1: @include version.texi
2:
3: @c @ifnottex
4: This file documents vmgen (Gforth @value{VERSION}).
5:
6: @chapter Introduction
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
32: as for efficiency reasons. The virtual machine code is typically passed
33: between front end and virtual machine interpreter in memory, like in a
34: load-and-go compiler; this avoids the complexity and time cost of
35: writing the code to a file and reading it again.
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
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).
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:
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.
102:
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:
133: @chapter Concepts
134:
135: @c --------------------------------------------------------------------
136: @section Front-end and virtual machine interpreter
137:
138: @cindex front-end
139: Interpretive systems are typically divided into a @emph{front end} that
140: parses the input language and produces an intermediate representation
141: for the program, and an interpreter that executes the intermediate
142: representation of the program.
143:
144: @cindex virtual machine
145: @cindex VM
146: @cindex instruction, VM
147: For efficient interpreters the intermediate representation of choice is
148: virtual machine code (rather than, e.g., an abstract syntax tree).
149: @emph{Virtual machine} (VM) code consists of VM instructions arranged
150: sequentially in memory; they are executed in sequence by the VM
151: interpreter, except for VM branch instructions, which implement control
152: structures. The conceptual similarity to real machine code results in
153: the name @emph{virtual machine}.
154:
155: In this framework, vmgen supports building the VM interpreter and any
156: other component dealing with VM instructions. It does not have any
157: support for the front end, apart from VM code generation support. The
158: front end can be implemented with classical compiler front-end
159: techniques, supported by tools like @command{flex} and @command{bison}.
160:
161: The intermediate representation is usually just internal to the
162: interpreter, but some systems also support saving it to a file, either
163: as an image file, or in a full-blown linkable file format (e.g., JVM).
164: Vmgen currently has no special support for such features, but the
165: information in the instruction descriptions can be helpful, and we are
166: open for feature requests and suggestions.
167:
168: @section Data handling
169:
170: @cindex stack machine
171: @cindex register machine
172: Most VMs use one or more stacks for passing temporary data between VM
173: instructions. Another option is to use a register machine architecture
174: for the virtual machine; however, this option is either slower or
175: significantly more complex to implement than a stack machine architecture.
176:
177: Vmgen has special support and optimizations for stack VMs, making their
178: implementation easy and efficient.
179:
180: You can also implement a register VM with vmgen (@pxref{Register
181: Machines}), and you will still profit from most vmgen features.
182:
183: @cindex stack item size
184: @cindex size, stack items
185: Stack items all have the same size, so they typically will be as wide as
186: an integer, pointer, or floating-point value. Vmgen supports treating
187: two consecutive stack items as a single value, but anything larger is
188: best kept in some other memory area (e.g., the heap), with pointers to
189: the data on the stack.
190:
191: @cindex instruction stream
192: @cindex immediate arguments
193: Another source of data is immediate arguments VM instructions (in the VM
194: instruction stream). The VM instruction stream is handled similar to a
195: stack in vmgen.
196:
197: @cindex garbage collection
198: @cindex reference counting
199: Vmgen has no built-in support for nor restrictions against @emph{garbage
200: collection}. If you need garbage collection, you need to provide it in
201: your run-time libraries. Using @emph{reference counting} is probably
202: harder, but might be possible (contact us if you are interested).
203: @c reference counting might be possible by including counting code in
204: @c the conversion macros.
205:
206: @c *************************************************************
207: @chapter Invoking vmgen
208:
209: The usual way to invoke vmgen is as follows:
210:
211: @example
212: vmgen @var{infile}
213: @end example
214:
215: Here @var{infile} is the VM instruction description file, which usually
216: ends in @file{.vmg}. The output filenames are made by taking the
217: basename of @file{infile} (i.e., the output files will be created in the
218: current working directory) and replacing @file{.vmg} with @file{-vm.i},
219: @file{-disasm.i}, @file{-gen.i}, @file{-labels.i}, @file{-profile.i},
220: and @file{-peephole.i}. E.g., @command{bison hack/foo.vmg} will create
221: @file{foo-vm.i} etc.
222:
223: The command-line options supported by vmgen are
224:
225: @table @option
226:
227: @cindex -h, command-line option
228: @cindex --help, command-line option
229: @item --help
230: @itemx -h
231: Print a message about the command-line options
232:
233: @cindex -v, command-line option
234: @cindex --version, command-line option
235: @item --version
236: @itemx -v
237: Print version and exit
238: @end table
239:
240: @c env vars GFORTHDIR GFORTHDATADIR
241:
242: @c ****************************************************************
243: @chapter Example
244:
245: @section Example overview
246:
247: There are two versions of the same example for using vmgen:
248: @file{vmgen-ex} and @file{vmgen-ex2} (you can also see Gforth as
249: example, but it uses additional (undocumented) features, and also
250: differs in some other respects). The example implements @emph{mini}, a
251: tiny Modula-2-like language with a small JavaVM-like virtual machine.
252: The difference between the examples is that @file{vmgen-ex} uses many
253: casts, and @file{vmgen-ex2} tries to avoids most casts and uses unions
254: instead.
255:
256: The files provided with each example are:
257:
258: @example
259: Makefile
260: README
261: disasm.c wrapper file
262: engine.c wrapper file
263: peephole.c wrapper file
264: profile.c wrapper file
265: mini-inst.vmg simple VM instructions
266: mini-super.vmg superinstructions (empty at first)
267: mini.h common declarations
268: mini.l scanner
269: mini.y front end (parser, VM code generator)
270: support.c main() and other support functions
271: fib.mini example mini program
272: simple.mini example mini program
273: test.mini example mini program (tests everything)
274: test.out test.mini output
275: stat.awk script for aggregating profile information
276: peephole-blacklist list of instructions not allowed in superinstructions
277: seq2rule.awk script for creating superinstructions
278: @end example
279:
280: For your own interpreter, you would typically copy the following files
281: and change little, if anything:
282:
283: @example
284: disasm.c wrapper file
285: engine.c wrapper file
286: peephole.c wrapper file
287: profile.c wrapper file
288: stat.awk script for aggregating profile information
289: seq2rule.awk script for creating superinstructions
290: @end example
291:
292: You would typically change much in or replace the following files:
293:
294: @example
295: Makefile
296: mini-inst.vmg simple VM instructions
297: mini.h common declarations
298: mini.l scanner
299: mini.y front end (parser, VM code generator)
300: support.c main() and other support functions
301: peephole-blacklist list of instructions not allowed in superinstructions
302: @end example
303:
304: You can build the example by @code{cd}ing into the example's directory,
305: and then typing @samp{make}; you can check that it works with @samp{make
306: check}. You can run run mini programs like this:
307:
308: @example
309: ./mini fib.mini
310: @end example
311:
312: To learn about the options, type @samp{./mini -h}.
313:
314: @section Using profiling to create superinstructions
315:
316: I have not added rules for this in the @file{Makefile} (there are many
317: options for selecting superinstructions, and I did not want to hardcode
318: one into the @file{Makefile}), but there are some supporting scripts, and
319: here's an example:
320:
321: Suppose you want to use @file{fib.mini} and @file{test.mini} as training
322: programs, you get the profiles like this:
323:
324: @example
325: make fib.prof test.prof #takes a few seconds
326: @end example
327:
328: You can aggregate these profiles with @file{stat.awk}:
329:
330: @example
331: awk -f stat.awk fib.prof test.prof
332: @end example
333:
334: The result contains lines like:
335:
336: @example
337: 2 16 36910041 loadlocal lit
338: @end example
339:
340: This means that the sequence @code{loadlocal lit} statically occurs a
341: total of 16 times in 2 profiles, with a dynamic execution count of
342: 36910041.
343:
344: The numbers can be used in various ways to select superinstructions.
345: E.g., if you just want to select all sequences with a dynamic
346: execution count exceeding 10000, you would use the following pipeline:
347:
348: @example
349: awk -f stat.awk fib.prof test.prof|
350: awk '$3>=10000'| #select sequences
351: fgrep -v -f peephole-blacklist| #eliminate wrong instructions
352: awk -f seq2rule.awk| #transform sequences into superinstruction rules
353: sort -k 3 >mini-super.vmg #sort sequences
354: @end example
355:
356: The file @file{peephole-blacklist} contains all instructions that
357: directly access a stack or stack pointer (for mini: @code{call},
358: @code{return}); the sort step is necessary to ensure that prefixes
359: preceed larger superinstructions.
360:
361: Now you can create a version of mini with superinstructions by just
362: saying @samp{make}
363:
364: @c ***************************************************************
365: @chapter Input File Format
366:
367: Vmgen takes as input a file containing specifications of virtual machine
368: instructions. This file usually has a name ending in @file{.vmg}.
369:
370: Most examples are taken from the example in @file{vmgen-ex}.
371:
372: @section Input File Grammar
373:
374: The grammar is in EBNF format, with @code{@var{a}|@var{b}} meaning
375: ``@var{a} or @var{b}'', @code{@{@var{c}@}} meaning 0 or more repetitions
376: of @var{c} and @code{[@var{d}]} meaning 0 or 1 repetitions of @var{d}.
377:
378: Vmgen input is not free-format, so you have to take care where you put
379: spaces and especially newlines; it's not as bad as makefiles, though:
380: any sequence of spaces and tabs is equivalent to a single space.
381:
382: @example
383: description: {instruction|comment|eval-escape}
384:
385: instruction: simple-inst|superinst
386:
387: simple-inst: ident " (" stack-effect " )" newline c-code newline newline
388:
389: stack-effect: {ident} " --" {ident}
390:
391: super-inst: ident " =" ident {ident}
392:
393: comment: "\ " text newline
394:
395: eval-escape: "\e " text newline
396: @end example
397: @c \+ \- \g \f \c
398:
399: Note that the @code{\}s in this grammar are meant literally, not as
400: C-style encodings for non-printable characters.
401:
402: The C code in @code{simple-inst} must not contain empty lines (because
403: vmgen would mistake that as the end of the simple-inst. The text in
404: @code{comment} and @code{eval-escape} must not contain a newline.
405: @code{Ident} must conform to the usual conventions of C identifiers
406: (otherwise the C compiler would choke on the vmgen output).
407:
408: Vmgen understands a few extensions beyond the grammar given here, but
409: these extensions are only useful for building Gforth. You can find a
410: description of the format used for Gforth in @file{prim}.
411:
412: @subsection
413: @c woanders?
414: The text in @code{eval-escape} is Forth code that is evaluated when
415: vmgen reads the line. If you do not know (and do not want to learn)
416: Forth, you can build the text according to the following grammar; these
417: rules are normally all Forth you need for using vmgen:
418:
419: @example
420: text: stack-decl|type-prefix-decl|stack-prefix-decl
421:
422: stack-decl: "stack " ident ident ident
423: type-prefix-decl:
424: 's" ' string '" ' ("single"|"double") ident "type-prefix" ident
425: stack-prefix-decl: ident "stack-prefix" string
426: @end example
427:
428: Note that the syntax of this code is not checked thoroughly (there are
429: many other Forth program fragments that could be written there).
430:
431: If you know Forth, the stack effects of the non-standard words involved
432: are:
433:
434: @example
435: stack ( "name" "pointer" "type" -- )
436: ( name execution: -- stack )
437: type-prefix ( addr u xt1 xt2 n stack "prefix" -- )
438: single ( -- xt1 xt2 n )
439: double ( -- xt1 xt2 n )
440: stack-prefix ( stack "prefix" -- )
441: @end example
442:
443:
444: @section Simple instructions
445:
446: We will use the following simple VM instruction description as example:
447:
448: @example
449: sub ( i1 i2 -- i )
450: i = i1-i2;
451: @end example
452:
453: The first line specifies the name of the VM instruction (@code{sub}) and
454: its stack effect (@code{i1 i2 -- i}). The rest of the description is
455: just plain C code.
456:
457: @cindex stack effect
458: The stack effect specifies that @code{sub} pulls two integers from the
459: data stack and puts them in the C variables @code{i1} and @code{i2} (with
460: the rightmost item (@code{i2}) taken from the top of stack) and later
461: pushes one integer (@code{i)) on the data stack (the rightmost item is
462: on the top afterwards).
463:
464: How do we know the type and stack of the stack items? Vmgen uses
465: prefixes, similar to Fortran; in contrast to Fortran, you have to
466: define the prefix first:
467:
468: @example
469: \E s" Cell" single data-stack type-prefix i
470: @end example
471:
472: This defines the prefix @code{i} to refer to the type @code{Cell}
473: (defined as @code{long} in @file{mini.h}) and, by default, to the
474: @code{data-stack}. It also specifies that this type takes one stack
475: item (@code{single}). The type prefix is part of the variable name.
476:
477: Before we can use @code{data-stack} in this way, we have to define it:
478:
479: @example
480: \E stack data-stack sp Cell
481: @end example
482: @c !! use something other than Cell
483:
484: This line defines the stack @code{data-stack}, which uses the stack
485: pointer @code{sp}, and each item has the basic type @code{Cell}; other
486: types have to fit into one or two @code{Cell}s (depending on whether the
487: type is @code{single} or @code{double} wide), and are converted from and
488: to Cells on accessing the @code{data-stack) with conversion macros
489: (@pxref{Conversion macros}). Stacks grow towards lower addresses in
490: vmgen-erated interpreters.
491:
492: We can override the default stack of a stack item by using a stack
493: prefix. E.g., consider the following instruction:
494:
495: @example
496: lit ( #i -- i )
497: @end example
498:
499: The VM instruction @code{lit} takes the item @code{i} from the
500: instruction stream (indicated by the prefix @code{#}), and pushes it on
501: the (default) data stack. The stack prefix is not part of the variable
502: name. Stack prefixes are defined like this:
503:
504: @example
505: \E inst-stream stack-prefix #
506: @end example
507:
508: This definition defines that the stack prefix @code{#} specifies the
509: ``stack'' @code{inst-stream}. Since the instruction stream behaves a
510: little differently than an ordinary stack, it is predefined, and you do
511: not need to define it.
512:
513: The instruction stream contains instructions and their immediate
514: arguments, so specifying that an argument comes from the instruction
515: stream indicates an immediate argument. Of course, instruction stream
516: arguments can only appear to the left of @code{--} in the stack effect.
517: If there are multiple instruction stream arguments, the leftmost is the
518: first one (just as the intuition suggests).
519:
520: @subsubsection C Code Macros
521:
522: Vmgen recognizes the following strings in the C code part of simple
523: instructions:
524:
525: @table @samp
526:
527: @item SET_IP
528: As far as vmgen is concerned, a VM instruction containing this ends a VM
529: basic block (used in profiling to delimit profiled sequences). On the C
530: level, this also sets the instruction pointer.
531:
532: @item SUPER_END
533: This ends a basic block (for profiling), without a SET_IP.
534:
535: @item TAIL;
536: Vmgen replaces @samp{TAIL;} with code for ending a VM instruction and
537: dispatching the next VM instruction. This happens automatically when
538: control reaches the end of the C code. If you want to have this in the
539: middle of the C code, you need to use @samp{TAIL;}. A typical example
540: is a conditional VM branch:
541:
542: @example
543: if (branch_condition) {
544: SET_IP(target); TAIL;
545: }
546: /* implicit tail follows here */
547: @end example
548:
549: In this example, @samp{TAIL;} is not strictly necessary, because there
550: is another one implicitly after the if-statement, but using it improves
551: branch prediction accuracy slightly and allows other optimizations.
552:
553: @item SUPER_CONTINUE
554: This indicates that the implicit tail at the end of the VM instruction
555: dispatches the sequentially next VM instruction even if there is a
556: @code{SET_IP} in the VM instruction. This enables an optimization that
557: is not yet implemented in the vmgen-ex code (but in Gforth). The
558: typical application is in conditional VM branches:
559:
560: @example
561: if (branch_condition) {
562: SET_IP(target); TAIL; /* now this TAIL is necessary */
563: }
564: SUPER_CONTINUE;
565: @end example
566:
567: @end table
568:
569: Note that vmgen is not smart about C-level tokenization, comments,
570: strings, or conditional compilation, so it will interpret even a
571: commented-out SUPER_END as ending a basic block (or, e.g.,
572: @samp{RETAIL;} as @samp{TAIL;}). Conversely, vmgen requires the literal
573: presence of these strings; vmgen will not see them if they are hiding in
574: a C preprocessor macro.
575:
576:
577: @subsubsection C Code restrictions
578:
579: Vmgen generates code and performs some optimizations under the
580: assumption that the user-supplied C code does not access the stack
581: pointers or stack items, and that accesses to the instruction pointer
582: only occur through special macros. In general you should heed these
583: restrictions. However, if you need to break these restrictions, read
584: the following.
585:
586: Accessing a stack or stack pointer directly can be a problem for several
587: reasons:
588:
589: @itemize
590:
591: @item
592: You may cache the top-of-stack item in a local variable (that is
593: allocated to a register). This is the most frequent source of trouble.
594: You can deal with it either by not using top-of-stack caching (slowdown
595: factor 1-1.4, depending on machine), or by inserting flushing code
596: (e.g., @samp{IF_spTOS(sp[...] = spTOS);}) at the start and reloading
597: code (e.g., @samp{IF_spTOS(spTOS = sp[0])}) at the end of problematic C
598: code. Vmgen inserts a stack pointer update before the start of the
599: user-supplied C code, so the flushing code has to use an index that
600: corrects for that. In the future, this flushing may be done
601: automatically by mentioning a special string in the C code.
602: @c sometimes flushing and/or reloading unnecessary
603:
604: @item
605: The vmgen-erated code loads the stack items from stack-pointer-indexed
606: memory into variables before the user-supplied C code, and stores them
607: from variables to stack-pointer-indexed memory afterwards. If you do
608: any writes to the stack through its stack pointer in your C code, it
609: will not affact the variables, and your write may be overwritten by the
610: stores after the C code. Similarly, a read from a stack using a stack
611: pointer will not reflect computations of stack items in the same VM
612: instruction.
613:
614: @item
615: Superinstructions keep stack items in variables across the whole
616: superinstruction. So you should not include VM instructions, that
617: access a stack or stack pointer, as components of superinstructions.
618:
619: @end itemize
620:
621: You should access the instruction pointer only through its special
622: macros (@samp{IP}, @samp{SET_IP}, @samp{IPTOS}); this ensure that these
623: macros can be implemented in several ways for best performance.
624: @samp{IP} points to the next instruction, and @samp{IPTOS} is its
625: contents.
626:
627:
628: @section Superinstructions
629:
630: Here is an example of a superinstruction definition:
631:
632: @example
633: lit_sub = lit sub
634: @end example
635:
636: @code{lit_sub} is the name of the superinstruction, and @code{lit} and
637: @code{sub} are its components. This superinstruction performs the same
638: action as the sequence @code{lit} and @code{sub}. It is generated
639: automatically by the VM code generation functions whenever that sequence
640: occurs, so you only need to add this definition if you want to use this
641: superinstruction (and even that can be partially automatized,
642: @pxref{...}).
643:
644: Vmgen requires that the component instructions are simple instructions
645: defined before superinstructions using the components. Currently, vmgen
646: also requires that all the subsequences at the start of a
647: superinstruction (prefixes) must be defined as superinstruction before
648: the superinstruction. I.e., if you want to define a superinstruction
649:
650: @example
651: sumof5 = add add add add
652: @end example
653:
654: you first have to define
655:
656: @example
657: add ( n1 n2 -- n )
658: n = n1+n2;
659:
660: sumof3 = add add
661: sumof4 = add add add
662: @end example
663:
664: Here, @code{sumof4} is the longest prefix of @code{sumof5}, and @code{sumof3}
665: is the longest prefix of @code{sumof4}.
666:
667: Note that vmgen assumes that only the code it generates accesses stack
668: pointers, the instruction pointer, and various stack items, and it
669: performs optimizations based on this assumption. Therefore, VM
670: instructions that change the instruction pointer should only be used as
671: last component; a VM instruction that accesses a stack pointer should
672: not be used as component at all. Vmgen does not check these
673: restrictions, they just result in bugs in your interpreter.
674:
675: @c ********************************************************************
676: @chapter Using the generated code
677:
678: The easiest way to create a working VM interpreter with vmgen is
679: probably to start with one of the examples, and modify it for your
680: purposes. This chapter is just the reference manual for the macros
681: etc. used by the generated code, and the other context expected by the
682: generated code, and what you can do with the various generated files.
683:
684: @section VM engine
685:
686: The VM engine is the VM interpreter that executes the VM code. It is
687: essential for an interpretive system.
688:
689: The main file generated for the VM interpreter is
690: @file{@var{name}-vm.i}. It uses the following macros and variables (and
691: you have to define them):
692:
693: @table @code
694:
695: @item LABEL(@var{inst_name})
696: This is used just before each VM instruction to provide a jump or
697: @code{switch} label (the @samp{:} is provided by vmgen). For switch
698: dispatch this should expand to @samp{case @var{label}}; for
699: threaded-code dispatch this should just expand to @samp{case
700: @var{label}}. In either case @var{label} is usually the @var{inst_name}
701: with some prefix or suffix to avoid naming conflicts.
702:
703: @item NAME(@var{inst_name_string})
704: Called on entering a VM instruction with a string containing the name of
705: the VM instruction as parameter. In normal execution this should be a
706: noop, but for tracing this usually prints the name, and possibly other
707: information (several VM registers in our example).
708:
709: @item DEF_CA
710: Usually empty. Called just inside a new scope at the start of a VM
711: instruction. Can be used to define variables that should be visible
712: during every VM instruction. If you define this macro as non-empty, you
713: have to provide the finishing @samp{;} in the macro.
714:
715: @item NEXT_P0 NEXT_P1 NEXT_P2
716: The three parts of instruction dispatch. They can be defined in
717: different ways for best performance on various processors (see
718: @file{engine.c} in the example or @file{engine/threaded.h} in Gforth).
719: @samp{NEXT_P0} is invoked right at the start of the VM isntruction (but
720: after @samp{DEF_CA}), @samp{NEXT_P1} right after the user-supplied C
721: code, and @samp{NEXT_P2} at the end. The actual jump has to be
722: performed by @samp{NEXT_P2}.
723:
724: The simplest variant is if @samp{NEXT_P2} does everything and the other
725: macros do nothing. Then also related macros like @samp{IP},
726: @samp{SET_IP}, @samp{IP}, @samp{INC_IP} and @samp{IPTOS} are very
727: straightforward to define. For switch dispatch this code consists just
728: of a jump to the dispatch code (@samp{goto next_inst;} in our example;
729: for direct threaded code it consists of something like
730: @samp{({cfa=*ip++; goto *cfa;})}.
731:
732: Pulling code (usually the @samp{cfa=*ip;}) up into @samp{NEXT_P1}
733: usually does not cause problems, but pulling things up into
734: @samp{NEXT_P0} usually requires changing the other macros (and, at least
735: for Gforth on Alpha, it does not buy much, because the compiler often
736: manages to schedule the relevant stuff up by itself). An even more
737: extreme variant is to pull code up even further, into, e.g., NEXT_P1 of
738: the previous VM instruction (prefetching, useful on PowerPCs).
739:
740: @item INC_IP(@var{n})
741: This increments IP by @var{n}.
742:
743: @item vm_@var{A}2@var{B}(a,b)
744: Type casting macro that assigns @samp{a} (of type @var{A}) to @samp{b}
745: (of type @var{B}). This is mainly used for getting stack items into
746: variables and back. So you need to define macros for every combination
747: of stack basic type (@code{Cell} in our example) and type-prefix types
748: used with that stack (in both directions). For the type-prefix type,
749: you use the type-prefix (not the C type string) as type name (e.g.,
750: @samp{vm_Cell2i}, not @samp{vm_Cell2Cell}). In addition, you have to
751: define a vm_@var{X}2@var{X} macro for the stack basic type (used in
752: superinstructions).
753:
754: The stack basic type for the predefined @samp{inst-stream} is
755: @samp{Cell}. If you want a stack with the same item size, making its
756: basic type @samp{Cell} usually reduces the number of macros you have to
757: define.
758:
759: Here our examples differ a lot: @file{vmgen-ex} uses casts in these
760: macros, whereas @file{vmgen-ex2} uses union-field selection (or
761: assignment to union fields).
762:
763: @item vm_two@var{A}2@var{B}(a1,a2,b)
764: @item vm_@var{B}2two@var{A}(b,a1,a2)
765: Conversions between two stack items (@code{a1}, @code{a2}) and a
766: variable @code{b} of a type that takes two stack items. This does not
767: occur in our small examples, but you can look at Gforth for examples.
768:
769: @item @var{stackpointer}
770: For each stack used, the stackpointer name given in the stack
771: declaration is used. For a regular stack this must be an l-expression;
772: typically it is a variable declared as a pointer to the stack's basic
773: type. For @samp{inst-stream}, the name is @samp{IP}, and it can be a
774: plain r-value; typically it is a macro that abstracts away the
775: differences between the various implementations of NEXT_P*.
776:
777: @item @var{stackpointer}TOS
778: The top-of-stack for the stack pointed to by @var{stackpointer}. If you
779: are using top-of-stack caching for that stack, this should be defined as
780: variable; if you are not using top-of-stack caching for that stack, this
781: should be a macro expanding to @samp{@var{stackpointer}[0]}. The stack
782: pointer for the predefined @samp{inst-stream} is called @samp{IP}, so
783: the top-of-stack is called @samp{IPTOS}.
784:
785: @item IF_@var{stackpointer}TOS(@var{expr})
786: Macro for executing @var{expr}, if top-of-stack caching is used for the
787: @var{stackpointer} stack. I.e., this should do @var{expr} if there is
788: top-of-stack caching for @var{stackpointer}; otherwise it should do
789: nothing.
790:
791: @item VM_DEBUG
792: If this is defined, the tracing code will be compiled in (slower
793: interpretation, but better debugging). Our example compiles two
794: versions of the engine, a fast-running one that cannot trace, and one
795: with potential tracing and profiling.
796:
797: @item vm_debug
798: Needed only if @samp{VM_DEBUG} is defined. If this variable contains
799: true, the VM instructions produce trace output. It can be turned on or
800: off at any time.
801:
802: @item vm_out
803: Needed only if @samp{VM_DEBUG} is defined. Specifies the file on which
804: to print the trace output (type @samp{FILE *}).
805:
806: @item printarg_@var{type}(@var{value})
807: Needed only if @samp{VM_DEBUG} is defined. Macro or function for
808: printing @var{value} in a way appropriate for the @var{type}. This is
809: used for printing the values of stack items during tracing. @var{Type}
810: is normally the type prefix specified in a @code{type-prefix} definition
811: (e.g., @samp{printarg_i}); in superinstructions it is currently the
812: basic type of the stack.
813:
814: @end table
815:
816: The file @file{@var{name}-labels.i} is used for enumerating or listing
817: all virtual machine instructions and uses the following macro:
818:
819: @table @samp
820:
821: @item INST_ADDR(@var{inst_name})
822: For switch dispatch, this is just the name of the switch label (the same
823: name as used in @samp{LABEL(@var{inst_name})}). For threaded-code
824: dispatch, this is the address of the label defined in
825: @samp{LABEL(@var{inst_name})}); the address is taken with @samp{&&}
826: (@pxref{labels-as-values}).
827:
828: @end table
829:
830:
831:
832: @section Stacks, types, and prefixes
833:
834:
835:
836: Invocation
837:
838: Input Syntax
839:
840: Concepts: Front end, VM, Stacks, Types, input stream
841:
842: Contact
843:
844:
845: Required changes:
846: vm_...2... -> two arguments
847: "vm_two...2...(arg1,arg2,arg3);" -> "vm_two...2...(arg3,arg1,arg2)" (no ";").
848: define INST_ADDR and LABEL
849: define VM_IS_INST also for disassembler
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