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4 <title>Stacker: An Example Of Using LLVM</title>
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8 <div class="doc_title">Stacker: An Example Of Using LLVM</div>
11 <li><a href="#abstract">Abstract</a></li>
12 <li><a href="#introduction">Introduction</a></li>
13 <li><a href="#lessons">Lessons I Learned About LLVM</a>
15 <li><a href="#value">Everything's a Value!</a></li>
16 <li><a href="#terminate">Terminate Those Blocks!</a></li>
17 <li><a href="#blocks">Concrete Blocks</a></li>
18 <li><a href="#push_back">push_back Is Your Friend</a></li>
19 <li><a href="#gep">The Wily GetElementPtrInst</a></li>
20 <li><a href="#linkage">Getting Linkage Types Right</a></li>
21 <li><a href="#constants">Constants Are Easier Than That!</a></li>
24 <li><a href="#lexicon">The Stacker Lexicon</a>
26 <li><a href="#stack">The Stack</a>
27 <li><a href="#punctuation">Punctuation</a>
28 <li><a href="#comments">Comments</a>
29 <li><a href="#literals">Literals</a>
30 <li><a href="#words">Words</a>
31 <li><a href="style">Standard Style</a>
32 <li><a href="#builtins">Built-Ins</a>
35 <li><a href="#example">Prime: A Complete Example</a></li>
36 <li><a href="#internal">Internal Code Details</a>
38 <li><a href="#directory">The Directory Structure </a></li>
39 <li><a href="#lexer">The Lexer</a></li>
40 <li><a href="#parser">The Parser</a></li>
41 <li><a href="#compiler">The Compiler</a></li>
42 <li><a href="#runtime">The Runtime</a></li>
43 <li><a href="#driver">Compiler Driver</a></li>
44 <li><a href="#tests">Test Programs</a></li>
45 <li><a href="#exercise">Exercise</a></li>
46 <li><a href="#todo">Things Remaining To Be Done</a></li>
50 <div class="doc_text">
51 <p><b>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a> </b></p>
55 <!-- ======================================================================= -->
56 <div class="doc_section"> <a name="abstract">Abstract </a></div>
57 <div class="doc_text">
58 <p>This document is another way to learn about LLVM. Unlike the
59 <a href="LangRef.html">LLVM Reference Manual</a> or
60 <a href="ProgrammersManual.html">LLVM Programmer's Manual</a>, here we learn
61 about LLVM through the experience of creating a simple programming language
62 named Stacker. Stacker was invented specifically as a demonstration of
63 LLVM. The emphasis in this document is not on describing the
64 intricacies of LLVM itself but on how to use it to build your own
67 <!-- ======================================================================= -->
68 <div class="doc_section"> <a name="introduction">Introduction</a> </div>
69 <div class="doc_text">
70 <p>Amongst other things, LLVM is a platform for compiler writers.
71 Because of its exceptionally clean and small IR (intermediate
72 representation), compiler writing with LLVM is much easier than with
73 other system. As proof, I wrote the entire compiler (language definition,
74 lexer, parser, code generator, etc.) in about <em>four days</em>!
75 That's important to know because it shows how quickly you can get a new
76 language running when using LLVM. Furthermore, this was the <em >first</em>
77 language the author ever created using LLVM. The learning curve is
78 included in that four days.</p>
79 <p>The language described here, Stacker, is Forth-like. Programs
80 are simple collections of word definitions, and the only thing definitions
81 can do is manipulate a stack or generate I/O. Stacker is not a "real"
82 programming language; it's very simple. Although it is computationally
83 complete, you wouldn't use it for your next big project. However,
84 the fact that it is complete, it's simple, and it <em>doesn't</em> have
85 a C-like syntax make it useful for demonstration purposes. It shows
86 that LLVM could be applied to a wide variety of languages.</p>
87 <p>The basic notions behind stacker is very simple. There's a stack of
88 integers (or character pointers) that the program manipulates. Pretty
89 much the only thing the program can do is manipulate the stack and do
90 some limited I/O operations. The language provides you with several
91 built-in words that manipulate the stack in interesting ways. To get
92 your feet wet, here's how you write the traditional "Hello, World"
93 program in Stacker:</p>
94 <p><code>: hello_world "Hello, World!" >s DROP CR ;<br>
95 : MAIN hello_world ;<br></code></p>
96 <p>This has two "definitions" (Stacker manipulates words, not
97 functions and words have definitions): <code>MAIN</code> and <code>
98 hello_world</code>. The <code>MAIN</code> definition is standard; it
99 tells Stacker where to start. Here, <code>MAIN</code> is defined to
100 simply invoke the word <code>hello_world</code>. The
101 <code>hello_world</code> definition tells stacker to push the
102 <code>"Hello, World!"</code> string on to the stack, print it out
103 (<code>>s</code>), pop it off the stack (<code>DROP</code>), and
104 finally print a carriage return (<code>CR</code>). Although
105 <code>hello_world</code> uses the stack, its net effect is null. Well
106 written Stacker definitions have that characteristic. </p>
107 <p>Exercise for the reader: how could you make this a one line program?</p>
109 <!-- ======================================================================= -->
110 <div class="doc_section"><a name="lessons"></a>Lessons I Learned About LLVM</div>
111 <div class="doc_text">
112 <p>Stacker was written for two purposes: </p>
114 <li>to get the author over the learning curve, and</li>
115 <li>to provide a simple example of how to write a compiler using LLVM.</li>
117 <p>During the development of Stacker, many lessons about LLVM were
118 learned. Those lessons are described in the following subsections.<p>
120 <!-- ======================================================================= -->
121 <div class="doc_subsection"><a name="value"></a>Everything's a Value!</div>
122 <div class="doc_text">
123 <p>Although I knew that LLVM uses a Single Static Assignment (SSA) format,
124 it wasn't obvious to me how prevalent this idea was in LLVM until I really
125 started using it. Reading the <a href="ProgrammersManual.html">
126 Programmer's Manual</a> and <a href="LangRef.html">Language Reference</a>,
127 I noted that most of the important LLVM IR (Intermediate Representation) C++
128 classes were derived from the Value class. The full power of that simple
129 design only became fully understood once I started constructing executable
130 expressions for Stacker.</p>
131 <p>This really makes your programming go faster. Think about compiling code
132 for the following C/C++ expression: <code>(a|b)*((x+1)/(y+1))</code>. Assuming
133 the values are on the stack in the order a, b, x, y, this could be
134 expressed in stacker as: <code>1 + SWAP 1 + / ROT2 OR *</code>.
135 You could write a function using LLVM that computes this expression like this: </p>
138 expression(BasicBlock* bb, Value* a, Value* b, Value* x, Value* y )
140 Instruction* tail = bb->getTerminator();
141 ConstantSInt* one = ConstantSInt::get( Type::IntTy, 1);
142 BinaryOperator* or1 =
143 BinaryOperator::create( Instruction::Or, a, b, "", tail );
144 BinaryOperator* add1 =
145 BinaryOperator::create( Instruction::Add, x, one, "", tail );
146 BinaryOperator* add2 =
147 BinaryOperator::create( Instruction::Add, y, one, "", tail );
148 BinaryOperator* div1 =
149 BinaryOperator::create( Instruction::Div, add1, add2, "", tail);
150 BinaryOperator* mult1 =
151 BinaryOperator::create( Instruction::Mul, or1, div1, "", tail );
156 <p>"Okay, big deal," you say? It is a big deal. Here's why. Note that I didn't
157 have to tell this function which kinds of Values are being passed in. They could be
158 <code>Instruction</code>s, <code>Constant</code>s, <code>GlobalVariable</code>s, or
159 any of the other subclasses of <code>Value</code> that LLVM supports.
160 Furthermore, if you specify Values that are incorrect for this sequence of
161 operations, LLVM will either notice right away (at compilation time) or the LLVM
162 Verifier will pick up the inconsistency when the compiler runs. In either case
163 LLVM prevents you from making a type error that gets passed through to the
164 generated program. This <em>really</em> helps you write a compiler that
165 always generates correct code!<p>
166 <p>The second point is that we don't have to worry about branching, registers,
167 stack variables, saving partial results, etc. The instructions we create
168 <em>are</em> the values we use. Note that all that was created in the above
169 code is a Constant value and five operators. Each of the instructions <em>is</em>
170 the resulting value of that instruction. This saves a lot of time.</p>
171 <p>The lesson is this: <em>SSA form is very powerful: there is no difference
172 between a value and the instruction that created it.</em> This is fully
173 enforced by the LLVM IR. Use it to your best advantage.</p>
175 <!-- ======================================================================= -->
176 <div class="doc_subsection"><a name="terminate"></a>Terminate Those Blocks!</div>
177 <div class="doc_text">
178 <p>I had to learn about terminating blocks the hard way: using the debugger
179 to figure out what the LLVM verifier was trying to tell me and begging for
180 help on the LLVMdev mailing list. I hope you avoid this experience.</p>
181 <p>Emblazon this rule in your mind:</p>
183 <li><em>All</em> <code>BasicBlock</code>s in your compiler <b>must</b> be
184 terminated with a terminating instruction (branch, return, etc.).
187 <p>Terminating instructions are a semantic requirement of the LLVM IR. There
188 is no facility for implicitly chaining together blocks placed into a function
189 in the order they occur. Indeed, in the general case, blocks will not be
190 added to the function in the order of execution because of the recursive
191 way compilers are written.</p>
192 <p>Furthermore, if you don't terminate your blocks, your compiler code will
193 compile just fine. You won't find out about the problem until you're running
194 the compiler and the module you just created fails on the LLVM Verifier.</p>
196 <!-- ======================================================================= -->
197 <div class="doc_subsection"><a name="blocks"></a>Concrete Blocks</div>
198 <div class="doc_text">
199 <p>After a little initial fumbling around, I quickly caught on to how blocks
200 should be constructed. In general, here's what I learned:
202 <li><em>Create your blocks early.</em> While writing your compiler, you
203 will encounter several situations where you know apriori that you will
204 need several blocks. For example, if-then-else, switch, while, and for
205 statements in C/C++ all need multiple blocks for expression in LVVM.
206 The rule is, create them early.</li>
207 <li><em>Terminate your blocks early.</em> This just reduces the chances
208 that you forget to terminate your blocks which is required (go
209 <a href="#terminate">here</a> for more).
210 <li><em>Use getTerminator() for instruction insertion.</em> I noticed early on
211 that many of the constructors for the Instruction classes take an optional
212 <code>insert_before</code> argument. At first, I thought this was a mistake
213 because clearly the normal mode of inserting instructions would be one at
214 a time <em>after</em> some other instruction, not <em>before</em>. However,
215 if you hold on to your terminating instruction (or use the handy dandy
216 <code>getTerminator()</code> method on a <code>BasicBlock</code>), it can
217 always be used as the <code>insert_before</code> argument to your instruction
218 constructors. This causes the instruction to automatically be inserted in
219 the RightPlace™ place, just before the terminating instruction. The
220 nice thing about this design is that you can pass blocks around and insert
221 new instructions into them without ever knowing what instructions came
222 before. This makes for some very clean compiler design.</li>
224 <p>The foregoing is such an important principal, its worth making an idiom:</p>
226 BasicBlock* bb = new BasicBlock();</li>
227 bb->getInstList().push_back( new Branch( ... ) );
228 new Instruction(..., bb->getTerminator() );
230 <p>To make this clear, consider the typical if-then-else statement
231 (see StackerCompiler::handle_if() method). We can set this up
232 in a single function using LLVM in the following way: </p>
234 using namespace llvm;
236 MyCompiler::handle_if( BasicBlock* bb, SetCondInst* condition )
238 // Create the blocks to contain code in the structure of if/then/else
239 BasicBlock* then_bb = new BasicBlock();
240 BasicBlock* else_bb = new BasicBlock();
241 BasicBlock* exit_bb = new BasicBlock();
243 // Insert the branch instruction for the "if"
244 bb->getInstList().push_back( new BranchInst( then_bb, else_bb, condition ) );
246 // Set up the terminating instructions
247 then->getInstList().push_back( new BranchInst( exit_bb ) );
248 else->getInstList().push_back( new BranchInst( exit_bb ) );
250 // Fill in the then part .. details excised for brevity
251 this->fill_in( then_bb );
253 // Fill in the else part .. details excised for brevity
254 this->fill_in( else_bb );
256 // Return a block to the caller that can be filled in with the code
257 // that follows the if/then/else construct.
261 <p>Presumably in the foregoing, the calls to the "fill_in" method would add
262 the instructions for the "then" and "else" parts. They would use the third part
263 of the idiom almost exclusively (inserting new instructions before the
264 terminator). Furthermore, they could even recurse back to <code>handle_if</code>
265 should they encounter another if/then/else statement, and it will just work.</p>
266 <p>Note how cleanly this all works out. In particular, the push_back methods on
267 the <code>BasicBlock</code>'s instruction list. These are lists of type
268 <code>Instruction</code> (which is also of type <code>Value</code>). To create
269 the "if" branch we merely instantiate a <code>BranchInst</code> that takes as
270 arguments the blocks to branch to and the condition to branch on. The
271 <code>BasicBlock</code> objects act like branch labels! This new
272 <code>BranchInst</code> terminates the <code>BasicBlock</code> provided
273 as an argument. To give the caller a way to keep inserting after calling
274 <code>handle_if</code>, we create an <code>exit_bb</code> block which is
276 to the caller. Note that the <code>exit_bb</code> block is used as the
277 terminator for both the <code>then_bb</code> and the <code>else_bb</code>
278 blocks. This guarantees that no matter what else <code>handle_if</code>
279 or <code>fill_in</code> does, they end up at the <code>exit_bb</code> block.
282 <!-- ======================================================================= -->
283 <div class="doc_subsection"><a name="push_back"></a>push_back Is Your Friend</div>
284 <div class="doc_text">
286 One of the first things I noticed is the frequent use of the "push_back"
287 method on the various lists. This is so common that it is worth mentioning.
288 The "push_back" inserts a value into an STL list, vector, array, etc. at the
289 end. The method might have also been named "insert_tail" or "append".
290 Although I've used STL quite frequently, my use of push_back wasn't very
291 high in other programs. In LLVM, you'll use it all the time.
294 <!-- ======================================================================= -->
295 <div class="doc_subsection"><a name="gep"></a>The Wily GetElementPtrInst</div>
296 <div class="doc_text">
298 It took a little getting used to and several rounds of postings to the LLVM
299 mailing list to wrap my head around this instruction correctly. Even though I had
300 read the Language Reference and Programmer's Manual a couple times each, I still
301 missed a few <em>very</em> key points:
304 <li>GetElementPtrInst gives you back a Value for the last thing indexed.</em>
305 <li>All global variables in LLVM are <em>pointers</em>.
306 <li>Pointers must also be dereferenced with the GetElementPtrInst instruction.
308 <p>This means that when you look up an element in the global variable (assuming
309 it's a struct or array), you <em>must</em> deference the pointer first! For many
310 things, this leads to the idiom:
313 std::vector<Value*> index_vector;
314 index_vector.push_back( ConstantSInt::get( Type::LongTy, 0 );
315 // ... push other indices ...
316 GetElementPtrInst* gep = new GetElementPtrInst( ptr, index_vector );
318 <p>For example, suppose we have a global variable whose type is [24 x int]. The
319 variable itself represents a <em>pointer</em> to that array. To subscript the
320 array, we need two indices, not just one. The first index (0) dereferences the
321 pointer. The second index subscripts the array. If you're a "C" programmer, this
322 will run against your grain because you'll naturally think of the global array
323 variable and the address of its first element as the same. That tripped me up
324 for a while until I realized that they really do differ .. by <em>type</em>.
325 Remember that LLVM is strongly typed. Everything has a type.
326 The "type" of the global variable is [24 x int]*. That is, it's
327 a pointer to an array of 24 ints. When you dereference that global variable with
328 a single (0) index, you now have a "[24 x int]" type. Although
329 the pointer value of the dereferenced global and the address of the zero'th element
330 in the array will be the same, they differ in their type. The zero'th element has
331 type "int" while the pointer value has type "[24 x int]".</p>
332 <p>Get this one aspect of LLVM right in your head, and you'll save yourself
333 a lot of compiler writing headaches down the road.</p>
335 <!-- ======================================================================= -->
336 <div class="doc_subsection"><a name="linkage"></a>Getting Linkage Types Right</div>
337 <div class="doc_text">
338 <p>Linkage types in LLVM can be a little confusing, especially if your compiler
339 writing mind has affixed firm concepts to particular words like "weak",
340 "external", "global", "linkonce", etc. LLVM does <em>not</em> use the precise
341 definitions of, say, ELF or GCC, even though they share common terms. To be fair,
342 the concepts are related and similar but not precisely the same. This can lead
343 you to think you know what a linkage type represents but in fact it is slightly
344 different. I recommend you read the
345 <a href="LangRef.html#linkage"> Language Reference on this topic</a> very
346 carefully. Then, read it again.<p>
347 <p>Here are some handy tips that I discovered along the way:</p>
349 <li><em>Unitialized means external.</em> That is, the symbol is declared in the current
350 module and can be used by that module, but it is not defined by that module.</li>
351 <li><em>Setting an initializer changes a global' linkage type.</em> Setting an
352 initializer changes a global's linkage type from whatever it was to a normal,
353 defined global (not external). You'll need to call the setLinkage() method to
354 reset it if you specify the initializer after the GlobalValue has been constructed.
355 This is important for LinkOnce and Weak linkage types.</li>
356 <li><em>Appending linkage can keep track of things.</em> Appending linkage can
357 be used to keep track of compilation information at runtime. It could be used,
358 for example, to build a full table of all the C++ virtual tables or hold the
359 C++ RTTI data, or whatever. Appending linkage can only be applied to arrays.
360 All arrays with the same name in each module are concatenated together at link
364 <!-- ======================================================================= -->
365 <div class="doc_subsection"><a name="constants"></a>Constants Are Easier Than That!</div>
366 <div class="doc_text">
368 Constants in LLVM took a little getting used to until I discovered a few utility
369 functions in the LLVM IR that make things easier. Here's what I learned: </p>
371 <li>Constants are Values like anything else and can be operands of instructions</li>
372 <li>Integer constants, frequently needed, can be created using the static "get"
373 methods of the ConstantInt, ConstantSInt, and ConstantUInt classes. The nice thing
374 about these is that you can "get" any kind of integer quickly.</li>
375 <li>There's a special method on Constant class which allows you to get the null
376 constant for <em>any</em> type. This is really handy for initializing large
377 arrays or structures, etc.</li>
380 <!-- ======================================================================= -->
381 <div class="doc_section"> <a name="lexicon">The Stacker Lexicon</a></div>
382 <div class="doc_text"><p>This section describes the Stacker language</p></div>
383 <div class="doc_subsection"><a name="stack"></a>The Stack</div>
384 <div class="doc_text">
385 <p>Stacker definitions define what they do to the global stack. Before
386 proceeding, a few words about the stack are in order. The stack is simply
387 a global array of 32-bit integers or pointers. A global index keeps track
388 of the location of the top of the stack. All of this is hidden from the
389 programmer, but it needs to be noted because it is the foundation of the
390 conceptual programming model for Stacker. When you write a definition,
391 you are, essentially, saying how you want that definition to manipulate
392 the global stack.</p>
393 <p>Manipulating the stack can be quite hazardous. There is no distinction
394 given and no checking for the various types of values that can be placed
395 on the stack. Automatic coercion between types is performed. In many
396 cases, this is useful. For example, a boolean value placed on the stack
397 can be interpreted as an integer with good results. However, using a
398 word that interprets that boolean value as a pointer to a string to
399 print out will almost always yield a crash. Stacker simply leaves it
400 to the programmer to get it right without any interference or hindering
401 on interpretation of the stack values. You've been warned. :) </p>
403 <!-- ======================================================================= -->
404 <div class="doc_subsection"> <a name="punctuation"></a>Punctuation</div>
405 <div class="doc_text">
406 <p>Punctuation in Stacker is very simple. The colon and semi-colon
407 characters are used to introduce and terminate a definition
408 (respectively). Except for <em>FORWARD</em> declarations, definitions
409 are all you can specify in Stacker. Definitions are read left to right.
410 Immediately after the colon comes the name of the word being defined.
411 The remaining words in the definition specify what the word does. The definition
412 is terminated by a semi-colon.</p>
413 <p>So, your typical definition will have the form:</p>
414 <pre><code>: name ... ;</code></pre>
415 <p>The <code>name</code> is up to you but it must start with a letter and contain
416 only letters, numbers, and underscore. Names are case sensitive and must not be
417 the same as the name of a built-in word. The <code>...</code> is replaced by
418 the stack manipulating words that you wish to define <code>name</code> as. <p>
420 <!-- ======================================================================= -->
421 <div class="doc_subsection"><a name="comments"></a>Comments</div>
422 <div class="doc_text">
423 <p>Stacker supports two types of comments. A hash mark (#) starts a comment
424 that extends to the end of the line. It is identical to the kind of comments
425 commonly used in shell scripts. A pair of parentheses also surround a comment.
426 In both cases, the content of the comment is ignored by the Stacker compiler. The
427 following does nothing in Stacker.
430 # This is a comment to end of line
431 ( This is an enclosed comment )
433 <p>See the <a href="#example">example</a> program to see comments in use in
436 <!-- ======================================================================= -->
437 <div class="doc_subsection"><a name="literals"></a>Literals</div>
438 <div class="doc_text">
439 <p>There are three kinds of literal values in Stacker: Integers, Strings,
440 and Booleans. In each case, the stack operation is to simply push the
441 value on to the stack. So, for example:<br/>
442 <code> 42 " is the answer." TRUE </code><br/>
443 will push three values on to the stack: the integer 42, the
444 string " is the answer.", and the boolean TRUE.</p>
446 <!-- ======================================================================= -->
447 <div class="doc_subsection"><a name="words"></a>Words</div>
448 <div class="doc_text">
449 <p>Each definition in Stacker is composed of a set of words. Words are
450 read and executed in order from left to right. There is very little
451 checking in Stacker to make sure you're doing the right thing with
452 the stack. It is assumed that the programmer knows how the stack
453 transformation he applies will affect the program.</p>
454 <p>Words in a definition come in two flavors: built-in and programmer
455 defined. Simply mentioning the name of a previously defined or declared
456 programmer-defined word causes that word's stack actions to be invoked. It
457 is somewhat like a function call in other languages. The built-in
458 words have various effects, described <a href="#builtins">below</a>.</p>
459 <p>Sometimes you need to call a word before it is defined. For this, you can
460 use the <code>FORWARD</code> declaration. It looks like this:</p>
461 <p><code>FORWARD name ;</code></p>
462 <p>This simply states to Stacker that "name" is the name of a definition
463 that is defined elsewhere. Generally it means the definition can be found
464 "forward" in the file. But, it doesn't have to be in the current compilation
465 unit. Anything declared with <code>FORWARD</code> is an external symbol for
468 <!-- ======================================================================= -->
469 <div class="doc_subsection"><a name="builtins"></a>Built In Words</div>
470 <div class="doc_text">
471 <p>The built-in words of the Stacker language are put in several groups
472 depending on what they do. The groups are as follows:</p>
474 <li><em>Logical</em>: These words provide the logical operations for
475 comparing stack operands.<br/>The words are: < > <= >=
476 = <> true false.</li>
477 <li><em>Bitwise</em>: These words perform bitwise computations on
478 their operands. <br/> The words are: << >> XOR AND NOT</li>
479 <li><em>Arithmetic</em>: These words perform arithmetic computations on
480 their operands. <br/> The words are: ABS NEG + - * / MOD */ ++ -- MIN MAX</li>
481 <li><em>Stack</em>These words manipulate the stack directly by moving
482 its elements around.<br/> The words are: DROP DROP2 NIP NIP2 DUP DUP2
483 SWAP SWAP2 OVER OVER2 ROT ROT2 RROT RROT2 TUCK TUCK2 PICK SELECT ROLL</li>
484 <li><em>Memory</em>These words allocate, free, and manipulate memory
485 areas outside the stack.<br/>The words are: MALLOC FREE GET PUT</li>
486 <li><em>Control</em>: These words alter the normal left to right flow
487 of execution.<br/>The words are: IF ELSE ENDIF WHILE END RETURN EXIT RECURSE</li>
488 <li><em>I/O</em>: These words perform output on the standard output
489 and input on the standard input. No other I/O is possible in Stacker.
490 <br/>The words are: SPACE TAB CR >s >d >c <s <d <c.</li>
492 <p>While you may be familiar with many of these operations from other
493 programming languages, a careful review of their semantics is important
494 for correct programming in Stacker. Of most importance is the effect
495 that each of these built-in words has on the global stack. The effect is
496 not always intuitive. To better describe the effects, we'll borrow from Forth the idiom of
497 describing the effect on the stack with:</p>
498 <p><code> BEFORE -- AFTER </code></p>
499 <p>That is, to the left of the -- is a representation of the stack before
500 the operation. To the right of the -- is a representation of the stack
501 after the operation. In the table below that describes the operation of
502 each of the built in words, we will denote the elements of the stack
503 using the following construction:</p>
505 <li><em>b</em> - a boolean truth value</li>
506 <li><em>w</em> - a normal integer valued word.</li>
507 <li><em>s</em> - a pointer to a string value</li>
508 <li><em>p</em> - a pointer to a malloc'd memory block</li>
511 <div class="doc_text" >
512 <table class="doc_table" style="border: 2px solid blue; border-collapse: collapse;" >
513 <tr class="doc_table"><td colspan="4" style="border: 2px solid blue">Definition Of Operation Of Built In Words</td></tr>
514 <tr class="doc_table"><td colspan="4" style="border: 2px solid blue"><b>LOGICAL OPERATIONS</b></td></tr>
515 <tr class="doc_table">
516 <td style="border: 2px solid blue"><u>Word</u></td>
517 <td style="border: 2px solid blue"><u>Name</u></td>
518 <td style="border: 2px solid blue"><u>Operation</u></td>
519 <td style="border: 2px solid blue"><u>Description</u></td>
521 <tr class="doc_table"><td style="border: 2px solid blue"><</td>
522 <td style="border: 2px solid blue">LT</td>
523 <td style="border: 2px solid blue">w1 w2 -- b</td>
524 <td style="border: 2px solid blue">Two values (w1 and w2) are popped off the stack and
525 compared. If w1 is less than w2, TRUE is pushed back on
526 the stack, otherwise FALSE is pushed back on the stack.</td>
528 <tr><td style="border: 2px solid blue">></td>
529 <td style="border: 2px solid blue">GT</td>
530 <td style="border: 2px solid blue">w1 w2 -- b</td>
531 <td style="border: 2px solid blue">Two values (w1 and w2) are popped off the stack and
532 compared. If w1 is greater than w2, TRUE is pushed back on
533 the stack, otherwise FALSE is pushed back on the stack.</td>
535 <tr><td style="border: 2px solid blue">>=</td>
536 <td style="border: 2px solid blue">GE</td>
537 <td style="border: 2px solid blue">w1 w2 -- b</td>
538 <td style="border: 2px solid blue">Two values (w1 and w2) are popped off the stack and
539 compared. If w1 is greater than or equal to w2, TRUE is
540 pushed back on the stack, otherwise FALSE is pushed back
543 <tr><td style="border: 2px solid blue"><=</td>
544 <td style="border: 2px solid blue">LE</td>
545 <td style="border: 2px solid blue">w1 w2 -- b</td>
546 <td style="border: 2px solid blue">Two values (w1 and w2) are popped off the stack and
547 compared. If w1 is less than or equal to w2, TRUE is
548 pushed back on the stack, otherwise FALSE is pushed back
551 <tr><td style="border: 2px solid blue">=</td>
552 <td style="border: 2px solid blue">EQ</td>
553 <td style="border: 2px solid blue">w1 w2 -- b</td>
554 <td style="border: 2px solid blue">Two values (w1 and w2) are popped off the stack and
555 compared. If w1 is equal to w2, TRUE is
556 pushed back on the stack, otherwise FALSE is pushed back
559 <tr><td style="border: 2px solid blue"><></td>
560 <td style="border: 2px solid blue">NE</td>
561 <td style="border: 2px solid blue">w1 w2 -- b</td>
562 <td style="border: 2px solid blue">Two values (w1 and w2) are popped off the stack and
563 compared. If w1 is equal to w2, TRUE is
564 pushed back on the stack, otherwise FALSE is pushed back
567 <tr><td style="border: 2px solid blue">FALSE</td>
568 <td style="border: 2px solid blue">FALSE</td>
569 <td style="border: 2px solid blue"> -- b</td>
570 <td style="border: 2px solid blue">The boolean value FALSE (0) is pushed on to the stack.</td>
572 <tr><td style="border: 2px solid blue">TRUE</td>
573 <td style="border: 2px solid blue">TRUE</td>
574 <td style="border: 2px solid blue"> -- b</td>
575 <td style="border: 2px solid blue">The boolean value TRUE (-1) is pushed on to the stack.</td>
577 <tr><td colspan="4"><b>BITWISE OPERATORS</b></td></tr>
579 <td style="border: 2px solid blue"><u>Word</u></td>
580 <td style="border: 2px solid blue"><u>Name</u></td>
581 <td style="border: 2px solid blue"><u>Operation</u></td>
582 <td style="border: 2px solid blue"><u>Description</u></td>
584 <tr><td style="border: 2px solid blue"><<</td>
585 <td style="border: 2px solid blue">SHL</td>
586 <td style="border: 2px solid blue">w1 w2 -- w1<<w2</td>
587 <td style="border: 2px solid blue">Two values (w1 and w2) are popped off the stack. The w2
588 operand is shifted left by the number of bits given by the
589 w1 operand. The result is pushed back to the stack.</td>
591 <tr><td style="border: 2px solid blue">>></td>
592 <td style="border: 2px solid blue">SHR</td>
593 <td style="border: 2px solid blue">w1 w2 -- w1>>w2</td>
594 <td style="border: 2px solid blue">Two values (w1 and w2) are popped off the stack. The w2
595 operand is shifted right by the number of bits given by the
596 w1 operand. The result is pushed back to the stack.</td>
598 <tr><td style="border: 2px solid blue">OR</td>
599 <td style="border: 2px solid blue">OR</td>
600 <td style="border: 2px solid blue">w1 w2 -- w2|w1</td>
601 <td style="border: 2px solid blue">Two values (w1 and w2) are popped off the stack. The values
602 are bitwise OR'd together and pushed back on the stack. This is
603 not a logical OR. The sequence 1 2 OR yields 3 not 1.</td>
605 <tr><td style="border: 2px solid blue">AND</td>
606 <td style="border: 2px solid blue">AND</td>
607 <td style="border: 2px solid blue">w1 w2 -- w2&w1</td>
608 <td style="border: 2px solid blue">Two values (w1 and w2) are popped off the stack. The values
609 are bitwise AND'd together and pushed back on the stack. This is
610 not a logical AND. The sequence 1 2 AND yields 0 not 1.</td>
612 <tr><td style="border: 2px solid blue">XOR</td>
613 <td style="border: 2px solid blue">XOR</td>
614 <td style="border: 2px solid blue">w1 w2 -- w2^w1</td>
615 <td style="border: 2px solid blue">Two values (w1 and w2) are popped off the stack. The values
616 are bitwise exclusive OR'd together and pushed back on the stack.
617 For example, The sequence 1 3 XOR yields 2.</td>
619 <tr><td colspan="4"><b>ARITHMETIC OPERATORS</b></td></tr>
621 <td style="border: 2px solid blue"><u>Word</u></td>
622 <td style="border: 2px solid blue"><u>Name</u></td>
623 <td style="border: 2px solid blue"><u>Operation</u></td>
624 <td style="border: 2px solid blue"><u>Description</u></td>
626 <tr><td style="border: 2px solid blue">ABS</td>
627 <td style="border: 2px solid blue">ABS</td>
628 <td style="border: 2px solid blue">w -- |w|</td>
629 <td style="border: 2px solid blue">One value s popped off the stack; its absolute value is computed
630 and then pushed on to the stack. If w1 is -1 then w2 is 1. If w1 is
631 1 then w2 is also 1.</td>
633 <tr><td style="border: 2px solid blue">NEG</td>
634 <td style="border: 2px solid blue">NEG</td>
635 <td style="border: 2px solid blue">w -- -w</td>
636 <td style="border: 2px solid blue">One value is popped off the stack which is negated and then
637 pushed back on to the stack. If w1 is -1 then w2 is 1. If w1 is
638 1 then w2 is -1.</td>
640 <tr><td style="border: 2px solid blue"> + </td>
641 <td style="border: 2px solid blue">ADD</td>
642 <td style="border: 2px solid blue">w1 w2 -- w2+w1</td>
643 <td style="border: 2px solid blue">Two values are popped off the stack. Their sum is pushed back
646 <tr><td style="border: 2px solid blue"> - </td>
647 <td style="border: 2px solid blue">SUB</td>
648 <td style="border: 2px solid blue">w1 w2 -- w2-w1</td>
649 <td style="border: 2px solid blue">Two values are popped off the stack. Their difference is pushed back
652 <tr><td style="border: 2px solid blue"> * </td>
653 <td style="border: 2px solid blue">MUL</td>
654 <td style="border: 2px solid blue">w1 w2 -- w2*w1</td>
655 <td style="border: 2px solid blue">Two values are popped off the stack. Their product is pushed back
658 <tr><td style="border: 2px solid blue"> / </td>
659 <td style="border: 2px solid blue">DIV</td>
660 <td style="border: 2px solid blue">w1 w2 -- w2/w1</td>
661 <td style="border: 2px solid blue">Two values are popped off the stack. Their quotient is pushed back
664 <tr><td style="border: 2px solid blue">MOD</td>
665 <td style="border: 2px solid blue">MOD</td>
666 <td style="border: 2px solid blue">w1 w2 -- w2%w1</td>
667 <td style="border: 2px solid blue">Two values are popped off the stack. Their remainder after division
668 of w1 by w2 is pushed back on to the stack</td>
670 <tr><td style="border: 2px solid blue"> */ </td>
671 <td style="border: 2px solid blue">STAR_SLAH</td>
672 <td style="border: 2px solid blue">w1 w2 w3 -- (w3*w2)/w1</td>
673 <td style="border: 2px solid blue">Three values are popped off the stack. The product of w1 and w2 is
674 divided by w3. The result is pushed back on to the stack.</td>
676 <tr><td style="border: 2px solid blue"> ++ </td>
677 <td style="border: 2px solid blue">INCR</td>
678 <td style="border: 2px solid blue">w -- w+1</td>
679 <td style="border: 2px solid blue">One value is popped off the stack. It is incremented by one and then
680 pushed back on to the stack.</td>
682 <tr><td style="border: 2px solid blue"> -- </td>
683 <td style="border: 2px solid blue">DECR</td>
684 <td style="border: 2px solid blue">w -- w-1</td>
685 <td style="border: 2px solid blue">One value is popped off the stack. It is decremented by one and then
686 pushed back on to the stack.</td>
688 <tr><td style="border: 2px solid blue">MIN</td>
689 <td style="border: 2px solid blue">MIN</td>
690 <td style="border: 2px solid blue">w1 w2 -- (w2<w1?w2:w1)</td>
691 <td style="border: 2px solid blue">Two values are popped off the stack. The larger one is pushed back
692 on to the stack.</td>
694 <tr><td style="border: 2px solid blue">MAX</td>
695 <td style="border: 2px solid blue">MAX</td>
696 <td style="border: 2px solid blue">w1 w2 -- (w2>w1?w2:w1)</td>
697 <td style="border: 2px solid blue">Two values are popped off the stack. The larger value is pushed back
698 on to the stack.</td>
700 <tr><td colspan="4"><b>STACK MANIPULATION OPERATORS</b></td></tr>
702 <td style="border: 2px solid blue"><u>Word</u></td>
703 <td style="border: 2px solid blue"><u>Name</u></td>
704 <td style="border: 2px solid blue"><u>Operation</u></td>
705 <td style="border: 2px solid blue"><u>Description</u></td>
707 <tr><td style="border: 2px solid blue">DROP</td>
708 <td style="border: 2px solid blue">DROP</td>
709 <td style="border: 2px solid blue">w -- </td>
710 <td style="border: 2px solid blue">One value is popped off the stack.</td>
712 <tr><td style="border: 2px solid blue">DROP2</td>
713 <td style="border: 2px solid blue">DROP2</td>
714 <td style="border: 2px solid blue">w1 w2 -- </td>
715 <td style="border: 2px solid blue">Two values are popped off the stack.</td>
717 <tr><td style="border: 2px solid blue">NIP</td>
718 <td style="border: 2px solid blue">NIP</td>
719 <td style="border: 2px solid blue">w1 w2 -- w2</td>
720 <td style="border: 2px solid blue">The second value on the stack is removed from the stack. That is,
721 a value is popped off the stack and retained. Then a second value is
722 popped and the retained value is pushed.</td>
724 <tr><td style="border: 2px solid blue">NIP2</td>
725 <td style="border: 2px solid blue">NIP2</td>
726 <td style="border: 2px solid blue">w1 w2 w3 w4 -- w3 w4</td>
727 <td style="border: 2px solid blue">The third and fourth values on the stack are removed from it. That is,
728 two values are popped and retained. Then two more values are popped and
729 the two retained values are pushed back on.</td>
731 <tr><td style="border: 2px solid blue">DUP</td>
732 <td style="border: 2px solid blue">DUP</td>
733 <td style="border: 2px solid blue">w1 -- w1 w1</td>
734 <td style="border: 2px solid blue">One value is popped off the stack. That value is then pushed on to
735 the stack twice to duplicate the top stack vaue.</td>
737 <tr><td style="border: 2px solid blue">DUP2</td>
738 <td style="border: 2px solid blue">DUP2</td>
739 <td style="border: 2px solid blue">w1 w2 -- w1 w2 w1 w2</td>
740 <td style="border: 2px solid blue">The top two values on the stack are duplicated. That is, two vaues
741 are popped off the stack. They are alternately pushed back on the
742 stack twice each.</td>
744 <tr><td style="border: 2px solid blue">SWAP</td>
745 <td style="border: 2px solid blue">SWAP</td>
746 <td style="border: 2px solid blue">w1 w2 -- w2 w1</td>
747 <td style="border: 2px solid blue">The top two stack items are reversed in their order. That is, two
748 values are popped off the stack and pushed back on to the stack in
749 the opposite order they were popped.</td>
751 <tr><td style="border: 2px solid blue">SWAP2</td>
752 <td style="border: 2px solid blue">SWAP2</td>
753 <td style="border: 2px solid blue">w1 w2 w3 w4 -- w3 w4 w2 w1</td>
754 <td style="border: 2px solid blue">The top four stack items are swapped in pairs. That is, two values
755 are popped and retained. Then, two more values are popped and retained.
756 The values are pushed back on to the stack in the reverse order but
759 <tr><td style="border: 2px solid blue">OVER</td>
760 <td style="border: 2px solid blue">OVER</td>
761 <td style="border: 2px solid blue">w1 w2-- w1 w2 w1</td>
762 <td style="border: 2px solid blue">Two values are popped from the stack. They are pushed back
763 on to the stack in the order w1 w2 w1. This seems to cause the
764 top stack element to be duplicated "over" the next value.</td>
766 <tr><td style="border: 2px solid blue">OVER2</td>
767 <td style="border: 2px solid blue">OVER2</td>
768 <td style="border: 2px solid blue">w1 w2 w3 w4 -- w1 w2 w3 w4 w1 w2</td>
769 <td style="border: 2px solid blue">The third and fourth values on the stack are replicated on to the
770 top of the stack</td>
772 <tr><td style="border: 2px solid blue">ROT</td>
773 <td style="border: 2px solid blue">ROT</td>
774 <td style="border: 2px solid blue">w1 w2 w3 -- w2 w3 w1</td>
775 <td style="border: 2px solid blue">The top three values are rotated. That is, three value are popped
776 off the stack. They are pushed back on to the stack in the order
779 <tr><td style="border: 2px solid blue">ROT2</td>
780 <td style="border: 2px solid blue">ROT2</td>
781 <td style="border: 2px solid blue">w1 w2 w3 w4 w5 w6 -- w3 w4 w5 w6 w1 w2</td>
782 <td style="border: 2px solid blue">Like ROT but the rotation is done using three pairs instead of
785 <tr><td style="border: 2px solid blue">RROT</td>
786 <td style="border: 2px solid blue">RROT</td>
787 <td style="border: 2px solid blue">w1 w2 w3 -- w2 w3 w1</td>
788 <td style="border: 2px solid blue">Reverse rotation. Like ROT, but it rotates the other way around.
789 Essentially, the third element on the stack is moved to the top
792 <tr><td style="border: 2px solid blue">RROT2</td>
793 <td style="border: 2px solid blue">RROT2</td>
794 <td style="border: 2px solid blue">w1 w2 w3 w4 w5 w6 -- w3 w4 w5 w6 w1 w2</td>
795 <td style="border: 2px solid blue">Double reverse rotation. Like RROT but the rotation is done using
796 three pairs instead of three singles. The fifth and sixth stack
797 elements are moved to the first and second positions</td>
799 <tr><td style="border: 2px solid blue">TUCK</td>
800 <td style="border: 2px solid blue">TUCK</td>
801 <td style="border: 2px solid blue">w1 w2 -- w2 w1 w2</td>
802 <td style="border: 2px solid blue">Similar to OVER except that the second operand is being
803 replicated. Essentially, the first operand is being "tucked"
804 in between two instances of the second operand. Logically, two
805 values are popped off the stack. They are placed back on the
806 stack in the order w2 w1 w2.</td>
808 <tr><td style="border: 2px solid blue">TUCK2</td>
809 <td style="border: 2px solid blue">TUCK2</td>
810 <td style="border: 2px solid blue">w1 w2 w3 w4 -- w3 w4 w1 w2 w3 w4</td>
811 <td style="border: 2px solid blue">Like TUCK but a pair of elements is tucked over two pairs.
812 That is, the top two elements of the stack are duplicated and
813 inserted into the stack at the fifth and positions.</td>
815 <tr><td style="border: 2px solid blue">PICK</td>
816 <td style="border: 2px solid blue">PICK</td>
817 <td style="border: 2px solid blue">x0 ... Xn n -- x0 ... Xn x0</td>
818 <td style="border: 2px solid blue">The top of the stack is used as an index into the remainder of
819 the stack. The element at the nth position replaces the index
820 (top of stack). This is useful for cycling through a set of
821 values. Note that indexing is zero based. So, if n=0 then you
822 get the second item on the stack. If n=1 you get the third, etc.
823 Note also that the index is replaced by the n'th value. </td>
825 <tr><td style="border: 2px solid blue">SELECT</td>
826 <td style="border: 2px solid blue">SELECT</td>
827 <td style="border: 2px solid blue">m n X0..Xm Xm+1 .. Xn -- Xm</td>
828 <td style="border: 2px solid blue">This is like PICK but the list is removed and you need to specify
829 both the index and the size of the list. Careful with this one,
830 the wrong value for n can blow away a huge amount of the stack.</td>
832 <tr><td style="border: 2px solid blue">ROLL</td>
833 <td style="border: 2px solid blue">ROLL</td>
834 <td style="border: 2px solid blue">x0 x1 .. xn n -- x1 .. xn x0</td>
835 <td style="border: 2px solid blue"><b>Not Implemented</b>. This one has been left as an exercise to
836 the student. See <a href="#exercise">Exercise</a>. ROLL requires
837 a value, "n", to be on the top of the stack. This value specifies how
838 far into the stack to "roll". The n'th value is <em>moved</em> (not
839 copied) from its location and replaces the "n" value on the top of the
840 stack. In this way, all the values between "n" and x0 roll up the stack.
841 The operation of ROLL is a generalized ROT. The "n" value specifies
842 how much to rotate. That is, ROLL with n=1 is the same as ROT and
843 ROLL with n=2 is the same as ROT2.</td>
845 <tr><td colspan="4"><b>MEMORY OPERATORS</b></td></tr>
847 <td style="border: 2px solid blue"><u>Word</u></td>
848 <td style="border: 2px solid blue"><u>Name</u></td>
849 <td style="border: 2px solid blue"><u>Operation</u></td>
850 <td style="border: 2px solid blue"><u>Description</u></td>
852 <tr><td style="border: 2px solid blue">MALLOC</td>
853 <td style="border: 2px solid blue">MALLOC</td>
854 <td style="border: 2px solid blue">w1 -- p</td>
855 <td style="border: 2px solid blue">One value is popped off the stack. The value is used as the size
856 of a memory block to allocate. The size is in bytes, not words.
857 The memory allocation is completed and the address of the memory
858 block is pushed on to the stack.</td>
860 <tr><td style="border: 2px solid blue">FREE</td>
861 <td style="border: 2px solid blue">FREE</td>
862 <td style="border: 2px solid blue">p -- </td>
863 <td style="border: 2px solid blue">One pointer value is popped off the stack. The value should be
864 the address of a memory block created by the MALLOC operation. The
865 associated memory block is freed. Nothing is pushed back on the
866 stack. Many bugs can be created by attempting to FREE something
867 that isn't a pointer to a MALLOC allocated memory block. Make
868 sure you know what's on the stack. One way to do this is with
869 the following idiom:<br/>
870 <code>64 MALLOC DUP DUP (use ptr) DUP (use ptr) ... FREE</code>
871 <br/>This ensures that an extra copy of the pointer is placed on
872 the stack (for the FREE at the end) and that every use of the
873 pointer is preceded by a DUP to retain the copy for FREE.</td>
875 <tr><td style="border: 2px solid blue">GET</td>
876 <td style="border: 2px solid blue">GET</td>
877 <td style="border: 2px solid blue">w1 p -- w2 p</td>
878 <td style="border: 2px solid blue">An integer index and a pointer to a memory block are popped of
879 the block. The index is used to index one byte from the memory
880 block. That byte value is retained, the pointer is pushed again
881 and the retained value is pushed. Note that the pointer value
882 s essentially retained in its position so this doesn't count
883 as a "use ptr" in the FREE idiom.</td>
885 <tr><td style="border: 2px solid blue">PUT</td>
886 <td style="border: 2px solid blue">PUT</td>
887 <td style="border: 2px solid blue">w1 w2 p -- p </td>
888 <td style="border: 2px solid blue">An integer value is popped of the stack. This is the value to
889 be put into a memory block. Another integer value is popped of
890 the stack. This is the indexed byte in the memory block. A
891 pointer to the memory block is popped off the stack. The
892 first value (w1) is then converted to a byte and written
893 to the element of the memory block(p) at the index given
894 by the second value (w2). The pointer to the memory block is
895 pushed back on the stack so this doesn't count as a "use ptr"
896 in the FREE idiom.</td>
898 <tr><td colspan="4"><b>CONTROL FLOW OPERATORS</b></td></tr>
900 <td style="border: 2px solid blue"><u>Word</u></td>
901 <td style="border: 2px solid blue"><u>Name</u></td>
902 <td style="border: 2px solid blue"><u>Operation</u></td>
903 <td style="border: 2px solid blue"><u>Description</u></td>
905 <tr><td style="border: 2px solid blue">RETURN</td>
906 <td style="border: 2px solid blue">RETURN</td>
907 <td style="border: 2px solid blue"> -- </td>
908 <td style="border: 2px solid blue">The currently executing definition returns immediately to its caller.
909 Note that there is an implicit <code>RETURN</code> at the end of each
910 definition, logically located at the semi-colon. The sequence
911 <code>RETURN ;</code> is valid but redundant.</td>
913 <tr><td style="border: 2px solid blue">EXIT</td>
914 <td style="border: 2px solid blue">EXIT</td>
915 <td style="border: 2px solid blue">w1 -- </td>
916 <td style="border: 2px solid blue">A return value for the program is popped off the stack. The program is
917 then immediately terminated. This is normally an abnormal exit from the
918 program. For a normal exit (when <code>MAIN</code> finishes), the exit
919 code will always be zero in accordance with UNIX conventions.</td>
921 <tr><td style="border: 2px solid blue">RECURSE</td>
922 <td style="border: 2px solid blue">RECURSE</td>
923 <td style="border: 2px solid blue"> -- </td>
924 <td style="border: 2px solid blue">The currently executed definition is called again. This operation is
925 needed since the definition of a word doesn't exist until the semi colon
926 is reacher. Attempting something like:<br/>
927 <code> : recurser recurser ; </code><br/> will yield and error saying that
928 "recurser" is not defined yet. To accomplish the same thing, change this
930 <code> : recurser RECURSE ; </code></td>
932 <tr><td style="border: 2px solid blue">IF (words...) ENDIF</td>
933 <td style="border: 2px solid blue">IF (words...) ENDIF</td>
934 <td style="border: 2px solid blue">b -- </td>
935 <td style="border: 2px solid blue">A boolean value is popped of the stack. If it is non-zero then the "words..."
936 are executed. Otherwise, execution continues immediately following the ENDIF.</td>
938 <tr><td style="border: 2px solid blue">IF (words...) ELSE (words...) ENDIF</td>
939 <td style="border: 2px solid blue">IF (words...) ELSE (words...) ENDIF</td>
940 <td style="border: 2px solid blue">b -- </td>
941 <td style="border: 2px solid blue">A boolean value is popped of the stack. If it is non-zero then the "words..."
942 between IF and ELSE are executed. Otherwise the words between ELSE and ENDIF are
943 executed. In either case, after the (words....) have executed, execution continues
944 immediately following the ENDIF. </td>
946 <tr><td style="border: 2px solid blue">WHILE (words...) END</td>
947 <td style="border: 2px solid blue">WHILE (words...) END</td>
948 <td style="border: 2px solid blue">b -- b </td>
949 <td style="border: 2px solid blue">The boolean value on the top of the stack is examined. If it is non-zero then the
950 "words..." between WHILE and END are executed. Execution then begins again at the WHILE where another
951 boolean is popped off the stack. To prevent this operation from eating up the entire
952 stack, you should push on to the stack (just before the END) a boolean value that indicates
953 whether to terminate. Note that since booleans and integers can be coerced you can
954 use the following "for loop" idiom:<br/>
955 <code>(push count) WHILE (words...) -- END</code><br/>
957 <code>10 WHILE DUP >d -- END</code><br/>
958 This will print the numbers from 10 down to 1. 10 is pushed on the stack. Since that is
959 non-zero, the while loop is entered. The top of the stack (10) is duplicated and then
960 printed out with >d. The top of the stack is decremented, yielding 9 and control is
961 transfered back to the WHILE keyword. The process starts all over again and repeats until
962 the top of stack is decremented to 0 at which the WHILE test fails and control is
963 transfered to the word after the END.</td>
965 <tr><td colspan="4"><b>INPUT & OUTPUT OPERATORS</b></td></tr>
967 <td style="border: 2px solid blue"><u>Word</u></td>
968 <td style="border: 2px solid blue"><u>Name</u></td>
969 <td style="border: 2px solid blue"><u>Operation</u></td>
970 <td style="border: 2px solid blue"><u>Description</u></td>
972 <tr><td style="border: 2px solid blue">SPACE</td>
973 <td style="border: 2px solid blue">SPACE</td>
974 <td style="border: 2px solid blue"> -- </td>
975 <td style="border: 2px solid blue">A space character is put out. There is no stack effect.</td>
977 <tr><td style="border: 2px solid blue">TAB</td>
978 <td style="border: 2px solid blue">TAB</td>
979 <td style="border: 2px solid blue"> -- </td>
980 <td style="border: 2px solid blue">A tab character is put out. There is no stack effect.</td>
982 <tr><td style="border: 2px solid blue">CR</td>
983 <td style="border: 2px solid blue">CR</td>
984 <td style="border: 2px solid blue"> -- </td>
985 <td style="border: 2px solid blue">A carriage return character is put out. There is no stack effect.</td>
987 <tr><td style="border: 2px solid blue">>s</td>
988 <td style="border: 2px solid blue">OUT_STR</td>
989 <td style="border: 2px solid blue"> -- </td>
990 <td style="border: 2px solid blue">A string pointer is popped from the stack. It is put out.</td>
992 <tr><td style="border: 2px solid blue">>d</td>
993 <td style="border: 2px solid blue">OUT_STR</td>
994 <td style="border: 2px solid blue"> -- </td>
995 <td style="border: 2px solid blue">A value is popped from the stack. It is put out as a decimal integer.</td>
997 <tr><td style="border: 2px solid blue">>c</td>
998 <td style="border: 2px solid blue">OUT_CHR</td>
999 <td style="border: 2px solid blue"> -- </td>
1000 <td style="border: 2px solid blue">A value is popped from the stack. It is put out as an ASCII character.</td>
1002 <tr><td style="border: 2px solid blue"><s</td>
1003 <td style="border: 2px solid blue">IN_STR</td>
1004 <td style="border: 2px solid blue"> -- s </td>
1005 <td style="border: 2px solid blue">A string is read from the input via the scanf(3) format string " %as". The
1006 resulting string is pushed on to the stack.</td>
1008 <tr><td style="border: 2px solid blue"><d</td>
1009 <td style="border: 2px solid blue">IN_STR</td>
1010 <td style="border: 2px solid blue"> -- w </td>
1011 <td style="border: 2px solid blue">An integer is read from the input via the scanf(3) format string " %d". The
1012 resulting value is pushed on to the stack</td>
1014 <tr><td style="border: 2px solid blue"><c</td>
1015 <td style="border: 2px solid blue">IN_CHR</td>
1016 <td style="border: 2px solid blue"> -- w </td>
1017 <td style="border: 2px solid blue">A single character is read from the input via the scanf(3) format string
1018 " %c". The value is converted to an integer and pushed on to the stack.</td>
1020 <tr><td style="border: 2px solid blue">DUMP</td>
1021 <td style="border: 2px solid blue">DUMP</td>
1022 <td style="border: 2px solid blue"> -- </td>
1023 <td style="border: 2px solid blue">The stack contents are dumped to standard output. This is useful for
1024 debugging your definitions. Put DUMP at the beginning and end of a definition
1025 to see instantly the net effect of the definition.</td>
1029 <!-- ======================================================================= -->
1030 <div class="doc_section"> <a name="example">Prime: A Complete Example</a></div>
1031 <div class="doc_text">
1032 <p>The following fully documented program highlights many features of both
1033 the Stacker language and what is possible with LLVM. The program has two modes
1034 of operation. If you provide numeric arguments to the program, it checks to see
1035 if those arguments are prime numbers and prints out the results. Without any
1036 arguments, the program prints out any prime numbers it finds between 1 and one
1037 million (there's a lot of them!). The source code comments below tell the
1038 remainder of the story.
1041 <div class="doc_text">
1043 ################################################################################
1045 # Brute force prime number generator
1047 # This program is written in classic Stacker style, that being the style of a
1048 # stack. Start at the bottom and read your way up !
1050 # Reid Spencer - Nov 2003
1051 ################################################################################
1052 # Utility definitions
1053 ################################################################################
1055 : it_is_a_prime TRUE ;
1056 : it_is_not_a_prime FALSE ;
1057 : continue_loop TRUE ;
1060 ################################################################################
1061 # This definition tries an actual division of a candidate prime number. It
1062 # determines whether the division loop on this candidate should continue or
1065 # div - the divisor to try
1066 # p - the prime number we are working on
1068 # cont - should we continue the loop ?
1069 # div - the next divisor to try
1070 # p - the prime number we are working on
1071 ################################################################################
1073 DUP2 ( save div and p )
1074 SWAP ( swap to put divisor second on stack)
1075 MOD 0 = ( get remainder after division and test for 0 )
1077 exit_loop ( remainder = 0, time to exit )
1079 continue_loop ( remainder != 0, keep going )
1083 ################################################################################
1084 # This function tries one divisor by calling try_dividing. But, before doing
1085 # that it checks to see if the value is 1. If it is, it does not bother with
1086 # the division because prime numbers are allowed to be divided by one. The
1087 # top stack value (cont) is set to determine if the loop should continue on
1088 # this prime number or not.
1090 # cont - should we continue the loop (ignored)?
1091 # div - the divisor to try
1092 # p - the prime number we are working on
1094 # cont - should we continue the loop ?
1095 # div - the next divisor to try
1096 # p - the prime number we are working on
1097 ################################################################################
1099 DROP ( drop the loop continuation )
1100 DUP ( save the divisor )
1101 1 = IF ( see if divisor is == 1 )
1102 exit_loop ( no point dividing by 1 )
1104 try_dividing ( have to keep going )
1106 SWAP ( get divisor on top )
1108 SWAP ( put loop continuation back on top )
1111 ################################################################################
1112 # The number on the stack (p) is a candidate prime number that we must test to
1113 # determine if it really is a prime number. To do this, we divide it by every
1114 # number from one p-1 to 1. The division is handled in the try_one_divisor
1115 # definition which returns a loop continuation value (which we also seed with
1116 # the value 1). After the loop, we check the divisor. If it decremented all
1117 # the way to zero then we found a prime, otherwise we did not find one.
1119 # p - the prime number to check
1121 # yn - boolean indicating if its a prime or not
1122 # p - the prime number checked
1123 ################################################################################
1125 DUP ( duplicate to get divisor value ) )
1126 -- ( first divisor is one less than p )
1127 1 ( continue the loop )
1129 try_one_divisor ( see if its prime )
1131 DROP ( drop the continuation value )
1132 0 = IF ( test for divisor == 1 )
1133 it_is_a_prime ( we found one )
1135 it_is_not_a_prime ( nope, this one is not a prime )
1139 ################################################################################
1140 # This definition determines if the number on the top of the stack is a prime
1141 # or not. It does this by testing if the value is degenerate (<= 3) and
1142 # responding with yes, its a prime. Otherwise, it calls try_harder to actually
1143 # make some calculations to determine its primeness.
1145 # p - the prime number to check
1147 # yn - boolean indicating if its a prime or not
1148 # p - the prime number checked
1149 ################################################################################
1151 DUP ( save the prime number )
1152 3 >= IF ( see if its <= 3 )
1153 it_is_a_prime ( its <= 3 just indicate its prime )
1155 try_harder ( have to do a little more work )
1159 ################################################################################
1160 # This definition is called when it is time to exit the program, after we have
1161 # found a sufficiently large number of primes.
1164 ################################################################################
1166 "Finished" >s CR ( say we are finished )
1167 0 EXIT ( exit nicely )
1170 ################################################################################
1171 # This definition checks to see if the candidate is greater than the limit. If
1172 # it is, it terminates the program by calling done. Otherwise, it increments
1173 # the value and calls is_prime to determine if the candidate is a prime or not.
1174 # If it is a prime, it prints it. Note that the boolean result from is_prime is
1175 # gobbled by the following IF which returns the stack to just contining the
1176 # prime number just considered.
1178 # p - one less than the prime number to consider
1180 # p+1 - the prime number considered
1181 ################################################################################
1183 DUP ( save the prime number to consider )
1184 1000000 < IF ( check to see if we are done yet )
1185 done ( we are done, call "done" )
1187 ++ ( increment to next prime number )
1188 is_prime ( see if it is a prime )
1190 print ( it is, print it )
1194 ################################################################################
1195 # This definition starts at one, prints it out and continues into a loop calling
1196 # consider_prime on each iteration. The prime number candidate we are looking at
1197 # is incremented by consider_prime.
1200 ################################################################################
1202 "Prime Numbers: " >s CR ( say hello )
1203 DROP ( get rid of that pesky string )
1204 1 ( stoke the fires )
1205 print ( print the first one, we know its prime )
1206 WHILE ( loop while the prime to consider is non zero )
1207 consider_prime ( consider one prime number )
1211 ################################################################################
1213 ################################################################################
1215 >d ( Print the prime number )
1216 " is prime." ( push string to output )
1218 CR ( print carriage return )
1223 >d ( Print the prime number )
1224 " is NOT prime." ( push string to put out )
1225 >s ( put out the string )
1226 CR ( print carriage return )
1230 ################################################################################
1231 # This definition processes a single command line argument and determines if it
1232 # is a prime number or not.
1234 # n - number of arguments
1235 # arg1 - the prime numbers to examine
1237 # n-1 - one less than number of arguments
1238 # arg2 - we processed one argument
1239 ################################################################################
1241 -- ( decrement loop counter )
1242 SWAP ( get the argument value )
1243 is_prime IF ( determine if its prime )
1248 DROP ( done with that argument )
1251 ################################################################################
1252 # The MAIN program just prints a banner and processes its arguments.
1254 # n - number of arguments
1255 # ... - the arguments
1256 ################################################################################
1258 WHILE ( while there are more arguments )
1259 do_one_argument ( process one argument )
1263 ################################################################################
1264 # The MAIN program just prints a banner and processes its arguments.
1266 ################################################################################
1268 NIP ( get rid of the program name )
1269 -- ( reduce number of arguments )
1270 DUP ( save the arg counter )
1271 1 <= IF ( See if we got an argument )
1272 process_arguments ( tell user if they are prime )
1274 find_primes ( see how many we can find )
1276 0 ( push return code )
1281 <!-- ======================================================================= -->
1282 <div class="doc_section"> <a name="internal">Internals</a></div>
1283 <div class="doc_text">
1284 <p><b>This section is under construction.</b>
1285 <p>In the mean time, you can always read the code! It has comments!</p>
1287 <!-- ======================================================================= -->
1288 <div class="doc_subsection"> <a name="directory">Directory Structure</a></div>
1289 <div class="doc_text">
1290 <p>The source code, test programs, and sample programs can all be found
1291 under the LLVM "projects" directory. You will need to obtain the LLVM sources
1292 to find it (either via anonymous CVS or a tarball. See the
1293 <a href="GettingStarted.html">Getting Started</a> document).</p>
1294 <p>Under the "projects" directory there is a directory named "Stacker". That
1295 directory contains everything, as follows:</p>
1297 <li><em>lib</em> - contains most of the source code
1299 <li><em>lib/compiler</em> - contains the compiler library
1300 <li><em>lib/runtime</em> - contains the runtime library
1302 <li><em>test</em> - contains the test programs</li>
1303 <li><em>tools</em> - contains the Stacker compiler main program, stkrc
1305 <li><em>lib/stkrc</em> - contains the Stacker compiler main program
1307 <li><em>sample</em> - contains the sample programs</li>
1310 <!-- ======================================================================= -->
1311 <div class="doc_subsection"><a name="lexer"></a>The Lexer</div>
1312 <div class="doc_text">
1313 <p>See projects/Stacker/lib/compiler/Lexer.l</p>
1315 <!-- ======================================================================= -->
1316 <div class="doc_subsection"><a name="parser"></a>The Parser</div>
1317 <div class="doc_text">
1318 <p>See projects/Stacker/lib/compiler/StackerParser.y</p>
1320 <!-- ======================================================================= -->
1321 <div class="doc_subsection"><a name="compiler"></a>The Compiler</div>
1322 <div class="doc_text">
1323 <p>See projects/Stacker/lib/compiler/StackerCompiler.cpp</p>
1325 <!-- ======================================================================= -->
1326 <div class="doc_subsection"><a name="runtime"></a>The Runtime</div>
1327 <div class="doc_text">
1328 <p>See projects/Stacker/lib/runtime/stacker_rt.c</p>
1330 <!-- ======================================================================= -->
1331 <div class="doc_subsection"><a name="driver"></a>Compiler Driver</div>
1332 <div class="doc_text">
1333 <p>See projects/Stacker/tools/stkrc/stkrc.cpp</p>
1335 <!-- ======================================================================= -->
1336 <div class="doc_subsection"><a name="tests"></a>Test Programs</div>
1337 <div class="doc_text">
1338 <p>See projects/Stacker/test/*.st</p>
1340 <!-- ======================================================================= -->
1341 <div class="doc_subsection"> <a name="exercise">Exercise</a></div>
1342 <div class="doc_text">
1343 <p>As you may have noted from a careful inspection of the Built-In word
1344 definitions, the ROLL word is not implemented. This word was left out of
1345 Stacker on purpose so that it can be an exercise for the student. The exercise
1346 is to implement the ROLL functionality (in your own workspace) and build a test
1347 program for it. If you can implement ROLL, you understand Stacker and probably
1348 a fair amount about LLVM since this is one of the more complicated Stacker
1349 operations. The work will almost be completely limited to the
1350 <a href="#compiler">compiler</a>.
1351 <p>The ROLL word is already recognized by both the lexer and parser but ignored
1352 by the compiler. That means you don't have to futz around with figuring out how
1353 to get the keyword recognized. It already is. The part of the compiler that
1354 you need to implement is the <code>ROLL</code> case in the
1355 <code>StackerCompiler::handle_word(int)</code> method.</p> See the implementations
1356 of PICk and SELECT in the same method to get some hints about how to complete
1360 <!-- ======================================================================= -->
1361 <div class="doc_subsection"> <a name="todo">Things Remaining To Be Done</a></div>
1362 <div class="doc_text">
1363 <p>The initial implementation of Stacker has several deficiencies. If you're
1364 interested, here are some things that could be implemented better:</p>
1366 <li>Write an LLVM pass to compute the correct stack depth needed by the
1367 program. Currently the stack is set to a fixed number which means programs
1368 with large numbers of definitions might fail.</li>
1369 <li>Enhance to run on 64-bit platforms like SPARC. Right now the size of a
1370 pointer on 64-bit machines will cause incorrect results because of the 32-bit
1371 size of a stack element currently supported. This feature was not implemented
1372 because LLVM needs a union type to be able to support the different sizes
1373 correctly (portably and efficiently).</li>
1374 <li>Write an LLVM pass to optimize the use of the global stack. The code
1375 emitted currently is somewhat wasteful. It gets cleaned up a lot by existing
1376 passes but more could be done.</li>
1377 <li>Add -O -O1 -O2 and -O3 optimization switches to the compiler driver to
1378 allow LLVM optimization without using "opt."</li>
1379 <li>Make the compiler driver use the LLVM linking facilities (with IPO) before
1380 depending on GCC to do the final link.</li>
1381 <li>Clean up parsing. It doesn't handle errors very well.</li>
1382 <li>Rearrange the StackerCompiler.cpp code to make better use of inserting
1383 instructions before a block's terminating instruction. I didn't figure this
1384 technique out until I was nearly done with LLVM. As it is, its a bad example
1385 of how to insert instructions!</li>
1386 <li>Provide for I/O to arbitrary files instead of just stdin/stdout.</li>
1387 <li>Write additional built-in words; with inspiration from FORTH</li>
1388 <li>Write additional sample Stacker programs.</li>
1389 <li>Add your own compiler writing experiences and tips in the
1390 <a href="#lessons">Lessons I Learned About LLVM</a> section.</li>
1393 <!-- ======================================================================= -->
1395 <div class="doc_footer">
1396 <address><a href="mailto:rspencer@x10sys.com">Reid Spencer</a></address>
1397 <a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a>
1398 <br>Last modified: $Date$ </div>