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Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	1	<!DOCTYPE HTML PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
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				4	<title>Stacker: An Example Of Using LLVM</title>
				5	<link rel="stylesheet" href="llvm.css" type="text/css">
				6	</head>
				7	<body>
				8	<div class="doc_title">Stacker: An Example Of Using LLVM</div>
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	9	<hr>
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	10	<ol>
				11	<li><a href="#abstract">Abstract</a></li>
				12	<li><a href="#introduction">Introduction</a></li>
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	13	<li><a href="#lessons">Lessons I Learned About LLVM</a>
				14	<ol>
				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>
				22	</ol>
				23	</li>
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	24	<li><a href="#lexicon">The Stacker Lexicon</a>
				25	<ol>
				26	<li><a href="#stack">The Stack</a>
				27	<li><a href="#punctuation">Punctuation</a>
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	28	<li><a href="#comments">Comments</a>
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	29	<li><a href="#literals">Literals</a>
				30	<li><a href="#words">Words</a>
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	31	<li><a href="style">Standard Style</a>
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	32	<li><a href="#builtins">Built-Ins</a>
				33	</ol>
				34	</li>
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	35	<li><a href="#example">Prime: A Complete Example</a></li>
				36	<li><a href="#internal">Internal Code Details</a>
				37	<ol>
				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>
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	45	<li><a href="#exercise">Exercise</a></li>
				46	<li><a href="#todo">Things Remaining To Be Done</a></li>
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	47	</ol>
				48	</li>
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	49	</ol>
				50	<div class="doc_text">
				51	<p><b>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a> </b></p>
				52	<p> </p>
				53	</div>
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	54	<hr>
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	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
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	60	<a href="ProgrammersManual.html">LLVM Programmer's Manual</a>, 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
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	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
				65	compiler system.</p>
				66	</div>
				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, the author of Stacker wrote the entire
				74	compiler (language definition, lexer, parser, code generator, etc.) in
				75	about <em>four days</em>! That's important to know because it shows
				76	how quickly you can get a new
				77	language up when using LLVM. Furthermore, this was the <em >first</em>
				78	language the author ever created using LLVM. The learning curve is
				79	included in that four days.</p>
				80	<p>The language described here, Stacker, is Forth-like. Programs
				81	are simple collections of word definitions and the only thing definitions
				82	can do is manipulate a stack or generate I/O. Stacker is not a "real"
				83	programming language; its very simple. Although it is computationally
				84	complete, you wouldn't use it for your next big project. However,
				85	the fact that it is complete, its simple, and it <em>doesn't</em> have
				86	a C-like syntax make it useful for demonstration purposes. It shows
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	87	that LLVM could be applied to a wide variety of languages.</p>
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	88	<p>The basic notions behind stacker is very simple. There's a stack of
				89	integers (or character pointers) that the program manipulates. Pretty
				90	much the only thing the program can do is manipulate the stack and do
				91	some limited I/O operations. The language provides you with several
				92	built-in words that manipulate the stack in interesting ways. To get
				93	your feet wet, here's how you write the traditional "Hello, World"
				94	program in Stacker:</p>
				95	<p><code>: hello_world "Hello, World!" >s DROP CR ;<br>
				96	: MAIN hello_world ;<br></code></p>
				97	<p>This has two "definitions" (Stacker manipulates words, not
				98	functions and words have definitions): <code>MAIN</code> and <code>
				99	hello_world</code>. The <code>MAIN</code> definition is standard, it
				100	tells Stacker where to start. Here, <code>MAIN</code> is defined to
				101	simply invoke the word <code>hello_world</code>. The
				102	<code>hello_world</code> definition tells stacker to push the
				103	<code>"Hello, World!"</code> string onto the stack, print it out
				104	(<code>>s</code>), pop it off the stack (<code>DROP</code>), and
				105	finally print a carriage return (<code>CR</code>). Although
				106	<code>hello_world</code> uses the stack, its net effect is null. Well
				107	written Stacker definitions have that characteristic. </p>
				108	<p>Exercise for the reader: how could you make this a one line program?</p>
				109	</div>
				110	<!-- ======================================================================= -->
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	111	<div class="doc_section"><a name="lessons"></a>Lessons I Learned About LLVM</div>
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	112	<div class="doc_text">
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	113	<p>Stacker was written for two purposes: </p>
				114	<ol>
				115	<li>to get the author over the learning curve, and</li>
				116	<li>to provide a simple example of how to write a compiler using LLVM.</li>
				117	</ol>
				118	<p>During the development of Stacker, many lessons about LLVM were
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	119	learned. Those lessons are described in the following subsections.<p>
				120	</div>
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	121	<!-- ======================================================================= -->
				122	<div class="doc_subsection"><a name="value"></a>Everything's a Value!</div>
				123	<div class="doc_text">
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	124	<p>Although I knew that LLVM uses a Single Static Assignment (SSA) format,
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	125	it wasn't obvious to me how prevalent this idea was in LLVM until I really
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	126	started using it. Reading the <a href="ProgrammersManual.html">
				127	Programmer's Manual</a> and <a href="LangRef.html">Language Reference</a>
				128	I noted that most of the important LLVM IR (Intermediate Representation) C++
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	129	classes were derived from the Value class. The full power of that simple
				130	design only became fully understood once I started constructing executable
				131	expressions for Stacker.</p>
				132	<p>This really makes your programming go faster. Think about compiling code
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	133	for the following C/C++ expression: <code>(a\|b)*((x+1)/(y+1))</code>. Assuming
				134	the values are on the stack in the order a, b, x, y, this could be
				135	expressed in stacker as: <code>1 + SWAP 1 + / ROT2 OR *</code>.
				136	You could write a function using LLVM that computes this expression like this: </p>
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	137	<pre><code>
				138	Value*
				139	expression(BasicBlockbb, Value a, Value* b, Value* x, Value* y )
				140	{
				141	Instruction* tail = bb->getTerminator();
				142	ConstantSInt* one = ConstantSInt::get( Type::IntTy, 1);
				143	BinaryOperator* or1 =
				144	new BinaryOperator::create( Instruction::Or, a, b, "", tail );
				145	BinaryOperator* add1 =
				146	new BinaryOperator::create( Instruction::Add, x, one, "", tail );
				147	BinaryOperator* add2 =
				148	new BinaryOperator::create( Instruction::Add, y, one, "", tail );
				149	BinaryOperator* div1 =
				150	new BinaryOperator::create( Instruction::Div, add1, add2, "", tail);
				151	BinaryOperator* mult1 =
				152	new BinaryOperator::create( Instruction::Mul, or1, div1, "", tail );
				153
				154	return mult1;
				155	}
				156	</code></pre>
				157	<p>"Okay, big deal," you say. It is a big deal. Here's why. Note that I didn't
				158	have to tell this function which kinds of Values are being passed in. They could be
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	159	<code>Instruction</code>s, <code>Constant</code>s, <code>GlobalVariable</code>s,
				160	etc. 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 no case will
				163	you make a type error that gets passed through to the generated program.
				164	This <em>really</em> helps you write a compiler that always generates correct code!<p>
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	165	<p>The second point is that we don't have to worry about branching, registers,
				166	stack variables, saving partial results, etc. The instructions we create
				167	<em>are</em> the values we use. Note that all that was created in the above
				168	code is a Constant value and five operators. Each of the instructions <em>is</em>
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	169	the resulting value of that instruction. This saves a lot of time.</p>
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	170	<p>The lesson is this: <em>SSA form is very powerful: there is no difference
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	171	between a value and the instruction that created it.</em> This is fully
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	172	enforced by the LLVM IR. Use it to your best advantage.</p>
				173	</div>
				174	<!-- ======================================================================= -->
				175	<div class="doc_subsection"><a name="terminate"></a>Terminate Those Blocks!</div>
				176	<div class="doc_text">
				177	<p>I had to learn about terminating blocks the hard way: using the debugger
				178	to figure out what the LLVM verifier was trying to tell me and begging for
				179	help on the LLVMdev mailing list. I hope you avoid this experience.</p>
				180	<p>Emblazon this rule in your mind:</p>
				181	<ul>
				182	<li><em>All</em> <code>BasicBlock</code>s in your compiler <b>must</b> be
				183	terminated with a terminating instruction (branch, return, etc.).
				184	</li>
				185	</ul>
				186	<p>Terminating instructions are a semantic requirement of the LLVM IR. There
				187	is no facility for implicitly chaining together blocks placed into a function
				188	in the order they occur. Indeed, in the general case, blocks will not be
				189	added to the function in the order of execution because of the recursive
				190	way compilers are written.</p>
				191	<p>Furthermore, if you don't terminate your blocks, your compiler code will
				192	compile just fine. You won't find out about the problem until you're running
				193	the compiler and the module you just created fails on the LLVM Verifier.</p>
				194	</div>
				195	<!-- ======================================================================= -->
				196	<div class="doc_subsection"><a name="blocks"></a>Concrete Blocks</div>
				197	<div class="doc_text">
				198	<p>After a little initial fumbling around, I quickly caught on to how blocks
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	199	should be constructed. In general, here's what I learned:
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	200	<ol>
				201	<li><em>Create your blocks early.</em> While writing your compiler, you
				202	will encounter several situations where you know apriori that you will
				203	need several blocks. For example, if-then-else, switch, while and for
				204	statements in C/C++ all need multiple blocks for expression in LVVM.
				205	The rule is, create them early.</li>
				206	<li><em>Terminate your blocks early.</em> This just reduces the chances
				207	that you forget to terminate your blocks which is required (go
				208	<a href="#terminate">here</a> for more).
				209	<li><em>Use getTerminator() for instruction insertion.</em> I noticed early on
				210	that many of the constructors for the Instruction classes take an optional
				211	<code>insert_before</code> argument. At first, I thought this was a mistake
				212	because clearly the normal mode of inserting instructions would be one at
				213	a time <em>after</em> some other instruction, not <em>before</em>. However,
				214	if you hold on to your terminating instruction (or use the handy dandy
				215	<code>getTerminator()</code> method on a <code>BasicBlock</code>), it can
				216	always be used as the <code>insert_before</code> argument to your instruction
				217	constructors. This causes the instruction to automatically be inserted in
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	218	the RightPlace™ place, just before the terminating instruction. The
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	219	nice thing about this design is that you can pass blocks around and insert
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	220	new instructions into them without ever knowing what instructions came
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	221	before. This makes for some very clean compiler design.</li>
				222	</ol>
				223	<p>The foregoing is such an important principal, its worth making an idiom:</p>
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	224	<pre><code>
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	225	BasicBlock* bb = new BasicBlock();</li>
				226	bb->getInstList().push_back( new Branch( ... ) );
				227	new Instruction(..., bb->getTerminator() );
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	228	</code></pre>
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	229	<p>To make this clear, consider the typical if-then-else statement
				230	(see StackerCompiler::handle_if() method). We can set this up
				231	in a single function using LLVM in the following way: </p>
				232	<pre>
				233	using namespace llvm;
				234	BasicBlock*
				235	MyCompiler::handle_if( BasicBlock* bb, SetCondInst* condition )
				236	{
				237	// Create the blocks to contain code in the structure of if/then/else
				238	BasicBlock* then = new BasicBlock();
				239	BasicBlock* else = new BasicBlock();
				240	BasicBlock* exit = new BasicBlock();
				241
				242	// Insert the branch instruction for the "if"
				243	bb->getInstList().push_back( new BranchInst( then, else, condition ) );
				244
				245	// Set up the terminating instructions
				246	then->getInstList().push_back( new BranchInst( exit ) );
				247	else->getInstList().push_back( new BranchInst( exit ) );
				248
				249	// Fill in the then part .. details excised for brevity
				250	this->fill_in( then );
				251
				252	// Fill in the else part .. details excised for brevity
				253	this->fill_in( else );
				254
				255	// Return a block to the caller that can be filled in with the code
				256	// that follows the if/then/else construct.
				257	return exit;
				258	}
				259	</pre>
				260	<p>Presumably in the foregoing, the calls to the "fill_in" method would add
				261	the instructions for the "then" and "else" parts. They would use the third part
				262	of the idiom almost exclusively (inserting new instructions before the
				263	terminator). Furthermore, they could even recurse back to <code>handle_if</code>
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	264	should they encounter another if/then/else statement and it will just work.</p>
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	265	<p>Note how cleanly this all works out. In particular, the push_back methods on
				266	the <code>BasicBlock</code>'s instruction list. These are lists of type
				267	<code>Instruction</code> which also happen to be <code>Value</code>s. To create
				268	the "if" branch we merely instantiate a <code>BranchInst</code> that takes as
				269	arguments the blocks to branch to and the condition to branch on. The blocks
				270	act like branch labels! This new <code>BranchInst</code> terminates
				271	the <code>BasicBlock</code> provided as an argument. To give the caller a way
				272	to keep inserting after calling <code>handle_if</code> we create an "exit" block
				273	which is returned to the caller. Note that the "exit" block is used as the
				274	terminator for both the "then" and the "else" blocks. This gaurantees that no
				275	matter what else "handle_if" or "fill_in" does, they end up at the "exit" block.
				276	</p>
				277	</div>
				278	<!-- ======================================================================= -->
				279	<div class="doc_subsection"><a name="push_back"></a>push_back Is Your Friend</div>
				280	<div class="doc_text">
				281	<p>
				282	One of the first things I noticed is the frequent use of the "push_back"
				283	method on the various lists. This is so common that it is worth mentioning.
				284	The "push_back" inserts a value into an STL list, vector, array, etc. at the
				285	end. The method might have also been named "insert_tail" or "append".
				286	Althought I've used STL quite frequently, my use of push_back wasn't very
				287	high in other programs. In LLVM, you'll use it all the time.
				288	</p>
				289	</div>
				290	<!-- ======================================================================= -->
				291	<div class="doc_subsection"><a name="gep"></a>The Wily GetElementPtrInst</div>
				292	<div class="doc_text">
				293	<p>
				294	It took a little getting used to and several rounds of postings to the LLVM
				295	mail list to wrap my head around this instruction correctly. Even though I had
				296	read the Language Reference and Programmer's Manual a couple times each, I still
				297	missed a few <em>very</em> key points:
				298	</p>
				299	<ul>
				300	<li>GetElementPtrInst gives you back a Value for the last thing indexed</em>
				301	<li>All global variables in LLVM are <em>pointers</em>.
				302	<li>Pointers must also be dereferenced with the GetElementPtrInst instruction.
				303	</ul>
				304	<p>This means that when you look up an element in the global variable (assuming
				305	its a struct or array), you <em>must</em> deference the pointer first! For many
				306	things, this leads to the idiom:
				307	</p>
				308	<pre><code>
				309	std::vector<Value*> index_vector;
				310	index_vector.push_back( ConstantSInt::get( Type::LongTy, 0 );
				311	// ... push other indices ...
				312	GetElementPtrInst* gep = new GetElementPtrInst( ptr, index_vector );
				313	</code></pre>
				314	<p>For example, suppose we have a global variable whose type is [24 x int]. The
				315	variable itself represents a <em>pointer</em> to that array. To subscript the
				316	array, we need two indices, not just one. The first index (0) dereferences the
				317	pointer. The second index subscripts the array. If you're a "C" programmer, this
				318	will run against your grain because you'll naturally think of the global array
				319	variable and the address of its first element as the same. That tripped me up
				320	for a while until I realized that they really do differ .. by <em>type</em>.
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	321	Remember that LLVM is a strongly typed language itself. Everything
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	322	has a type. The "type" of the global variable is [24 x int]*. That is, its
				323	a pointer to an array of 24 ints. When you dereference that global variable with
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	324	a single (0) index, you now have a "[24 x int]" type. Although
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	325	the pointer value of the dereferenced global and the address of the zero'th element
				326	in the array will be the same, they differ in their type. The zero'th element has
				327	type "int" while the pointer value has type "[24 x int]".</p>
				328	<p>Get this one aspect of LLVM right in your head and you'll save yourself
				329	a lot of compiler writing headaches down the road.</p>
				330	</div>
				331	<!-- ======================================================================= -->
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	332	<div class="doc_subsection"><a name="linkage"></a>Getting Linkage Types Right</div>
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	333	<div class="doc_text">
				334	<p>Linkage types in LLVM can be a little confusing, especially if your compiler
				335	writing mind has affixed very hard concepts to particular words like "weak",
				336	"external", "global", "linkonce", etc. LLVM does <em>not</em> use the precise
				337	definitions of say ELF or GCC even though they share common terms. To be fair,
				338	the concepts are related and similar but not precisely the same. This can lead
				339	you to think you know what a linkage type represents but in fact it is slightly
				340	different. I recommend you read the
				341	<a href="LangRef.html#linkage"> Language Reference on this topic</a> very
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	342	carefully. Then, read it again.<p>
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	343	<p>Here are some handy tips that I discovered along the way:</p>
				344	<ul>
				345	<li>Unitialized means external. That is, the symbol is declared in the current
				346	module and can be used by that module but it is not defined by that module.</li>
				347	<li>Setting an initializer changes a global's linkage type from whatever it was
				348	to a normal, defind global (not external). You'll need to call the setLinkage()
				349	method to reset it if you specify the initializer after the GlobalValue has been
				350	constructed. This is important for LinkOnce and Weak linkage types.</li>
				351	<li>Appending linkage can be used to keep track of compilation information at
				352	runtime. It could be used, for example, to build a full table of all the C++
				353	virtual tables or hold the C++ RTTI data, or whatever. Appending linkage can
				354	only be applied to arrays. The arrays are concatenated together at link time.</li>
				355	</ul>
				356	</div>
				357	<!-- ======================================================================= -->
				358	<div class="doc_subsection"><a name="constants"></a>Constants Are Easier Than That!</div>
				359	<div class="doc_text">
				360	<p>
				361	Constants in LLVM took a little getting used to until I discovered a few utility
				362	functions in the LLVM IR that make things easier. Here's what I learned: </p>
				363	<ul>
				364	<li>Constants are Values like anything else and can be operands of instructions</li>
				365	<li>Integer constants, frequently needed can be created using the static "get"
				366	methods of the ConstantInt, ConstantSInt, and ConstantUInt classes. The nice thing
				367	about these is that you can "get" any kind of integer quickly.</li>
				368	<li>There's a special method on Constant class which allows you to get the null
				369	constant for <em>any</em> type. This is really handy for initializing large
				370	arrays or structures, etc.</li>
				371	</ul>
				372	</div>
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	373	<!-- ======================================================================= -->
				374	<div class="doc_section"> <a name="lexicon">The Stacker Lexicon</a></div>
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	375	<div class="doc_text"><p>This section describes the Stacker language</p></div>
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	376	<div class="doc_subsection"><a name="stack"></a>The Stack</div>
				377	<div class="doc_text">
				378	<p>Stacker definitions define what they do to the global stack. Before
				379	proceeding, a few words about the stack are in order. The stack is simply
				380	a global array of 32-bit integers or pointers. A global index keeps track
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	381	of the location of the top of the stack. All of this is hidden from the
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	382	programmer but it needs to be noted because it is the foundation of the
				383	conceptual programming model for Stacker. When you write a definition,
				384	you are, essentially, saying how you want that definition to manipulate
				385	the global stack.</p>
				386	<p>Manipulating the stack can be quite hazardous. There is no distinction
				387	given and no checking for the various types of values that can be placed
				388	on the stack. Automatic coercion between types is performed. In many
				389	cases this is useful. For example, a boolean value placed on the stack
				390	can be interpreted as an integer with good results. However, using a
				391	word that interprets that boolean value as a pointer to a string to
				392	print out will almost always yield a crash. Stacker simply leaves it
				393	to the programmer to get it right without any interference or hindering
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	394	on interpretation of the stack values. You've been warned. :) </p>
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	395	</div>
				396	<!-- ======================================================================= -->
				397	<div class="doc_subsection"> <a name="punctuation"></a>Punctuation</div>
				398	<div class="doc_text">
				399	<p>Punctuation in Stacker is very simple. The colon and semi-colon
				400	characters are used to introduce and terminate a definition
				401	(respectively). Except for <em>FORWARD</em> declarations, definitions
				402	are all you can specify in Stacker. Definitions are read left to right.
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	403	Immediately after the colon comes the name of the word being defined.
				404	The remaining words in the definition specify what the word does. The definition
				405	is terminated by a semi-colon.</p>
				406	<p>So, your typical definition will have the form:</p>
				407	<pre><code>: name ... ;</code></pre>
				408	<p>The <code>name</code> is up to you but it must start with a letter and contain
				409	only letters numbers and underscore. Names are case sensitive and must not be
				410	the same as the name of a built-in word. The <code>...</code> is replaced by
				411	the stack manipulting words that you wish define <code>name</code> as. <p>
				412	</div>
				413	<!-- ======================================================================= -->
				414	<div class="doc_subsection"><a name="comments"></a>Comments</div>
				415	<div class="doc_text">
				416	<p>Stacker supports two types of comments. A hash mark (#) starts a comment
				417	that extends to the end of the line. It is identical to the kind of comments
				418	commonly used in shell scripts. A pair of parentheses also surround a comment.
				419	In both cases, the content of the comment is ignored by the Stacker compiler. The
				420	following does nothing in Stacker.
				421	</p>
				422	<pre><code>
				423	# This is a comment to end of line
				424	( This is an enclosed comment )
				425	</code></pre>
				426	<p>See the <a href="#example">example</a> program to see how this works in
				427	a real program.</p>
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	428	</div>
				429	<!-- ======================================================================= -->
				430	<div class="doc_subsection"><a name="literals"></a>Literals</div>
				431	<div class="doc_text">
				432	<p>There are three kinds of literal values in Stacker. Integer, Strings,
				433	and Booleans. In each case, the stack operation is to simply push the
				434	value onto the stack. So, for example:<br/>
				435	<code> 42 " is the answer." TRUE </code><br/>
				436	will push three values onto the stack: the integer 42, the
				437	string " is the answer." and the boolean TRUE.</p>
				438	</div>
				439	<!-- ======================================================================= -->
				440	<div class="doc_subsection"><a name="words"></a>Words</div>
				441	<div class="doc_text">
				442	<p>Each definition in Stacker is composed of a set of words. Words are
				443	read and executed in order from left to right. There is very little
				444	checking in Stacker to make sure you're doing the right thing with
				445	the stack. It is assumed that the programmer knows how the stack
				446	transformation he applies will affect the program.</p>
				447	<p>Words in a definition come in two flavors: built-in and programmer
				448	defined. Simply mentioning the name of a previously defined or declared
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	449	programmer-defined word causes that word's definition to be invoked. It
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	450	is somewhat like a function call in other languages. The built-in
				451	words have various effects, described below.</p>
				452	<p>Sometimes you need to call a word before it is defined. For this, you can
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	453	use the <code>FORWARD</code> declaration. It looks like this:</p>
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	454	<p><code>FORWARD name ;</code></p>
				455	<p>This simply states to Stacker that "name" is the name of a definition
				456	that is defined elsewhere. Generally it means the definition can be found
				457	"forward" in the file. But, it doesn't have to be in the current compilation
				458	unit. Anything declared with <code>FORWARD</code> is an external symbol for
				459	linking.</p>
				460	</div>
				461	<!-- ======================================================================= -->
				462	<div class="doc_subsection"><a name="builtins"></a>Built In Words</div>
				463	<div class="doc_text">
				464	<p>The built-in words of the Stacker language are put in several groups
				465	depending on what they do. The groups are as follows:</p>
				466	<ol>
				467	<li><em>Logical</em>These words provide the logical operations for
				468	comparing stack operands.<br/>The words are: < > <= >=
				469	= <> true false.</li>
				470	<li><em>Bitwise</em>These words perform bitwise computations on
				471	their operands. <br/> The words are: << >> XOR AND NOT</li>
				472	<li><em>Arithmetic</em>These words perform arithmetic computations on
				473	their operands. <br/> The words are: ABS NEG + - * / MOD */ ++ -- MIN MAX</li>
				474	<li><em>Stack</em>These words manipulate the stack directly by moving
				475	its elements around.<br/> The words are: DROP DUP SWAP OVER ROT DUP2 DROP2 PICK TUCK</li>
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	476	<li><em>Memory</em>These words allocate, free and manipulate memory
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	477	areas outside the stack.<br/>The words are: MALLOC FREE GET PUT</li>
				478	<li><em>Control</em>These words alter the normal left to right flow
				479	of execution.<br/>The words are: IF ELSE ENDIF WHILE END RETURN EXIT RECURSE</li>
				480	<li><em>I/O</em> These words perform output on the standard output
				481	and input on the standard input. No other I/O is possible in Stacker.
				482	<br/>The words are: SPACE TAB CR >s >d >c <s <d <c.</li>
				483	</ol>
				484	<p>While you may be familiar with many of these operations from other
				485	programming languages, a careful review of their semantics is important
				486	for correct programming in Stacker. Of most importance is the effect
				487	that each of these built-in words has on the global stack. The effect is
				488	not always intuitive. To better describe the effects, we'll borrow from Forth the idiom of
				489	describing the effect on the stack with:</p>
				490	<p><code> BEFORE -- AFTER </code></p>
				491	<p>That is, to the left of the -- is a representation of the stack before
				492	the operation. To the right of the -- is a representation of the stack
				493	after the operation. In the table below that describes the operation of
				494	each of the built in words, we will denote the elements of the stack
				495	using the following construction:</p>
				496	<ol>
				497	<li><em>b</em> - a boolean truth value</li>
				498	<li><em>w</em> - a normal integer valued word.</li>
				499	<li><em>s</em> - a pointer to a string value</li>
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	500	<li><em>p</em> - a pointer to a malloc'd memory block</li>
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	501	</ol>
				502	</div>
				503	<div class="doc_text">
				504	<table class="doc_table" >
				505	<tr class="doc_table"><td colspan="4">Definition Of Operation Of Built In Words</td></tr>
				506	<tr class="doc_table"><td colspan="4">LOGICAL OPERATIONS</td></tr>
				507	<tr class="doc_table"><td>Word</td><td>Name</td><td>Operation</td><td>Description</td></tr>
				508	<tr class="doc_table"><td><</td>
				509	<td>LT</td>
				510	<td>w1 w2 -- b</td>
				511	<td>Two values (w1 and w2) are popped off the stack and
				512	compared. If w1 is less than w2, TRUE is pushed back on
				513	the stack, otherwise FALSE is pushed back on the stack.</td>
				514	</tr>
				515	<tr><td>></td>
				516	<td>GT</td>
				517	<td>w1 w2 -- b</td>
				518	<td>Two values (w1 and w2) are popped off the stack and
				519	compared. If w1 is greater than w2, TRUE is pushed back on
				520	the stack, otherwise FALSE is pushed back on the stack.</td>
				521	</tr>
				522	<tr><td>>=</td>
				523	<td>GE</td>
				524	<td>w1 w2 -- b</td>
				525	<td>Two values (w1 and w2) are popped off the stack and
				526	compared. If w1 is greater than or equal to w2, TRUE is
				527	pushed back on the stack, otherwise FALSE is pushed back
				528	on the stack.</td>
				529	</tr>
				530	<tr><td><=</td>
				531	<td>LE</td>
				532	<td>w1 w2 -- b</td>
				533	<td>Two values (w1 and w2) are popped off the stack and
				534	compared. If w1 is less than or equal to w2, TRUE is
				535	pushed back on the stack, otherwise FALSE is pushed back
				536	on the stack.</td>
				537	</tr>
				538	<tr><td>=</td>
				539	<td>EQ</td>
				540	<td>w1 w2 -- b</td>
				541	<td>Two values (w1 and w2) are popped off the stack and
				542	compared. If w1 is equal to w2, TRUE is
				543	pushed back on the stack, otherwise FALSE is pushed back
				544	</td>
				545	</tr>
				546	<tr><td><></td>
				547	<td>NE</td>
				548	<td>w1 w2 -- b</td>
				549	<td>Two values (w1 and w2) are popped off the stack and
				550	compared. If w1 is equal to w2, TRUE is
				551	pushed back on the stack, otherwise FALSE is pushed back
				552	</td>
				553	</tr>
				554	<tr><td>FALSE</td>
				555	<td>FALSE</td>
				556	<td> -- b</td>
				557	<td>The boolean value FALSE (0) is pushed onto the stack.</td>
				558	</tr>
				559	<tr><td>TRUE</td>
				560	<td>TRUE</td>
				561	<td> -- b</td>
				562	<td>The boolean value TRUE (-1) is pushed onto the stack.</td>
				563	</tr>
				564	<tr><td colspan="4">BITWISE OPERATIONS</td></tr>
				565	<tr><td>Word</td><td>Name</td><td>Operation</td><td>Description</td></tr>
				566	<tr><td><<</td>
				567	<td>SHL</td>
				568	<td>w1 w2 -- w1<<w2</td>
				569	<td>Two values (w1 and w2) are popped off the stack. The w2
				570	operand is shifted left by the number of bits given by the
				571	w1 operand. The result is pushed back to the stack.</td>
				572	</tr>
				573	<tr><td>>></td>
				574	<td>SHR</td>
				575	<td>w1 w2 -- w1>>w2</td>
				576	<td>Two values (w1 and w2) are popped off the stack. The w2
				577	operand is shifted right by the number of bits given by the
				578	w1 operand. The result is pushed back to the stack.</td>
				579	</tr>
				580	<tr><td>OR</td>
				581	<td>OR</td>
				582	<td>w1 w2 -- w2\|w1</td>
				583	<td>Two values (w1 and w2) are popped off the stack. The values
				584	are bitwise OR'd together and pushed back on the stack. This is
				585	not a logical OR. The sequence 1 2 OR yields 3 not 1.</td>
				586	</tr>
				587	<tr><td>AND</td>
				588	<td>AND</td>
				589	<td>w1 w2 -- w2&w1</td>
				590	<td>Two values (w1 and w2) are popped off the stack. The values
				591	are bitwise AND'd together and pushed back on the stack. This is
				592	not a logical AND. The sequence 1 2 AND yields 0 not 1.</td>
				593	</tr>
				594	<tr><td>XOR</td>
				595	<td>XOR</td>
				596	<td>w1 w2 -- w2^w1</td>
				597	<td>Two values (w1 and w2) are popped off the stack. The values
				598	are bitwise exclusive OR'd together and pushed back on the stack.
				599	For example, The sequence 1 3 XOR yields 2.</td>
				600	</tr>
				601	<tr><td colspan="4">ARITHMETIC OPERATIONS</td></tr>
				602	<tr><td>Word</td><td>Name</td><td>Operation</td><td>Description</td></tr>
				603	<tr><td>ABS</td>
				604	<td>ABS</td>
				605	<td>w -- \|w\|</td>
				606	<td>One value s popped off the stack; its absolute value is computed
				607	and then pushed onto the stack. If w1 is -1 then w2 is 1. If w1 is
				608	1 then w2 is also 1.</td>
				609	</tr>
				610	<tr><td>NEG</td>
				611	<td>NEG</td>
				612	<td>w -- -w</td>
				613	<td>One value is popped off the stack which is negated and then
				614	pushed back onto the stack. If w1 is -1 then w2 is 1. If w1 is
				615	1 then w2 is -1.</td>
				616	</tr>
				617	<tr><td> + </td>
				618	<td>ADD</td>
				619	<td>w1 w2 -- w2+w1</td>
				620	<td>Two values are popped off the stack. Their sum is pushed back
				621	onto the stack</td>
				622	</tr>
				623	<tr><td> - </td>
				624	<td>SUB</td>
				625	<td>w1 w2 -- w2-w1</td>
				626	<td>Two values are popped off the stack. Their difference is pushed back
				627	onto the stack</td>
				628	</tr>
				629	<tr><td> * </td>
				630	<td>MUL</td>
				631	<td>w1 w2 -- w2*w1</td>
				632	<td>Two values are popped off the stack. Their product is pushed back
				633	onto the stack</td>
				634	</tr>
				635	<tr><td> / </td>
				636	<td>DIV</td>
				637	<td>w1 w2 -- w2/w1</td>
				638	<td>Two values are popped off the stack. Their quotient is pushed back
				639	onto the stack</td>
				640	</tr>
				641	<tr><td>MOD</td>
				642	<td>MOD</td>
				643	<td>w1 w2 -- w2%w1</td>
				644	<td>Two values are popped off the stack. Their remainder after division
				645	of w1 by w2 is pushed back onto the stack</td>
				646	</tr>
				647	<tr><td> */ </td>
				648	<td>STAR_SLAH</td>
				649	<td>w1 w2 w3 -- (w3*w2)/w1</td>
				650	<td>Three values are popped off the stack. The product of w1 and w2 is
				651	divided by w3. The result is pushed back onto the stack.</td>
				652	</tr>
				653	<tr><td> ++ </td>
				654	<td>INCR</td>
				655	<td>w -- w+1</td>
				656	<td>One value is popped off the stack. It is incremented by one and then
				657	pushed back onto the stack.</td>
				658	</tr>
				659	<tr><td> -- </td>
				660	<td>DECR</td>
				661	<td>w -- w-1</td>
				662	<td>One value is popped off the stack. It is decremented by one and then
				663	pushed back onto the stack.</td>
				664	</tr>
				665	<tr><td>MIN</td>
				666	<td>MIN</td>
				667	<td>w1 w2 -- (w2<w1?w2:w1)</td>
				668	<td>Two values are popped off the stack. The larger one is pushed back
				669	onto the stack.</td>
				670	</tr>
				671	<tr><td>MAX</td>
				672	<td>MAX</td>
				673	<td>w1 w2 -- (w2>w1?w2:w1)</td>
				674	<td>Two values are popped off the stack. The larger value is pushed back
				675	onto the stack.</td>
				676	</tr>
				677	<tr><td colspan="4">STACK MANIPULATION OPERATIONS</td></tr>
				678	<tr><td>Word</td><td>Name</td><td>Operation</td><td>Description</td></tr>
				679	<tr><td>DROP</td>
				680	<td>DROP</td>
				681	<td>w -- </td>
				682	<td>One value is popped off the stack.</td>
				683	</tr>
				684	<tr><td>DROP2</td>
				685	<td>DROP2</td>
				686	<td>w1 w2 -- </td>
				687	<td>Two values are popped off the stack.</td>
				688	</tr>
				689	<tr><td>NIP</td>
				690	<td>NIP</td>
				691	<td>w1 w2 -- w2</td>
				692	<td>The second value on the stack is removed from the stack. That is,
				693	a value is popped off the stack and retained. Then a second value is
				694	popped and the retained value is pushed.</td>
				695	</tr>
				696	<tr><td>NIP2</td>
				697	<td>NIP2</td>
				698	<td>w1 w2 w3 w4 -- w3 w4</td>
				699	<td>The third and fourth values on the stack are removed from it. That is,
				700	two values are popped and retained. Then two more values are popped and
				701	the two retained values are pushed back on.</td>
				702	</tr>
				703	<tr><td>DUP</td>
				704	<td>DUP</td>
				705	<td>w1 -- w1 w1</td>
				706	<td>One value is popped off the stack. That value is then pushed onto
				707	the stack twice to duplicate the top stack vaue.</td>
				708	</tr>
				709	<tr><td>DUP2</td>
				710	<td>DUP2</td>
				711	<td>w1 w2 -- w1 w2 w1 w2</td>
				712	<td>The top two values on the stack are duplicated. That is, two vaues
				713	are popped off the stack. They are alternately pushed back on the
				714	stack twice each.</td>
				715	</tr>
				716	<tr><td>SWAP</td>
				717	<td>SWAP</td>
				718	<td>w1 w2 -- w2 w1</td>
				719	<td>The top two stack items are reversed in their order. That is, two
				720	values are popped off the stack and pushed back onto the stack in
				721	the opposite order they were popped.</td>
				722	</tr>
				723	<tr><td>SWAP2</td>
				724	<td>SWAP2</td>
				725	<td>w1 w2 w3 w4 -- w3 w4 w2 w1</td>
				726	<td>The top four stack items are swapped in pairs. That is, two values
				727	are popped and retained. Then, two more values are popped and retained.
				728	The values are pushed back onto the stack in the reverse order but
				729	in pairs.</p>
				730	</tr>
				731	<tr><td>OVER</td>
				732	<td>OVER</td>
				733	<td>w1 w2-- w1 w2 w1</td>
				734	<td>Two values are popped from the stack. They are pushed back
				735	onto the stack in the order w1 w2 w1. This seems to cause the
				736	top stack element to be duplicated "over" the next value.</td>
				737	</tr>
				738	<tr><td>OVER2</td>
				739	<td>OVER2</td>
				740	<td>w1 w2 w3 w4 -- w1 w2 w3 w4 w1 w2</td>
				741	<td>The third and fourth values on the stack are replicated onto the
				742	top of the stack</td>
				743	</tr>
				744	<tr><td>ROT</td>
				745	<td>ROT</td>
				746	<td>w1 w2 w3 -- w2 w3 w1</td>
				747	<td>The top three values are rotated. That is, three value are popped
				748	off the stack. They are pushed back onto the stack in the order
				749	w1 w3 w2.</td>
				750	</tr>
				751	<tr><td>ROT2</td>
				752	<td>ROT2</td>
				753	<td>w1 w2 w3 w4 w5 w6 -- w3 w4 w5 w6 w1 w2</td>
				754	<td>Like ROT but the rotation is done using three pairs instead of
				755	three singles.</td>
				756	</tr>
				757	<tr><td>RROT</td>
				758	<td>RROT</td>
				759	<td>w1 w2 w3 -- w2 w3 w1</td>
				760	<td>Reverse rotation. Like ROT, but it rotates the other way around.
				761	Essentially, the third element on the stack is moved to the top
				762	of the stack.</td>
				763	</tr>
				764	<tr><td>RROT2</td>
				765	<td>RROT2</td>
				766	<td>w1 w2 w3 w4 w5 w6 -- w3 w4 w5 w6 w1 w2</td>
				767	<td>Double reverse rotation. Like RROT but the rotation is done using
				768	three pairs instead of three singles. The fifth and sixth stack
				769	elements are moved to the first and second positions</td>
				770	</tr>
				771	<tr><td>TUCK</td>
				772	<td>TUCK</td>
				773	<td>w1 w2 -- w2 w1 w2</td>
				774	<td>Similar to OVER except that the second operand is being
				775	replicated. Essentially, the first operand is being "tucked"
				776	in between two instances of the second operand. Logically, two
				777	values are popped off the stack. They are placed back on the
				778	stack in the order w2 w1 w2.</td>
				779	</tr>
				780	<tr><td>TUCK2</td>
				781	<td>TUCK2</td>
				782	<td>w1 w2 w3 w4 -- w3 w4 w1 w2 w3 w4</td>
				783	<td>Like TUCK but a pair of elements is tucked over two pairs.
				784	That is, the top two elements of the stack are duplicated and
				785	inserted into the stack at the fifth and positions.</td>
				786	</tr>
				787	<tr><td>PICK</td>
				788	<td>PICK</td>
				789	<td>x0 ... Xn n -- x0 ... Xn x0</td>
				790	<td>The top of the stack is used as an index into the remainder of
				791	the stack. The element at the nth position replaces the index
				792	(top of stack). This is useful for cycling through a set of
				793	values. Note that indexing is zero based. So, if n=0 then you
				794	get the second item on the stack. If n=1 you get the third, etc.
				795	Note also that the index is replaced by the n'th value. </td>
				796	</tr>
				797	<tr><td>SELECT</td>
				798	<td>SELECT</td>
				799	<td>m n X0..Xm Xm+1 .. Xn -- Xm</td>
				800	<td>This is like PICK but the list is removed and you need to specify
				801	both the index and the size of the list. Careful with this one,
				802	the wrong value for n can blow away a huge amount of the stack.</td>
				803	</tr>
				804	<tr><td>ROLL</td>
				805	<td>ROLL</td>
				806	<td>x0 x1 .. xn n -- x1 .. xn x0</td>
				807	<td><b>Not Implemented</b>. This one has been left as an exercise to
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	808	the student. See <a href="#exercise">Exercise</a>. ROLL requires
				809	a value, "n", to be on the top of the stack. This value specifies how
				810	far into the stack to "roll". The n'th value is <em>moved</em> (not
				811	copied) from its location and replaces the "n" value on the top of the
				812	stack. In this way, all the values between "n" and x0 roll up the stack.
				813	The operation of ROLL is a generalized ROT. The "n" value specifies
				814	how much to rotate. That is, ROLL with n=1 is the same as ROT and
				815	ROLL with n=2 is the same as ROT2.</td>
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	816	</tr>
				817	<tr><td colspan="4">MEMORY OPERATIONS</td></tr>
				818	<tr><td>Word</td><td>Name</td><td>Operation</td><td>Description</td></tr>
				819	<tr><td>MALLOC</td>
				820	<td>MALLOC</td>
				821	<td>w1 -- p</td>
				822	<td>One value is popped off the stack. The value is used as the size
				823	of a memory block to allocate. The size is in bytes, not words.
				824	The memory allocation is completed and the address of the memory
				825	block is pushed onto the stack.</td>
				826	</tr>
				827	<tr><td>FREE</td>
				828	<td>FREE</td>
				829	<td>p -- </td>
				830	<td>One pointer value is popped off the stack. The value should be
				831	the address of a memory block created by the MALLOC operation. The
				832	associated memory block is freed. Nothing is pushed back on the
				833	stack. Many bugs can be created by attempting to FREE something
				834	that isn't a pointer to a MALLOC allocated memory block. Make
				835	sure you know what's on the stack. One way to do this is with
				836	the following idiom:<br/>
				837	<code>64 MALLOC DUP DUP (use ptr) DUP (use ptr) ... FREE</code>
				838	<br/>This ensures that an extra copy of the pointer is placed on
				839	the stack (for the FREE at the end) and that every use of the
				840	pointer is preceded by a DUP to retain the copy for FREE.</td>
				841	</tr>
				842	<tr><td>GET</td>
				843	<td>GET</td>
				844	<td>w1 p -- w2 p</td>
				845	<td>An integer index and a pointer to a memory block are popped of
				846	the block. The index is used to index one byte from the memory
				847	block. That byte value is retained, the pointer is pushed again
				848	and the retained value is pushed. Note that the pointer value
				849	s essentially retained in its position so this doesn't count
				850	as a "use ptr" in the FREE idiom.</td>
				851	</tr>
				852	<tr><td>PUT</td>
				853	<td>PUT</td>
				854	<td>w1 w2 p -- p </td>
				855	<td>An integer value is popped of the stack. This is the value to
				856	be put into a memory block. Another integer value is popped of
				857	the stack. This is the indexed byte in the memory block. A
				858	pointer to the memory block is popped off the stack. The
				859	first value (w1) is then converted to a byte and written
				860	to the element of the memory block(p) at the index given
				861	by the second value (w2). The pointer to the memory block is
				862	pushed back on the stack so this doesn't count as a "use ptr"
				863	in the FREE idiom.</td>
				864	</tr>
				865	<tr><td colspan="4">CONTROL FLOW OPERATIONS</td></tr>
				866	<tr><td>Word</td><td>Name</td><td>Operation</td><td>Description</td></tr>
				867	<tr><td>RETURN</td>
				868	<td>RETURN</td>
				869	<td> -- </td>
				870	<td>The currently executing definition returns immediately to its caller.
				871	Note that there is an implicit <code>RETURN</code> at the end of each
				872	definition, logically located at the semi-colon. The sequence
				873	<code>RETURN ;</code> is valid but redundant.</td>
				874	</tr>
				875	<tr><td>EXIT</td>
				876	<td>EXIT</td>
				877	<td>w1 -- </td>
				878	<td>A return value for the program is popped off the stack. The program is
				879	then immediately terminated. This is normally an abnormal exit from the
				880	program. For a normal exit (when <code>MAIN</code> finishes), the exit
				881	code will always be zero in accordance with UNIX conventions.</td>
				882	</tr>
				883	<tr><td>RECURSE</td>
				884	<td>RECURSE</td>
				885	<td> -- </td>
				886	<td>The currently executed definition is called again. This operation is
				887	needed since the definition of a word doesn't exist until the semi colon
				888	is reacher. Attempting something like:<br/>
				889	<code> : recurser recurser ; </code><br/> will yield and error saying that
				890	"recurser" is not defined yet. To accomplish the same thing, change this
				891	to:<br/>
				892	<code> : recurser RECURSE ; </code></td>
				893	</tr>
				894	<tr><td>IF (words...) ENDIF</td>
				895	<td>IF (words...) ENDIF</td>
				896	<td>b -- </td>
				897	<td>A boolean value is popped of the stack. If it is non-zero then the "words..."
				898	are executed. Otherwise, execution continues immediately following the ENDIF.</td>
				899	</tr>
				900	<tr><td>IF (words...) ELSE (words...) ENDIF</td>
				901	<td>IF (words...) ELSE (words...) ENDIF</td>
				902	<td>b -- </td>
				903	<td>A boolean value is popped of the stack. If it is non-zero then the "words..."
				904	between IF and ELSE are executed. Otherwise the words between ELSE and ENDIF are
				905	executed. In either case, after the (words....) have executed, execution continues
				906	immediately following the ENDIF. </td>
				907	</tr>
				908	<tr><td>WHILE (words...) END</td>
				909	<td>WHILE (words...) END</td>
				910	<td>b -- b </td>
				911	<td>The boolean value on the top of the stack is examined. If it is non-zero then the
				912	"words..." between WHILE and END are executed. Execution then begins again at the WHILE where another
				913	boolean is popped off the stack. To prevent this operation from eating up the entire
				914	stack, you should push onto the stack (just before the END) a boolean value that indicates
				915	whether to terminate. Note that since booleans and integers can be coerced you can
				916	use the following "for loop" idiom:<br/>
				917	<code>(push count) WHILE (words...) -- END</code><br/>
				918	For example:<br/>
				919	<code>10 WHILE DUP >d -- END</code><br/>
				920	This will print the numbers from 10 down to 1. 10 is pushed on the stack. Since that is
				921	non-zero, the while loop is entered. The top of the stack (10) is duplicated and then
				922	printed out with >d. The top of the stack is decremented, yielding 9 and control is
				923	transfered back to the WHILE keyword. The process starts all over again and repeats until
				924	the top of stack is decremented to 0 at which the WHILE test fails and control is
				925	transfered to the word after the END.</td>
				926	</tr>
				927	<tr><td colspan="4">INPUT & OUTPUT OPERATIONS</td></tr>
				928	<tr><td>Word</td><td>Name</td><td>Operation</td><td>Description</td></tr>
				929	<tr><td>SPACE</td>
				930	<td>SPACE</td>
				931	<td> -- </td>
				932	<td>A space character is put out. There is no stack effect.</td>
				933	</tr>
				934	<tr><td>TAB</td>
				935	<td>TAB</td>
				936	<td> -- </td>
				937	<td>A tab character is put out. There is no stack effect.</td>
				938	</tr>
				939	<tr><td>CR</td>
				940	<td>CR</td>
				941	<td> -- </td>
				942	<td>A carriage return character is put out. There is no stack effect.</td>
				943	</tr>
				944	<tr><td>>s</td>
				945	<td>OUT_STR</td>
				946	<td> -- </td>
				947	<td>A string pointer is popped from the stack. It is put out.</td>
				948	</tr>
				949	<tr><td>>d</td>
				950	<td>OUT_STR</td>
				951	<td> -- </td>
				952	<td>A value is popped from the stack. It is put out as a decimal integer.</td>
				953	</tr>
				954	<tr><td>>c</td>
				955	<td>OUT_CHR</td>
				956	<td> -- </td>
				957	<td>A value is popped from the stack. It is put out as an ASCII character.</td>
				958	</tr>
				959	<tr><td><s</td>
				960	<td>IN_STR</td>
				961	<td> -- s </td>
				962	<td>A string is read from the input via the scanf(3) format string " %as". The
				963	resulting string is pushed onto the stack.</td>
				964	</tr>
				965	<tr><td><d</td>
				966	<td>IN_STR</td>
				967	<td> -- w </td>
				968	<td>An integer is read from the input via the scanf(3) format string " %d". The
				969	resulting value is pushed onto the stack</td>
				970	</tr>
				971	<tr><td><c</td>
				972	<td>IN_CHR</td>
				973	<td> -- w </td>
				974	<td>A single character is read from the input via the scanf(3) format string
				975	" %c". The value is converted to an integer and pushed onto the stack.</td>
				976	</tr>
				977	<tr><td>DUMP</td>
				978	<td>DUMP</td>
				979	<td> -- </td>
				980	<td>The stack contents are dumped to standard output. This is useful for
				981	debugging your definitions. Put DUMP at the beginning and end of a definition
				982	to see instantly the net effect of the definition.</td>
				983	</tr>
				984	</table>
				985	</div>
				986	<!-- ======================================================================= -->
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	987	<div class="doc_section"> <a name="example">Prime: A Complete Example</a></div>
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	988	<div class="doc_text">
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	989	<p>The following fully documented program highlights many features of both
				990	the Stacker language and what is possible with LLVM. The program has two modes
				991	of operations. If you provide numeric arguments to the program, it checks to see
				992	if those arguments are prime numbers, prints out the results. Without any
				993	aruments, the program prints out any prime numbers it finds between 1 and one
				994	million (there's a log of them!). The source code comments below tell the
				995	remainder of the story.
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	996	</p>
				997	</div>
				998	<div class="doc_text">
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	999	<pre><code>
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	1000	################################################################################
				1001	#
				1002	# Brute force prime number generator
				1003	#
				1004	# This program is written in classic Stacker style, that being the style of a
				1005	# stack. Start at the bottom and read your way up !
				1006	#
				1007	# Reid Spencer - Nov 2003
				1008	################################################################################
				1009	# Utility definitions
				1010	################################################################################
				1011	: print >d CR ;
				1012	: it_is_a_prime TRUE ;
				1013	: it_is_not_a_prime FALSE ;
				1014	: continue_loop TRUE ;
				1015	: exit_loop FALSE;
				1016
				1017	################################################################################
				1018	# This definition tryies an actual division of a candidate prime number. It
				1019	# determines whether the division loop on this candidate should continue or
				1020	# not.
				1021	# STACK<:
				1022	# div - the divisor to try
				1023	# p - the prime number we are working on
				1024	# STACK>:
				1025	# cont - should we continue the loop ?
				1026	# div - the next divisor to try
				1027	# p - the prime number we are working on
				1028	################################################################################
				1029	: try_dividing
				1030	DUP2 ( save div and p )
				1031	SWAP ( swap to put divisor second on stack)
				1032	MOD 0 = ( get remainder after division and test for 0 )
				1033	IF
				1034	exit_loop ( remainder = 0, time to exit )
				1035	ELSE
				1036	continue_loop ( remainder != 0, keep going )
				1037	ENDIF
				1038	;
				1039
				1040	################################################################################
				1041	# This function tries one divisor by calling try_dividing. But, before doing
				1042	# that it checks to see if the value is 1. If it is, it does not bother with
				1043	# the division because prime numbers are allowed to be divided by one. The
				1044	# top stack value (cont) is set to determine if the loop should continue on
				1045	# this prime number or not.
				1046	# STACK<:
				1047	# cont - should we continue the loop (ignored)?
				1048	# div - the divisor to try
				1049	# p - the prime number we are working on
				1050	# STACK>:
				1051	# cont - should we continue the loop ?
				1052	# div - the next divisor to try
				1053	# p - the prime number we are working on
				1054	################################################################################
				1055	: try_one_divisor
				1056	DROP ( drop the loop continuation )
				1057	DUP ( save the divisor )
				1058	1 = IF ( see if divisor is == 1 )
				1059	exit_loop ( no point dividing by 1 )
				1060	ELSE
				1061	try_dividing ( have to keep going )
				1062	ENDIF
				1063	SWAP ( get divisor on top )
				1064	-- ( decrement it )
				1065	SWAP ( put loop continuation back on top )
				1066	;
				1067
				1068	################################################################################
				1069	# The number on the stack (p) is a candidate prime number that we must test to
				1070	# determine if it really is a prime number. To do this, we divide it by every
				1071	# number from one p-1 to 1. The division is handled in the try_one_divisor
				1072	# definition which returns a loop continuation value (which we also seed with
				1073	# the value 1). After the loop, we check the divisor. If it decremented all
				1074	# the way to zero then we found a prime, otherwise we did not find one.
				1075	# STACK<:
				1076	# p - the prime number to check
				1077	# STACK>:
				1078	# yn - boolean indiating if its a prime or not
				1079	# p - the prime number checked
				1080	################################################################################
				1081	: try_harder
				1082	DUP ( duplicate to get divisor value ) )
				1083	-- ( first divisor is one less than p )
				1084	1 ( continue the loop )
				1085	WHILE
				1086	try_one_divisor ( see if its prime )
				1087	END
				1088	DROP ( drop the continuation value )
				1089	0 = IF ( test for divisor == 1 )
				1090	it_is_a_prime ( we found one )
				1091	ELSE
				1092	it_is_not_a_prime ( nope, this one is not a prime )
				1093	ENDIF
				1094	;
				1095
				1096	################################################################################
				1097	# This definition determines if the number on the top of the stack is a prime
				1098	# or not. It does this by testing if the value is degenerate (<= 3) and
				1099	# responding with yes, its a prime. Otherwise, it calls try_harder to actually
				1100	# make some calculations to determine its primeness.
				1101	# STACK<:
				1102	# p - the prime number to check
				1103	# STACK>:
				1104	# yn - boolean indicating if its a prime or not
				1105	# p - the prime number checked
				1106	################################################################################
				1107	: is_prime
				1108	DUP ( save the prime number )
				1109	3 >= IF ( see if its <= 3 )
				1110	it_is_a_prime ( its <= 3 just indicate its prime )
				1111	ELSE
				1112	try_harder ( have to do a little more work )
				1113	ENDIF
				1114	;
				1115
				1116	################################################################################
				1117	# This definition is called when it is time to exit the program, after we have
				1118	# found a sufficiently large number of primes.
				1119	# STACK<: ignored
				1120	# STACK>: exits
				1121	################################################################################
				1122	: done
				1123	"Finished" >s CR ( say we are finished )
				1124	0 EXIT ( exit nicely )
				1125	;
				1126
				1127	################################################################################
				1128	# This definition checks to see if the candidate is greater than the limit. If
				1129	# it is, it terminates the program by calling done. Otherwise, it increments
				1130	# the value and calls is_prime to determine if the candidate is a prime or not.
				1131	# If it is a prime, it prints it. Note that the boolean result from is_prime is
				1132	# gobbled by the following IF which returns the stack to just contining the
				1133	# prime number just considered.
				1134	# STACK<:
				1135	# p - one less than the prime number to consider
				1136	# STACK>
				1137	# p+1 - the prime number considered
				1138	################################################################################
				1139	: consider_prime
				1140	DUP ( save the prime number to consider )
				1141	1000000 < IF ( check to see if we are done yet )
				1142	done ( we are done, call "done" )
				1143	ENDIF
				1144	++ ( increment to next prime number )
				1145	is_prime ( see if it is a prime )
				1146	IF
				1147	print ( it is, print it )
				1148	ENDIF
				1149	;
				1150
				1151	################################################################################
				1152	# This definition starts at one, prints it out and continues into a loop calling
				1153	# consider_prime on each iteration. The prime number candidate we are looking at
				1154	# is incremented by consider_prime.
				1155	# STACK<: empty
				1156	# STACK>: empty
				1157	################################################################################
				1158	: find_primes
				1159	"Prime Numbers: " >s CR ( say hello )
				1160	DROP ( get rid of that pesky string )
				1161	1 ( stoke the fires )
				1162	print ( print the first one, we know its prime )
				1163	WHILE ( loop while the prime to consider is non zero )
				1164	consider_prime ( consider one prime number )
				1165	END
				1166	;
				1167
				1168	################################################################################
				1169	#
				1170	################################################################################
				1171	: say_yes
				1172	>d ( Print the prime number )
				1173	" is prime." ( push string to output )
				1174	>s ( output it )
				1175	CR ( print carriage return )
				1176	DROP ( pop string )
				1177	;
				1178
				1179	: say_no
				1180	>d ( Print the prime number )
				1181	" is NOT prime." ( push string to put out )
				1182	>s ( put out the string )
				1183	CR ( print carriage return )
				1184	DROP ( pop string )
				1185	;
				1186
				1187	################################################################################
				1188	# This definition processes a single command line argument and determines if it
				1189	# is a prime number or not.
				1190	# STACK<:
				1191	# n - number of arguments
				1192	# arg1 - the prime numbers to examine
				1193	# STACK>:
				1194	# n-1 - one less than number of arguments
				1195	# arg2 - we processed one argument
				1196	################################################################################
				1197	: do_one_argument
				1198	-- ( decrement loop counter )
				1199	SWAP ( get the argument value )
				1200	is_prime IF ( determine if its prime )
				1201	say_yes ( uhuh )
				1202	ELSE
				1203	say_no ( nope )
				1204	ENDIF
				1205	DROP ( done with that argument )
				1206	;
				1207
				1208	################################################################################
				1209	# The MAIN program just prints a banner and processes its arguments.
				1210	# STACK<:
				1211	# n - number of arguments
				1212	# ... - the arguments
				1213	################################################################################
				1214	: process_arguments
				1215	WHILE ( while there are more arguments )
				1216	do_one_argument ( process one argument )
				1217	END
				1218	;
				1219
				1220	################################################################################
				1221	# The MAIN program just prints a banner and processes its arguments.
				1222	# STACK<: arguments
				1223	################################################################################
				1224	: MAIN
				1225	NIP ( get rid of the program name )
				1226	-- ( reduce number of arguments )
				1227	DUP ( save the arg counter )
				1228	1 <= IF ( See if we got an argument )
				1229	process_arguments ( tell user if they are prime )
				1230	ELSE
				1231	find_primes ( see how many we can find )
				1232	ENDIF
				1233	0 ( push return code )
				1234	;
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	1235	</code>
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	1236	</pre>
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	1237	</div>
				1238	<!-- ======================================================================= -->
Brian Gaeke	07e89e4	2003-11-24 17:03:38 +0000	[diff] [blame]	1239	<div class="doc_section"> <a name="internal">Internals</a></div>
				1240	<div class="doc_text">
				1241	<p><b>This section is under construction.</b>
				1242	<p>In the mean time, you can always read the code! It has comments!</p>
				1243	</div>
				1244	<!-- ======================================================================= -->
				1245	<div class="doc_subsection"> <a name="directory">Directory Structure</a></div>
				1246	<div class="doc_text">
				1247	<p>The source code, test programs, and sample programs can all be found
				1248	under the LLVM "projects" directory. You will need to obtain the LLVM sources
				1249	to find it (either via anonymous CVS or a tarball. See the
				1250	<a href="GettingStarted.html">Getting Started</a> document).</p>
				1251	<p>Under the "projects" directory there is a directory named "stacker". That
				1252	directory contains everything, as follows:</p>
				1253	<ul>
				1254	<li><em>lib</em> - contains most of the source code
				1255	<ul>
				1256	<li><em>lib/compiler</em> - contains the compiler library
				1257	<li><em>lib/runtime</em> - contains the runtime library
				1258	</ul></li>
				1259	<li><em>test</em> - contains the test programs</li>
				1260	<li><em>tools</em> - contains the Stacker compiler main program, stkrc
				1261	<ul>
				1262	<li><em>lib/stkrc</em> - contains the Stacker compiler main program
				1263	</ul</li>
				1264	<li><em>sample</em> - contains the sample programs</li>
				1265	</ul>
				1266	</div>
				1267	<!-- ======================================================================= -->
				1268	<div class="doc_subsection"><a name="lexer"></a>The Lexer</div>
				1269	<div class="doc_text">
				1270	<p>See projects/Stacker/lib/compiler/Lexer.l</p>
				1271	</p></div>
				1272	<!-- ======================================================================= -->
				1273	<div class="doc_subsection"><a name="parser"></a>The Parser</div>
				1274	<div class="doc_text">
				1275	<p>See projects/Stacker/lib/compiler/StackerParser.y</p>
				1276	</p></div>
				1277	<!-- ======================================================================= -->
				1278	<div class="doc_subsection"><a name="compiler"></a>The Compiler</div>
				1279	<div class="doc_text">
				1280	<p>See projects/Stacker/lib/compiler/StackerCompiler.cpp</p>
				1281	</p></div>
				1282	<!-- ======================================================================= -->
				1283	<div class="doc_subsection"><a name="runtime"></a>The Runtime</div>
				1284	<div class="doc_text">
				1285	<p>See projects/Stacker/lib/runtime/stacker_rt.c</p>
				1286	</p></div>
				1287	<!-- ======================================================================= -->
				1288	<div class="doc_subsection"><a name="driver"></a>Compiler Driver</div>
				1289	<div class="doc_text">
				1290	<p>See projects/Stacker/tools/stkrc/stkrc.cpp</p>
				1291	</p></div>
				1292	<!-- ======================================================================= -->
				1293	<div class="doc_subsection"><a name="tests"></a>Test Programs</div>
				1294	<div class="doc_text">
				1295	<p>See projects/Stacker/test/*.st</p>
				1296	</p></div>
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	1297	<!-- ======================================================================= -->
Chris Lattner	e46d601	2003-11-25 01:35:06 +0000	[diff] [blame^]	1298	<div class="doc_subsection"> <a name="exercise">Exercise</a></div>
				1299	<div class="doc_text">
				1300	<p>As you may have noted from a careful inspection of the Built-In word
				1301	definitions, the ROLL word is not implemented. This word was left out of
				1302	Stacker on purpose so that it can be an exercise for the student. The exercise
				1303	is to implement the ROLL functionality (in your own workspace) and build a test
				1304	program for it. If you can implement ROLL you understand Stacker and probably
				1305	a fair amount about LLVM since this is one of the more complicated Stacker
				1306	operations. The work will almost be completely limited to the
				1307	<a href="#compiler">compiler</a>.
				1308	<p>The ROLL word is already recognized by both the lexer and parser but ignored
				1309	by the compiler. That means you don't have to futz around with figuring out how
				1310	to get the keyword recognized. It already is. The part of the compiler that
				1311	you need to implement is the <code>ROLL</code> case in the
				1312	<code>StackerCompiler::handle_word(int)</code> method.</p> See the implementations
				1313	of PICk and SELECT in the same method to get some hints about how to complete
				1314	this exercise.<p>
				1315	<p>Good luck!</p>
				1316	</div>
				1317	<!-- ======================================================================= -->
				1318	<div class="doc_subsection"> <a name="todo">Things Remaining To Be Done</a></div>
				1319	<div class="doc_text">
				1320	<p>The initial implementation of Stacker has several deficiencies. If you're
				1321	interested, here are some things that could be implemented better:</p>
				1322	<ol>
				1323	<li>Write an LLVM pass to compute the correct stack depth needed by the
				1324	program.</li>
				1325	<li>Write an LLVM pass to optimize the use of the global stack. The code
				1326	emitted currently is somewhat wasteful. It gets cleaned up a lot by existing
				1327	passes but more could be done.</li>
				1328	<li>Add -O -O1 -O2 and -O3 optimization switches to the compiler driver to
				1329	allow LLVM optimization without using "opt"</li>
				1330	<li>Make the compiler driver use the LLVM linking facilities (with IPO) before
				1331	depending on GCC to do the final link.</li>
				1332	<li>Clean up parsing. It doesn't handle errors very well.</li>
				1333	<li>Rearrange the StackerCompiler.cpp code to make better use of inserting
				1334	instructions before a block's terminating instruction. I didn't figure this
				1335	technique out until I was nearly done with LLVM. As it is, its a bad example
				1336	of how to insert instructions!</li>
				1337	<li>Provide for I/O to arbitrary files instead of just stdin/stdout.</li>
				1338	<li>Write additional built-in words.</li>
				1339	<li>Write additional sample Stacker programs.</li>
				1340	<li>Add your own compiler writing experiences and tips in the <a href="lessons">
				1341	Lessons I Learned About LLVM</a> section.</li>
				1342	</ol>
				1343	</div>
				1344	<!-- ======================================================================= -->
Brian Gaeke	9018148	2003-11-24 02:52:51 +0000	[diff] [blame]	1345	<hr>
				1346	<div class="doc_footer">
				1347	<address><a href="mailto:rspencer@x10sys.com">Reid Spencer</a></address>
				1348	<a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a>
				1349	<br>Last modified: $Date$ </div>
				1350	</body>
				1351	</html>