Annotation of gforth/doc/vmgen.texi, revision 1.6
1.1 anton 1: @include version.texi
2:
3: @c @ifnottex
4: This file documents vmgen (Gforth @value{VERSION}).
5:
1.2 anton 6: @chapter Introduction
1.1 anton 7:
8: Vmgen is a tool for writing efficient interpreters. It takes a simple
9: virtual machine description and generates efficient C code for dealing
10: with the virtual machine code in various ways (in particular, executing
11: it). The run-time efficiency of the resulting interpreters is usually
12: within a factor of 10 of machine code produced by an optimizing
13: compiler.
14:
15: The interpreter design strategy supported by vmgen is to divide the
16: interpreter into two parts:
17:
18: @itemize @bullet
19:
20: @item The @emph{front end} takes the source code of the language to be
21: implemented, and translates it into virtual machine code. This is
22: similar to an ordinary compiler front end; typically an interpreter
23: front-end performs no optimization, so it is relatively simple to
24: implement and runs fast.
25:
26: @item The @emph{virtual machine interpreter} executes the virtual
27: machine code.
28:
29: @end itemize
30:
31: Such a division is usually used in interpreters, for modularity as well
1.6 ! anton 32: as for efficiency. The virtual machine code is typically passed between
! 33: front end and virtual machine interpreter in memory, like in a
1.1 anton 34: load-and-go compiler; this avoids the complexity and time cost of
35: writing the code to a file and reading it again.
36:
37: A @emph{virtual machine} (VM) represents the program as a sequence of
38: @emph{VM instructions}, following each other in memory, similar to real
39: machine code. Control flow occurs through VM branch instructions, like
40: in a real machine.
41:
42: In this setup, vmgen can generate most of the code dealing with virtual
43: machine instructions from a simple description of the virtual machine
44: instructions (@pxref...), in particular:
45:
46: @table @emph
47:
48: @item VM instruction execution
49:
50: @item VM code generation
51: Useful in the front end.
52:
53: @item VM code decompiler
54: Useful for debugging the front end.
55:
56: @item VM code tracing
57: Useful for debugging the front end and the VM interpreter. You will
58: typically provide other means for debugging the user's programs at the
59: source level.
60:
61: @item VM code profiling
62: Useful for optimizing the VM insterpreter with superinstructions
63: (@pxref...).
64:
65: @end table
66:
67: VMgen supports efficient interpreters though various optimizations, in
68: particular
69:
70: @itemize
71:
72: @item Threaded code
73:
74: @item Caching the top-of-stack in a register
75:
76: @item Combining VM instructions into superinstructions
77:
78: @item
79: Replicating VM (super)instructions for better BTB prediction accuracy
80: (not yet in vmgen-ex, but already in Gforth).
81:
82: @end itemize
83:
84: As a result, vmgen-based interpreters are only about an order of
85: magintude slower than native code from an optimizing C compiler on small
86: benchmarks; on large benchmarks, which spend more time in the run-time
1.2 anton 87: system, the slowdown is often less (e.g., the slowdown of a
88: Vmgen-generated JVM interpreter over the best JVM JIT compiler we
89: measured is only a factor of 2-3 for large benchmarks; some other JITs
90: and all other interpreters we looked at were slower than our
91: interpreter).
1.1 anton 92:
93: VMs are usually designed as stack machines (passing data between VM
94: instructions on a stack), and vmgen supports such designs especially
95: well; however, you can also use vmgen for implementing a register VM and
96: still benefit from most of the advantages offered by vmgen.
97:
1.2 anton 98: There are many potential uses of the instruction descriptions that are
99: not implemented at the moment, but we are open for feature requests, and
100: we will implement new features if someone asks for them; so the feature
101: list above is not exhaustive.
1.1 anton 102:
1.2 anton 103: @c *********************************************************************
104: @chapter Why interpreters?
105:
106: Interpreters are a popular language implementation technique because
107: they combine all three of the following advantages:
108:
109: @itemize
110:
111: @item Ease of implementation
112:
113: @item Portability
114:
115: @item Fast edit-compile-run cycle
116:
117: @end itemize
118:
119: The main disadvantage of interpreters is their run-time speed. However,
120: there are huge differences between different interpreters in this area:
121: the slowdown over optimized C code on programs consisting of simple
122: operations is typically a factor of 10 for the more efficient
123: interpreters, and a factor of 1000 for the less efficient ones (the
124: slowdown for programs executing complex operations is less, because the
125: time spent in libraries for executing complex operations is the same in
126: all implementation strategies).
127:
128: Vmgen makes it even easier to implement interpreters. It also supports
129: techniques for building efficient interpreters.
130:
131: @c ********************************************************************
132: @chapter Concepts
133:
134: @c --------------------------------------------------------------------
135: @section Front-end and virtual machine interpreter
136:
137: @cindex front-end
138: Interpretive systems are typically divided into a @emph{front end} that
139: parses the input language and produces an intermediate representation
140: for the program, and an interpreter that executes the intermediate
141: representation of the program.
142:
143: @cindex virtual machine
144: @cindex VM
145: @cindex instruction, VM
146: For efficient interpreters the intermediate representation of choice is
147: virtual machine code (rather than, e.g., an abstract syntax tree).
148: @emph{Virtual machine} (VM) code consists of VM instructions arranged
149: sequentially in memory; they are executed in sequence by the VM
150: interpreter, except for VM branch instructions, which implement control
151: structures. The conceptual similarity to real machine code results in
152: the name @emph{virtual machine}.
153:
154: In this framework, vmgen supports building the VM interpreter and any
155: other component dealing with VM instructions. It does not have any
156: support for the front end, apart from VM code generation support. The
157: front end can be implemented with classical compiler front-end
1.3 anton 158: techniques, supported by tools like @command{flex} and @command{bison}.
1.2 anton 159:
160: The intermediate representation is usually just internal to the
161: interpreter, but some systems also support saving it to a file, either
162: as an image file, or in a full-blown linkable file format (e.g., JVM).
163: Vmgen currently has no special support for such features, but the
164: information in the instruction descriptions can be helpful, and we are
165: open for feature requests and suggestions.
1.3 anton 166:
167: @section Data handling
168:
169: @cindex stack machine
170: @cindex register machine
171: Most VMs use one or more stacks for passing temporary data between VM
172: instructions. Another option is to use a register machine architecture
173: for the virtual machine; however, this option is either slower or
174: significantly more complex to implement than a stack machine architecture.
175:
176: Vmgen has special support and optimizations for stack VMs, making their
177: implementation easy and efficient.
178:
179: You can also implement a register VM with vmgen (@pxref{Register
180: Machines}), and you will still profit from most vmgen features.
181:
182: @cindex stack item size
183: @cindex size, stack items
184: Stack items all have the same size, so they typically will be as wide as
185: an integer, pointer, or floating-point value. Vmgen supports treating
186: two consecutive stack items as a single value, but anything larger is
187: best kept in some other memory area (e.g., the heap), with pointers to
188: the data on the stack.
189:
190: @cindex instruction stream
191: @cindex immediate arguments
192: Another source of data is immediate arguments VM instructions (in the VM
193: instruction stream). The VM instruction stream is handled similar to a
194: stack in vmgen.
195:
196: @cindex garbage collection
197: @cindex reference counting
198: Vmgen has no built-in support for nor restrictions against @emph{garbage
199: collection}. If you need garbage collection, you need to provide it in
200: your run-time libraries. Using @emph{reference counting} is probably
201: harder, but might be possible (contact us if you are interested).
202: @c reference counting might be possible by including counting code in
203: @c the conversion macros.
204:
1.6 ! anton 205: @section Dispatch
! 206:
! 207: Understanding this section is probably not necessary for using vmgen,
! 208: but it may help. You may want to skip it now, and read it if you find statements about dispatch methods confusing.
! 209:
! 210: After executing one VM instruction, the VM interpreter has to dispatch
! 211: the next VM instruction (vmgen calls the dispatch routine @samp{NEXT}).
! 212: Vmgen supports two methods of dispatch:
! 213:
! 214: @table
! 215:
! 216: @item switch dispatch
! 217: In this method the VM interpreter contains a giant @code{switch}
! 218: statement, with one @code{case} for each VM instruction. The VM
! 219: instructions are represented by integers (e.g., produced by an
! 220: @code{enum}) in the VM code, and dipatch occurs by loading the next
! 221: integer from the VM code, @code{switch}ing on it, and continuing at the
! 222: appropriate @code{case}; after executing the VM instruction, jump back
! 223: to the dispatch code.
! 224:
! 225: @item threaded code
! 226: This method represents a VM instruction in the VM code by the address of
! 227: the start of the machine code fragment for executing the VM instruction.
! 228: Dispatch consists of loading this address, jumping to it, and
! 229: incrementing the VM instruction pointer. Typically the threaded-code
! 230: dispatch code is appended directly to the code for executing the VM
! 231: instruction. Threaded code cannot be implemented in ANSI C, but it can
! 232: be implemented using GNU C's labels-as-values extension (@pxref{labels
! 233: as values}).
! 234:
! 235: @end table
! 236:
1.3 anton 237: @c *************************************************************
238: @chapter Invoking vmgen
239:
240: The usual way to invoke vmgen is as follows:
241:
242: @example
243: vmgen @var{infile}
244: @end example
245:
246: Here @var{infile} is the VM instruction description file, which usually
247: ends in @file{.vmg}. The output filenames are made by taking the
248: basename of @file{infile} (i.e., the output files will be created in the
249: current working directory) and replacing @file{.vmg} with @file{-vm.i},
250: @file{-disasm.i}, @file{-gen.i}, @file{-labels.i}, @file{-profile.i},
251: and @file{-peephole.i}. E.g., @command{bison hack/foo.vmg} will create
252: @file{foo-vm.i} etc.
253:
254: The command-line options supported by vmgen are
255:
256: @table @option
257:
258: @cindex -h, command-line option
259: @cindex --help, command-line option
260: @item --help
261: @itemx -h
262: Print a message about the command-line options
263:
264: @cindex -v, command-line option
265: @cindex --version, command-line option
266: @item --version
267: @itemx -v
268: Print version and exit
269: @end table
270:
271: @c env vars GFORTHDIR GFORTHDATADIR
272:
1.5 anton 273: @c ****************************************************************
274: @chapter Example
275:
276: @section Example overview
277:
278: There are two versions of the same example for using vmgen:
279: @file{vmgen-ex} and @file{vmgen-ex2} (you can also see Gforth as
280: example, but it uses additional (undocumented) features, and also
281: differs in some other respects). The example implements @emph{mini}, a
282: tiny Modula-2-like language with a small JavaVM-like virtual machine.
283: The difference between the examples is that @file{vmgen-ex} uses many
284: casts, and @file{vmgen-ex2} tries to avoids most casts and uses unions
285: instead.
286:
287: The files provided with each example are:
288:
289: @example
290: Makefile
291: README
292: disasm.c wrapper file
293: engine.c wrapper file
294: peephole.c wrapper file
295: profile.c wrapper file
296: mini-inst.vmg simple VM instructions
297: mini-super.vmg superinstructions (empty at first)
298: mini.h common declarations
299: mini.l scanner
300: mini.y front end (parser, VM code generator)
301: support.c main() and other support functions
302: fib.mini example mini program
303: simple.mini example mini program
304: test.mini example mini program (tests everything)
305: test.out test.mini output
306: stat.awk script for aggregating profile information
307: peephole-blacklist list of instructions not allowed in superinstructions
308: seq2rule.awk script for creating superinstructions
309: @end example
310:
311: For your own interpreter, you would typically copy the following files
312: and change little, if anything:
313:
314: @example
315: disasm.c wrapper file
316: engine.c wrapper file
317: peephole.c wrapper file
318: profile.c wrapper file
319: stat.awk script for aggregating profile information
320: seq2rule.awk script for creating superinstructions
321: @end example
322:
323: You would typically change much in or replace the following files:
324:
325: @example
326: Makefile
327: mini-inst.vmg simple VM instructions
328: mini.h common declarations
329: mini.l scanner
330: mini.y front end (parser, VM code generator)
331: support.c main() and other support functions
332: peephole-blacklist list of instructions not allowed in superinstructions
333: @end example
334:
335: You can build the example by @code{cd}ing into the example's directory,
336: and then typing @samp{make}; you can check that it works with @samp{make
337: check}. You can run run mini programs like this:
338:
339: @example
340: ./mini fib.mini
341: @end example
342:
343: To learn about the options, type @samp{./mini -h}.
344:
345: @section Using profiling to create superinstructions
346:
347: I have not added rules for this in the @file{Makefile} (there are many
348: options for selecting superinstructions, and I did not want to hardcode
349: one into the @file{Makefile}), but there are some supporting scripts, and
350: here's an example:
351:
352: Suppose you want to use @file{fib.mini} and @file{test.mini} as training
353: programs, you get the profiles like this:
354:
355: @example
356: make fib.prof test.prof #takes a few seconds
357: @end example
358:
359: You can aggregate these profiles with @file{stat.awk}:
360:
361: @example
362: awk -f stat.awk fib.prof test.prof
363: @end example
364:
365: The result contains lines like:
366:
367: @example
368: 2 16 36910041 loadlocal lit
369: @end example
370:
371: This means that the sequence @code{loadlocal lit} statically occurs a
372: total of 16 times in 2 profiles, with a dynamic execution count of
373: 36910041.
374:
375: The numbers can be used in various ways to select superinstructions.
376: E.g., if you just want to select all sequences with a dynamic
377: execution count exceeding 10000, you would use the following pipeline:
378:
379: @example
380: awk -f stat.awk fib.prof test.prof|
381: awk '$3>=10000'| #select sequences
382: fgrep -v -f peephole-blacklist| #eliminate wrong instructions
383: awk -f seq2rule.awk| #transform sequences into superinstruction rules
384: sort -k 3 >mini-super.vmg #sort sequences
385: @end example
386:
387: The file @file{peephole-blacklist} contains all instructions that
388: directly access a stack or stack pointer (for mini: @code{call},
389: @code{return}); the sort step is necessary to ensure that prefixes
390: preceed larger superinstructions.
391:
392: Now you can create a version of mini with superinstructions by just
393: saying @samp{make}
394:
1.3 anton 395: @c ***************************************************************
396: @chapter Input File Format
397:
398: Vmgen takes as input a file containing specifications of virtual machine
399: instructions. This file usually has a name ending in @file{.vmg}.
400:
1.5 anton 401: Most examples are taken from the example in @file{vmgen-ex}.
1.3 anton 402:
403: @section Input File Grammar
404:
405: The grammar is in EBNF format, with @code{@var{a}|@var{b}} meaning
406: ``@var{a} or @var{b}'', @code{@{@var{c}@}} meaning 0 or more repetitions
407: of @var{c} and @code{[@var{d}]} meaning 0 or 1 repetitions of @var{d}.
408:
409: Vmgen input is not free-format, so you have to take care where you put
410: spaces and especially newlines; it's not as bad as makefiles, though:
411: any sequence of spaces and tabs is equivalent to a single space.
412:
413: @example
414: description: {instruction|comment|eval-escape}
415:
416: instruction: simple-inst|superinst
417:
418: simple-inst: ident " (" stack-effect " )" newline c-code newline newline
419:
420: stack-effect: {ident} " --" {ident}
421:
422: super-inst: ident " =" ident {ident}
423:
424: comment: "\ " text newline
425:
426: eval-escape: "\e " text newline
427: @end example
428: @c \+ \- \g \f \c
429:
430: Note that the @code{\}s in this grammar are meant literally, not as
1.5 anton 431: C-style encodings for non-printable characters.
1.3 anton 432:
433: The C code in @code{simple-inst} must not contain empty lines (because
434: vmgen would mistake that as the end of the simple-inst. The text in
435: @code{comment} and @code{eval-escape} must not contain a newline.
436: @code{Ident} must conform to the usual conventions of C identifiers
437: (otherwise the C compiler would choke on the vmgen output).
438:
439: Vmgen understands a few extensions beyond the grammar given here, but
440: these extensions are only useful for building Gforth. You can find a
441: description of the format used for Gforth in @file{prim}.
442:
443: @subsection
444: @c woanders?
445: The text in @code{eval-escape} is Forth code that is evaluated when
446: vmgen reads the line. If you do not know (and do not want to learn)
447: Forth, you can build the text according to the following grammar; these
448: rules are normally all Forth you need for using vmgen:
449:
450: @example
451: text: stack-decl|type-prefix-decl|stack-prefix-decl
452:
453: stack-decl: "stack " ident ident ident
454: type-prefix-decl:
455: 's" ' string '" ' ("single"|"double") ident "type-prefix" ident
456: stack-prefix-decl: ident "stack-prefix" string
457: @end example
458:
459: Note that the syntax of this code is not checked thoroughly (there are
460: many other Forth program fragments that could be written there).
461:
462: If you know Forth, the stack effects of the non-standard words involved
463: are:
464:
465: @example
466: stack ( "name" "pointer" "type" -- )
467: ( name execution: -- stack )
468: type-prefix ( addr u xt1 xt2 n stack "prefix" -- )
469: single ( -- xt1 xt2 n )
470: double ( -- xt1 xt2 n )
471: stack-prefix ( stack "prefix" -- )
472: @end example
473:
1.5 anton 474:
1.3 anton 475: @section Simple instructions
476:
477: We will use the following simple VM instruction description as example:
478:
479: @example
480: sub ( i1 i2 -- i )
481: i = i1-i2;
482: @end example
483:
484: The first line specifies the name of the VM instruction (@code{sub}) and
485: its stack effect (@code{i1 i2 -- i}). The rest of the description is
486: just plain C code.
487:
488: @cindex stack effect
489: The stack effect specifies that @code{sub} pulls two integers from the
1.5 anton 490: data stack and puts them in the C variables @code{i1} and @code{i2} (with
1.3 anton 491: the rightmost item (@code{i2}) taken from the top of stack) and later
492: pushes one integer (@code{i)) on the data stack (the rightmost item is
493: on the top afterwards).
494:
495: How do we know the type and stack of the stack items? Vmgen uses
496: prefixes, similar to Fortran; in contrast to Fortran, you have to
497: define the prefix first:
498:
499: @example
500: \E s" Cell" single data-stack type-prefix i
501: @end example
502:
503: This defines the prefix @code{i} to refer to the type @code{Cell}
504: (defined as @code{long} in @file{mini.h}) and, by default, to the
505: @code{data-stack}. It also specifies that this type takes one stack
506: item (@code{single}). The type prefix is part of the variable name.
507:
508: Before we can use @code{data-stack} in this way, we have to define it:
509:
510: @example
511: \E stack data-stack sp Cell
512: @end example
513: @c !! use something other than Cell
514:
515: This line defines the stack @code{data-stack}, which uses the stack
516: pointer @code{sp}, and each item has the basic type @code{Cell}; other
517: types have to fit into one or two @code{Cell}s (depending on whether the
518: type is @code{single} or @code{double} wide), and are converted from and
519: to Cells on accessing the @code{data-stack) with conversion macros
520: (@pxref{Conversion macros}). Stacks grow towards lower addresses in
1.5 anton 521: vmgen-erated interpreters.
1.3 anton 522:
523: We can override the default stack of a stack item by using a stack
524: prefix. E.g., consider the following instruction:
525:
526: @example
527: lit ( #i -- i )
528: @end example
529:
530: The VM instruction @code{lit} takes the item @code{i} from the
1.5 anton 531: instruction stream (indicated by the prefix @code{#}), and pushes it on
1.3 anton 532: the (default) data stack. The stack prefix is not part of the variable
533: name. Stack prefixes are defined like this:
534:
535: @example
536: \E inst-stream stack-prefix #
537: @end example
538:
1.5 anton 539: This definition defines that the stack prefix @code{#} specifies the
1.3 anton 540: ``stack'' @code{inst-stream}. Since the instruction stream behaves a
541: little differently than an ordinary stack, it is predefined, and you do
542: not need to define it.
543:
544: The instruction stream contains instructions and their immediate
545: arguments, so specifying that an argument comes from the instruction
546: stream indicates an immediate argument. Of course, instruction stream
547: arguments can only appear to the left of @code{--} in the stack effect.
548: If there are multiple instruction stream arguments, the leftmost is the
549: first one (just as the intuition suggests).
550:
1.5 anton 551: @subsubsection C Code Macros
552:
553: Vmgen recognizes the following strings in the C code part of simple
554: instructions:
555:
556: @table @samp
557:
558: @item SET_IP
559: As far as vmgen is concerned, a VM instruction containing this ends a VM
560: basic block (used in profiling to delimit profiled sequences). On the C
561: level, this also sets the instruction pointer.
562:
563: @item SUPER_END
564: This ends a basic block (for profiling), without a SET_IP.
565:
566: @item TAIL;
567: Vmgen replaces @samp{TAIL;} with code for ending a VM instruction and
568: dispatching the next VM instruction. This happens automatically when
569: control reaches the end of the C code. If you want to have this in the
570: middle of the C code, you need to use @samp{TAIL;}. A typical example
571: is a conditional VM branch:
572:
573: @example
574: if (branch_condition) {
575: SET_IP(target); TAIL;
576: }
577: /* implicit tail follows here */
578: @end example
579:
580: In this example, @samp{TAIL;} is not strictly necessary, because there
581: is another one implicitly after the if-statement, but using it improves
582: branch prediction accuracy slightly and allows other optimizations.
583:
584: @item SUPER_CONTINUE
585: This indicates that the implicit tail at the end of the VM instruction
586: dispatches the sequentially next VM instruction even if there is a
587: @code{SET_IP} in the VM instruction. This enables an optimization that
588: is not yet implemented in the vmgen-ex code (but in Gforth). The
589: typical application is in conditional VM branches:
590:
591: @example
592: if (branch_condition) {
593: SET_IP(target); TAIL; /* now this TAIL is necessary */
594: }
595: SUPER_CONTINUE;
596: @end example
597:
598: @end table
599:
600: Note that vmgen is not smart about C-level tokenization, comments,
601: strings, or conditional compilation, so it will interpret even a
602: commented-out SUPER_END as ending a basic block (or, e.g.,
603: @samp{RETAIL;} as @samp{TAIL;}). Conversely, vmgen requires the literal
604: presence of these strings; vmgen will not see them if they are hiding in
605: a C preprocessor macro.
606:
607:
608: @subsubsection C Code restrictions
609:
610: Vmgen generates code and performs some optimizations under the
611: assumption that the user-supplied C code does not access the stack
612: pointers or stack items, and that accesses to the instruction pointer
613: only occur through special macros. In general you should heed these
614: restrictions. However, if you need to break these restrictions, read
615: the following.
616:
617: Accessing a stack or stack pointer directly can be a problem for several
618: reasons:
619:
620: @itemize
621:
622: @item
623: You may cache the top-of-stack item in a local variable (that is
624: allocated to a register). This is the most frequent source of trouble.
625: You can deal with it either by not using top-of-stack caching (slowdown
626: factor 1-1.4, depending on machine), or by inserting flushing code
627: (e.g., @samp{IF_spTOS(sp[...] = spTOS);}) at the start and reloading
628: code (e.g., @samp{IF_spTOS(spTOS = sp[0])}) at the end of problematic C
629: code. Vmgen inserts a stack pointer update before the start of the
630: user-supplied C code, so the flushing code has to use an index that
631: corrects for that. In the future, this flushing may be done
632: automatically by mentioning a special string in the C code.
633: @c sometimes flushing and/or reloading unnecessary
634:
635: @item
636: The vmgen-erated code loads the stack items from stack-pointer-indexed
637: memory into variables before the user-supplied C code, and stores them
638: from variables to stack-pointer-indexed memory afterwards. If you do
639: any writes to the stack through its stack pointer in your C code, it
640: will not affact the variables, and your write may be overwritten by the
641: stores after the C code. Similarly, a read from a stack using a stack
642: pointer will not reflect computations of stack items in the same VM
643: instruction.
644:
645: @item
646: Superinstructions keep stack items in variables across the whole
647: superinstruction. So you should not include VM instructions, that
648: access a stack or stack pointer, as components of superinstructions.
649:
650: @end itemize
651:
652: You should access the instruction pointer only through its special
653: macros (@samp{IP}, @samp{SET_IP}, @samp{IPTOS}); this ensure that these
654: macros can be implemented in several ways for best performance.
655: @samp{IP} points to the next instruction, and @samp{IPTOS} is its
656: contents.
657:
658:
1.3 anton 659: @section Superinstructions
1.5 anton 660:
661: Here is an example of a superinstruction definition:
662:
663: @example
664: lit_sub = lit sub
665: @end example
666:
667: @code{lit_sub} is the name of the superinstruction, and @code{lit} and
668: @code{sub} are its components. This superinstruction performs the same
669: action as the sequence @code{lit} and @code{sub}. It is generated
670: automatically by the VM code generation functions whenever that sequence
671: occurs, so you only need to add this definition if you want to use this
672: superinstruction (and even that can be partially automatized,
673: @pxref{...}).
674:
675: Vmgen requires that the component instructions are simple instructions
676: defined before superinstructions using the components. Currently, vmgen
677: also requires that all the subsequences at the start of a
678: superinstruction (prefixes) must be defined as superinstruction before
679: the superinstruction. I.e., if you want to define a superinstruction
680:
681: @example
682: sumof5 = add add add add
683: @end example
684:
685: you first have to define
686:
687: @example
688: add ( n1 n2 -- n )
689: n = n1+n2;
690:
691: sumof3 = add add
692: sumof4 = add add add
693: @end example
694:
695: Here, @code{sumof4} is the longest prefix of @code{sumof5}, and @code{sumof3}
696: is the longest prefix of @code{sumof4}.
697:
698: Note that vmgen assumes that only the code it generates accesses stack
699: pointers, the instruction pointer, and various stack items, and it
700: performs optimizations based on this assumption. Therefore, VM
701: instructions that change the instruction pointer should only be used as
702: last component; a VM instruction that accesses a stack pointer should
703: not be used as component at all. Vmgen does not check these
704: restrictions, they just result in bugs in your interpreter.
705:
706: @c ********************************************************************
707: @chapter Using the generated code
708:
709: The easiest way to create a working VM interpreter with vmgen is
710: probably to start with one of the examples, and modify it for your
711: purposes. This chapter is just the reference manual for the macros
712: etc. used by the generated code, and the other context expected by the
713: generated code, and what you can do with the various generated files.
714:
1.6 ! anton 715:
1.5 anton 716: @section VM engine
717:
718: The VM engine is the VM interpreter that executes the VM code. It is
719: essential for an interpretive system.
720:
1.6 ! anton 721: Vmgen supports two methods of VM instruction dispatch: @emph{threaded
! 722: code} (fast, but gcc-specific), and @emph{switch dispatch} (slow, but
! 723: portable across C compilers); you can use conditional compilation
! 724: (@samp{defined(__GNUC__)}) to choose between these methods, and our
! 725: example does so.
! 726:
! 727: For both methods, the VM engine is contained in a C-level function.
! 728: Vmgen generates most of the contents of the function for you
! 729: (@file{@var{name}-vm.i}), but you have to define this function, and
! 730: macros and variables used in the engine, and initialize the variables.
! 731: In our example the engine function also includes
! 732: @file{@var{name}-labels.i} (@pxref{VM instruction table}).
! 733:
! 734: The following macros and variables are used in @file{@var{name}-vm.i}:
1.5 anton 735:
736: @table @code
737:
738: @item LABEL(@var{inst_name})
739: This is used just before each VM instruction to provide a jump or
740: @code{switch} label (the @samp{:} is provided by vmgen). For switch
741: dispatch this should expand to @samp{case @var{label}}; for
742: threaded-code dispatch this should just expand to @samp{case
743: @var{label}}. In either case @var{label} is usually the @var{inst_name}
744: with some prefix or suffix to avoid naming conflicts.
745:
746: @item NAME(@var{inst_name_string})
747: Called on entering a VM instruction with a string containing the name of
748: the VM instruction as parameter. In normal execution this should be a
749: noop, but for tracing this usually prints the name, and possibly other
750: information (several VM registers in our example).
751:
752: @item DEF_CA
753: Usually empty. Called just inside a new scope at the start of a VM
754: instruction. Can be used to define variables that should be visible
755: during every VM instruction. If you define this macro as non-empty, you
756: have to provide the finishing @samp{;} in the macro.
757:
758: @item NEXT_P0 NEXT_P1 NEXT_P2
759: The three parts of instruction dispatch. They can be defined in
760: different ways for best performance on various processors (see
761: @file{engine.c} in the example or @file{engine/threaded.h} in Gforth).
762: @samp{NEXT_P0} is invoked right at the start of the VM isntruction (but
763: after @samp{DEF_CA}), @samp{NEXT_P1} right after the user-supplied C
764: code, and @samp{NEXT_P2} at the end. The actual jump has to be
765: performed by @samp{NEXT_P2}.
766:
767: The simplest variant is if @samp{NEXT_P2} does everything and the other
768: macros do nothing. Then also related macros like @samp{IP},
769: @samp{SET_IP}, @samp{IP}, @samp{INC_IP} and @samp{IPTOS} are very
770: straightforward to define. For switch dispatch this code consists just
771: of a jump to the dispatch code (@samp{goto next_inst;} in our example;
772: for direct threaded code it consists of something like
773: @samp{({cfa=*ip++; goto *cfa;})}.
774:
775: Pulling code (usually the @samp{cfa=*ip;}) up into @samp{NEXT_P1}
776: usually does not cause problems, but pulling things up into
777: @samp{NEXT_P0} usually requires changing the other macros (and, at least
778: for Gforth on Alpha, it does not buy much, because the compiler often
779: manages to schedule the relevant stuff up by itself). An even more
780: extreme variant is to pull code up even further, into, e.g., NEXT_P1 of
781: the previous VM instruction (prefetching, useful on PowerPCs).
782:
783: @item INC_IP(@var{n})
784: This increments IP by @var{n}.
785:
786: @item vm_@var{A}2@var{B}(a,b)
787: Type casting macro that assigns @samp{a} (of type @var{A}) to @samp{b}
788: (of type @var{B}). This is mainly used for getting stack items into
789: variables and back. So you need to define macros for every combination
790: of stack basic type (@code{Cell} in our example) and type-prefix types
791: used with that stack (in both directions). For the type-prefix type,
792: you use the type-prefix (not the C type string) as type name (e.g.,
793: @samp{vm_Cell2i}, not @samp{vm_Cell2Cell}). In addition, you have to
794: define a vm_@var{X}2@var{X} macro for the stack basic type (used in
795: superinstructions).
796:
797: The stack basic type for the predefined @samp{inst-stream} is
798: @samp{Cell}. If you want a stack with the same item size, making its
799: basic type @samp{Cell} usually reduces the number of macros you have to
800: define.
801:
802: Here our examples differ a lot: @file{vmgen-ex} uses casts in these
803: macros, whereas @file{vmgen-ex2} uses union-field selection (or
804: assignment to union fields).
805:
806: @item vm_two@var{A}2@var{B}(a1,a2,b)
807: @item vm_@var{B}2two@var{A}(b,a1,a2)
808: Conversions between two stack items (@code{a1}, @code{a2}) and a
809: variable @code{b} of a type that takes two stack items. This does not
810: occur in our small examples, but you can look at Gforth for examples.
811:
812: @item @var{stackpointer}
813: For each stack used, the stackpointer name given in the stack
814: declaration is used. For a regular stack this must be an l-expression;
815: typically it is a variable declared as a pointer to the stack's basic
816: type. For @samp{inst-stream}, the name is @samp{IP}, and it can be a
817: plain r-value; typically it is a macro that abstracts away the
818: differences between the various implementations of NEXT_P*.
819:
820: @item @var{stackpointer}TOS
821: The top-of-stack for the stack pointed to by @var{stackpointer}. If you
822: are using top-of-stack caching for that stack, this should be defined as
823: variable; if you are not using top-of-stack caching for that stack, this
824: should be a macro expanding to @samp{@var{stackpointer}[0]}. The stack
825: pointer for the predefined @samp{inst-stream} is called @samp{IP}, so
826: the top-of-stack is called @samp{IPTOS}.
827:
828: @item IF_@var{stackpointer}TOS(@var{expr})
829: Macro for executing @var{expr}, if top-of-stack caching is used for the
830: @var{stackpointer} stack. I.e., this should do @var{expr} if there is
831: top-of-stack caching for @var{stackpointer}; otherwise it should do
832: nothing.
833:
834: @item VM_DEBUG
835: If this is defined, the tracing code will be compiled in (slower
836: interpretation, but better debugging). Our example compiles two
837: versions of the engine, a fast-running one that cannot trace, and one
838: with potential tracing and profiling.
839:
840: @item vm_debug
841: Needed only if @samp{VM_DEBUG} is defined. If this variable contains
842: true, the VM instructions produce trace output. It can be turned on or
843: off at any time.
844:
845: @item vm_out
846: Needed only if @samp{VM_DEBUG} is defined. Specifies the file on which
847: to print the trace output (type @samp{FILE *}).
848:
849: @item printarg_@var{type}(@var{value})
850: Needed only if @samp{VM_DEBUG} is defined. Macro or function for
851: printing @var{value} in a way appropriate for the @var{type}. This is
852: used for printing the values of stack items during tracing. @var{Type}
853: is normally the type prefix specified in a @code{type-prefix} definition
854: (e.g., @samp{printarg_i}); in superinstructions it is currently the
855: basic type of the stack.
856:
857: @end table
858:
1.6 ! anton 859:
! 860: @section{VM instruction table}
! 861:
! 862: For threaded code we also need to produce a table containing the labels
! 863: of all VM instructions. This is needed for VM code generation
! 864: (@pxref{VM code generation}), and it has to be done in the engine
! 865: function, because the labels are not visible outside. It then has to be
! 866: passed outside the function (and assigned to @samp{vm_prim}), to be used
! 867: by the VM code generation functions.
! 868:
! 869: This means that the engine function has to be called first to produce
! 870: the VM instruction table, and later, after generating VM code, it has to
! 871: be called again to execute the generated VM code (yes, this is ugly).
! 872: In our example program, these two modes of calling the engine function
! 873: are differentiated by the value of the parameter ip0 (if it equals 0,
! 874: then the table is passed out, otherwise the VM code is executed); in our
! 875: example, we pass the table out by assigning it to @samp{vm_prim} and
! 876: returning from @samp{engine}.
! 877:
! 878: In our example, we also build such a table for switch dispatch; this is
! 879: mainly done for uniformity.
! 880:
! 881: For switch dispatch, we also need to define the VM instruction opcodes
! 882: used as case labels in an @code{enum}.
! 883:
! 884: For both purposes (VM instruction table, and enum), the file
! 885: @file{@var{name}-labels.i} is generated by vmgen. You have to define
! 886: the following macro used in this file:
1.5 anton 887:
888: @table @samp
889:
890: @item INST_ADDR(@var{inst_name})
891: For switch dispatch, this is just the name of the switch label (the same
1.6 ! anton 892: name as used in @samp{LABEL(@var{inst_name})}), for both uses of
! 893: @file{@var{name}-labels.i}. For threaded-code dispatch, this is the
! 894: address of the label defined in @samp{LABEL(@var{inst_name})}); the
! 895: address is taken with @samp{&&} (@pxref{labels-as-values}).
1.5 anton 896:
897: @end table
898:
899:
1.6 ! anton 900: @section VM code generation
! 901:
! 902: Vmgen generates VM code generation functions in @file{@var{name}-gen.i}
! 903: that the front end can call to generate VM code. This is essential for
! 904: an interpretive system.
! 905:
! 906: For a VM instruction @samp{x ( #a b #c -- d )}, vmgen generates a
! 907: function with the prototype
! 908:
! 909: @example
! 910: void gen_x(Inst **ctp, a_type a, c_type c)
! 911: @end example
! 912:
! 913: The @code{ctp} argument points to a pointer to the next instruction.
! 914: @code{*ctp} is increased by the generation functions; i.e., you should
! 915: allocate memory for the code to be generated beforehand, and start with
! 916: *ctp set at the start of this memory area. Before running out of
! 917: memory, allocate a new area, and generate a VM-level jump to the new
! 918: area (this is not implemented in our examples).
! 919:
! 920: The other arguments correspond to the immediate arguments of the VM
! 921: instruction (with their appropriate types as defined in the
! 922: @code{type_prefix} declaration.
! 923:
! 924: The following types, variables, and functions are used in
! 925: @file{@var{name}-gen.i}:
! 926:
! 927: @table @samp
! 928:
! 929: @item Inst
! 930: The type of the VM instruction; if you use threaded code, this is
! 931: @code{void *}; for switch dispatch this is an integer type.
! 932:
! 933: @item vm_prim
! 934: The VM instruction table (type: @code{Inst *}, @pxref{VM instruction table}).
! 935:
! 936: @item gen_inst(Inst **ctp, Inst i)
! 937: This function compiles the instruction @code{i}. Take a look at it in
! 938: @file{vmgen-ex/peephole.c}. It is trivial when you don't want to use
! 939: superinstructions (just the last two lines of the example function), and
! 940: slightly more complicated in the example due to its ability to use
! 941: superinstructions (@pxref{Peephole optimization}).
! 942:
! 943: @item genarg_@var{type_prefix}(Inst **ctp, @var{type} @var{type_prefix})
! 944: This compiles an immediate argument of @var{type} (as defined in a
! 945: @code{type-prefix} definition). These functions are trivial to define
! 946: (see @file{vmgen-ex/support.c}). You need one of these functions for
! 947: every type that you use as immediate argument.
! 948:
! 949: @end table
! 950:
! 951: In addition to using these functions to generate code, you should call
! 952: @code{BB_BOUNDARY} at every basic block entry point if you ever want to
! 953: use superinstructions (or if you want to use the profiling supported by
! 954: vmgen; however, this is mainly useful for selecting superinstructions).
! 955: If you use @code{BB_BOUNDARY}, you should also define it (take a look at
! 956: its definition in @file{vmgen-ex/mini.y}).
! 957:
! 958: You do not need to call @code{BB_BOUNDARY} after branches, because you
! 959: will not define superinstructions that contain branches in the middle
! 960: (and if you did, and it would work, there would be no reason to end the
! 961: superinstruction at the branch), and because the branches announce
! 962: themselves to the profiler.
! 963:
! 964:
! 965: @section Peephole optimization
! 966:
! 967: You need peephole optimization only if you want to use
! 968: superinstructions. But having the code for it does not hurt much if you
! 969: do not use superinstructions.
! 970:
! 971: A simple greedy peephole optimization algorithm is used for
! 972: superinstruction selection: every time @code{gen_inst} compiles a VM
! 973: instruction, it looks if it can combine it with the last VM instruction
! 974: (which may also be a superinstruction resulting from a previous peephole
! 975: optimization); if so, it changes the last instruction to the combined
! 976: instruction instead of laying down @code{i} at the current @samp{*ctp}.
! 977:
! 978: The code for peephole optimization is in @file{vmgen-ex/peephole.c}.
! 979: You can use this file almost verbatim. Vmgen generates
! 980: @file{@var{file}-peephole.i} which contains data for the peephoile
! 981: optimizer.
! 982:
! 983: You have to call @samp{init_peeptable()} after initializing
! 984: @samp{vm_prim}, and before compiling any VM code to initialize data
! 985: structures for peephole optimization. After that, compiling with the VM
! 986: code generation functions will automatically combine VM instructions
! 987: into superinstructions. Since you do not want to combine instructions
! 988: across VM branch targets (otherwise there will not be a proper VM
! 989: instruction to branch to), you have to call @code{BB_BOUNDARY}
! 990: (@pxref{VM code generation}) at branch targets.
! 991:
! 992:
! 993: @section VM disassembler
! 994:
! 995: A VM code disassembler is optional for an interpretive system, but
! 996: highly recommended during its development and maintenance, because it is
! 997: very useful for detecting bugs in the front end (and for distinguishing
! 998: them from VM interpreter bugs).
! 999:
! 1000: Vmgen supports VM code disassembling by generating
! 1001: @file{@var{file}-disasm.i}. This code has to be wrapped into a
! 1002: function, as is done in @file{vmgen-ex/disasm.i}. You can use this file
! 1003: almost verbatim. In addition to @samp{vm_@var{A}2@var{B}(a,b)},
! 1004: @samp{vm_out}, @samp{printarg_@var{type}(@var{value})}, which are
! 1005: explained above, the following macros and variables are used in
! 1006: @file{@var{file}-disasm.i} (and you have to define them):
! 1007:
! 1008: @table @samp
! 1009:
! 1010: @item ip
! 1011: This variable points to the opcode of the current VM instruction.
! 1012:
! 1013: @item IP IPTOS
! 1014: @samp{IPTOS} is the first argument of the current VM instruction, and
! 1015: @samp{IP} points to it; this is just as in the engine, but here
! 1016: @samp{ip} points to the opcode of the VM instruction (in contrast to the
! 1017: engine, where @samp{ip} points to the next cell, or even one further).
! 1018:
! 1019: @item VM_IS_INST(Inst i, int n)
! 1020: Tests if the opcode @samp{i} is the same as the @samp{n}th entry in the
! 1021: VM instruction table.
! 1022:
! 1023: @end table
! 1024:
! 1025:
! 1026:
1.3 anton 1027:
1.2 anton 1028:
1029:
1030:
1031: Invocation
1032:
1033: Input Syntax
1034:
1035: Concepts: Front end, VM, Stacks, Types, input stream
1036:
1037: Contact
1.4 anton 1038:
1039:
1040: Required changes:
1041: vm_...2... -> two arguments
1042: "vm_two...2...(arg1,arg2,arg3);" -> "vm_two...2...(arg3,arg1,arg2)" (no ";").
1043: define INST_ADDR and LABEL
1044: define VM_IS_INST also for disassembler
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