: @cindex @file{-profile.i} output file
1.7 anton 1776:
1777: The VM profiler is designed for getting execution and occurence counts
1778: for VM instruction sequences, and these counts can then be used for
1779: selecting sequences as superinstructions. The VM profiler is probably
1.8 anton 1780: not useful as profiling tool for the interpretive system. I.e., the VM
1.7 anton 1781: profiler is useful for the developers, but not the users of the
1.8 anton 1782: interpretive system.
1.7 anton 1783:
1.8 anton 1784: The output of the profiler is: for each basic block (executed at least
1785: once), it produces the dynamic execution count of that basic block and
1786: all its subsequences; e.g.,
1.7 anton 1787:
1.8 anton 1788: @example
1789: 9227465 lit storelocal
1790: 9227465 storelocal branch
1791: 9227465 lit storelocal branch
1792: @end example
1.7 anton 1793:
1.8 anton 1794: I.e., a basic block consisting of @samp{lit storelocal branch} is
1795: executed 9227465 times.
1.6 anton 1796:
1.12 anton 1797: @cindex @file{stat.awk}
1798: @cindex @file{seq2rule.awk}
1.8 anton 1799: This output can be combined in various ways. E.g.,
1.12 anton 1800: @file{vmgen-ex/stat.awk} adds up the occurences of a given sequence wrt
1.8 anton 1801: dynamic execution, static occurence, and per-program occurence. E.g.,
1.3 anton 1802:
1.8 anton 1803: @example
1804: 2 16 36910041 loadlocal lit
1805: @end example
1.2 anton 1806:
1.12 anton 1807: @noindent
1.8 anton 1808: indicates that the sequence @samp{loadlocal lit} occurs in 2 programs,
1809: in 16 places, and has been executed 36910041 times. Now you can select
1810: superinstructions in any way you like (note that compile time and space
1811: typically limit the number of superinstructions to 100--1000). After
1812: you have done that, @file{vmgen/seq2rule.awk} turns lines of the form
1.11 anton 1813: above into rules for inclusion in a Vmgen input file. Note that this
1.8 anton 1814: script does not ensure that all prefixes are defined, so you have to do
1815: that in other ways. So, an overall script for turning profiles into
1816: superinstructions can look like this:
1.2 anton 1817:
1.8 anton 1818: @example
1819: awk -f stat.awk fib.prof test.prof|
1820: awk '$3>=10000'| #select sequences
1821: fgrep -v -f peephole-blacklist| #eliminate wrong instructions
1822: awk -f seq2rule.awk| #turn into superinstructions
1823: sort -k 3 >mini-super.vmg #sort sequences
1824: @end example
1.2 anton 1825:
1.8 anton 1826: Here the dynamic count is used for selecting sequences (preliminary
1827: results indicate that the static count gives better results, though);
1.12 anton 1828: the third line eliminates sequences containing instructions that must not
1.8 anton 1829: occur in a superinstruction, because they access a stack directly. The
1830: dynamic count selection ensures that all subsequences (including
1831: prefixes) of longer sequences occur (because subsequences have at least
1832: the same count as the longer sequences); the sort in the last line
1833: ensures that longer superinstructions occur after their prefixes.
1834:
1.12 anton 1835: But before using this, you have to have the profiler. Vmgen supports its
1.8 anton 1836: creation by generating @file{@var{file}-profile.i}; you also need the
1837: wrapper file @file{vmgen-ex/profile.c} that you can use almost verbatim.
1838:
1.12 anton 1839: @cindex @code{SUPER_END} in profiling
1840: @cindex @code{BB_BOUNDARY} in profiling
1.8 anton 1841: The profiler works by recording the targets of all VM control flow
1842: changes (through @code{SUPER_END} during execution, and through
1843: @code{BB_BOUNDARY} in the front end), and counting (through
1844: @code{SUPER_END}) how often they were targeted. After the program run,
1845: the numbers are corrected such that each VM basic block has the correct
1.12 anton 1846: count (entering a block without executing a branch does not increase the
1847: count, and the correction fixes that), then the subsequences of all
1848: basic blocks are printed. To get all this, you just have to define
1849: @code{SUPER_END} (and @code{BB_BOUNDARY}) appropriately, and call
1850: @code{vm_print_profile(FILE *file)} when you want to output the profile
1851: on @code{file}.
1.8 anton 1852:
1.12 anton 1853: @cindex @code{VM_IS_INST} in profiling
1854: The @file{@var{file}-profile.i} is similar to the disassembler file, and
1.8 anton 1855: it uses variables and functions defined in @file{vmgen-ex/profile.c},
1856: plus @code{VM_IS_INST} already defined for the VM disassembler
1857: (@pxref{VM disassembler}).
1858:
1.13 anton 1859: @c **********************************************************
1860: @node Hints, The future, Using the generated code, Top
1861: @chapter Hints
1862: @cindex hints
1863:
1864: @menu
1865: * Floating point:: and stacks
1866: @end menu
1867:
1868: @c --------------------------------------------------------------------
1869: @node Floating point, , Hints, Hints
1870: @section Floating point
1871:
1872: How should you deal with floating point values? Should you use the same
1873: stack as for integers/pointers, or a different one? This section
1874: discusses this issue with a view on execution speed.
1875:
1876: The simpler approach is to use a separate floating-point stack. This
1877: allows you to choose FP value size without considering the size of the
1878: integers/pointers, and you avoid a number of performance problems. The
1879: main downside is that this needs an FP stack pointer (and that may not
1880: fit in the register file on the 386 arhitecture, costing some
1881: performance, but comparatively little if you take the other option into
1882: account). If you use a separate FP stack (with stack pointer @code{fp}),
1883: using an fpTOS is helpful on most machines, but some spill the fpTOS
1884: register into memory, and fpTOS should not be used there.
1885:
1886: The other approach is to share one stack (pointed to by, say, @code{sp})
1887: between integer/pointer and floating-point values. This is ok if you do
1888: not use @code{spTOS}. If you do use @code{spTOS}, the compiler has to
1889: decide whether to put that variable into an integer or a floating point
1890: register, and the other type of operation becomes quite expensive on
1891: most machines (because moving values between integer and FP registers is
1892: quite expensive). If a value of one type has to be synthesized out of
1893: two values of the other type (@code{double} types), things are even more
1894: interesting.
1895:
1896: One way around this problem would be to not use the @code{spTOS}
1897: supported by Vmgen, but to use explicit top-of-stack variables (one for
1898: integers, one for FP values), and having a kind of accumulator+stack
1899: architecture (e.g., Ocaml bytecode uses this approach); however, this is
1900: a major change, and it's ramifications are not completely clear.
1.10 anton 1901:
1902: @c **********************************************************
1.13 anton 1903: @node The future, Changes, Hints, Top
1904: @chapter The future
1905: @cindex future ideas
1906:
1.21 anton 1907: We have a number of ideas for future versions of Vmgen. However, there
1.13 anton 1908: are so many possible things to do that we would like some feedback from
1909: you. What are you doing with Vmgen, what features are you missing, and
1910: why?
1911:
1912: One idea we are thinking about is to generate just one @file{.c} file
1913: instead of letting you copy and adapt all the wrapper files (you would
1914: still have to define stuff like the type-specific macros, and stack
1915: pointers etc. somewhere). The advantage would be that, if we change the
1916: wrapper files between versions, you would not need to integrate your
1917: changes and our changes to them; Vmgen would also be easier to use for
1918: beginners. The main disadvantage of that is that it would reduce the
1919: flexibility of Vmgen a little (well, those who like flexibility could
1920: still patch the resulting @file{.c} file, like they are now doing for
1921: the wrapper files). In any case, if you are doing things to the wrapper
1922: files that would cause problems in a generated-@file{.c}-file approach,
1923: please let us know.
1924:
1925: @c **********************************************************
1926: @node Changes, Contact, The future, Top
1.8 anton 1927: @chapter Changes
1.12 anton 1928: @cindex Changes from old versions
1.8 anton 1929:
1.19 anton 1930: User-visible changes between 0.5.9-20020822 and 0.5.9-20020901:
1931:
1932: The store optimization is now disabled by default, but can be enabled by
1933: the user (@pxref{Store Optimization}). Documentation for this
1934: optimization is also new.
1935:
1936: User-visible changes between 0.5.9-20010501 and 0.5.9-20020822:
1.17 anton 1937:
1938: There is now a manual (in info, HTML, Postscript, or plain text format).
1939:
1940: There is the vmgen-ex2 variant of the vmgen-ex example; the new
1941: variant uses a union type instead of lots of casting.
1942:
1943: Both variants of the example can now be compiled with an ANSI C compiler
1944: (using switch dispatch and losing quite a bit of performance); tested
1945: with @command{lcc}.
1946:
1.11 anton 1947: Users of the gforth-0.5.9-20010501 version of Vmgen need to change
1.8 anton 1948: several things in their source code to use the current version. I
1949: recommend keeping the gforth-0.5.9-20010501 version until you have
1950: completed the change (note that you can have several versions of Gforth
1951: installed at the same time). I hope to avoid such incompatible changes
1952: in the future.
1.2 anton 1953:
1.8 anton 1954: The required changes are:
1955:
1956: @table @code
1.13 anton 1957:
1958: @cindex @code{TAIL;}, changes
1959: @item TAIL;
1960: has been renamed into @code{INST_TAIL;} (less chance of an accidental
1961: match).
1.2 anton 1962:
1.12 anton 1963: @cindex @code{vm_@var{A}2@var{B}}, changes
1.8 anton 1964: @item vm_@var{A}2@var{B}
1965: now takes two arguments.
1966:
1.12 anton 1967: @cindex @code{vm_two@var{A}2@var{B}}, changes
1.8 anton 1968: @item vm_two@var{A}2@var{B}(b,a1,a2);
1969: changed to vm_two@var{A}2@var{B}(a1,a2,b) (note the absence of the @samp{;}).
1970:
1971: @end table
1.2 anton 1972:
1.8 anton 1973: Also some new macros have to be defined, e.g., @code{INST_ADDR}, and
1974: @code{LABEL}; some macros have to be defined in new contexts, e.g.,
1975: @code{VM_IS_INST} is now also needed in the disassembler.
1.4 anton 1976:
1.12 anton 1977: @c *********************************************************
1.10 anton 1978: @node Contact, Copying This Manual, Changes, Top
1.8 anton 1979: @chapter Contact
1.17 anton 1980:
1981: To report a bug, use
1982: @url{https://savannah.gnu.org/bugs/?func=addbug&group_id=2672}.
1983:
1984: For discussion on Vmgen (e.g., how to use it), use the mailing list
1985: @email{bug-vmgen@@mail.freesoftware.fsf.org} (use
1986: @url{http://mail.gnu.org/mailman/listinfo/help-vmgen} to subscribe).
1987:
1988: You can find vmgen information at
1989: @url{http://www.complang.tuwien.ac.at/anton/vmgen/}.
1.4 anton 1990:
1.12 anton 1991: @c ***********************************************************
1.10 anton 1992: @node Copying This Manual, Index, Contact, Top
1993: @appendix Copying This Manual
1994:
1995: @menu
1996: * GNU Free Documentation License:: License for copying this manual.
1997: @end menu
1998:
1.30 ! anton 1999: @node GNU Free Documentation License, , Copying This Manual, Copying This Manual
! 2000: @appendixsec GNU Free Documentation License
1.10 anton 2001: @include fdl.texi
2002:
2003:
2004: @node Index, , Copying This Manual, Top
2005: @unnumbered Index
2006:
2007: @printindex cp
2008:
2009: @bye
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