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1413 lines
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1413 lines
62 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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"http://www.w3.org/TR/html4/strict.dtd">
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<html>
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<head>
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<title>Stacker: An Example Of Using LLVM</title>
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<link rel="stylesheet" href="llvm.css" type="text/css">
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</head>
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<body>
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<div class="doc_title">Stacker: An Example Of Using LLVM</div>
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<ol>
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<li><a href="#abstract">Abstract</a></li>
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<li><a href="#introduction">Introduction</a></li>
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<li><a href="#lessons">Lessons I Learned About LLVM</a>
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<ol>
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<li><a href="#value">Everything's a Value!</a></li>
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<li><a href="#terminate">Terminate Those Blocks!</a></li>
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<li><a href="#blocks">Concrete Blocks</a></li>
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<li><a href="#push_back">push_back Is Your Friend</a></li>
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<li><a href="#gep">The Wily GetElementPtrInst</a></li>
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<li><a href="#linkage">Getting Linkage Types Right</a></li>
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<li><a href="#constants">Constants Are Easier Than That!</a></li>
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</ol></li>
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<li><a href="#lexicon">The Stacker Lexicon</a>
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<ol>
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<li><a href="#stack">The Stack</a></li>
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<li><a href="#punctuation">Punctuation</a></li>
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<li><a href="#comments">Comments</a></li>
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<li><a href="#literals">Literals</a></li>
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<li><a href="#words">Words</a></li>
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<li><a href="#style">Standard Style</a></li>
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<li><a href="#builtins">Built-Ins</a></li>
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</ol></li>
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<li><a href="#example">Prime: A Complete Example</a></li>
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<li><a href="#internal">Internal Code Details</a>
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<ol>
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<li><a href="#directory">The Directory Structure </a></li>
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<li><a href="#lexer">The Lexer</a></li>
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<li><a href="#parser">The Parser</a></li>
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<li><a href="#compiler">The Compiler</a></li>
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<li><a href="#runtime">The Runtime</a></li>
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<li><a href="#driver">Compiler Driver</a></li>
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<li><a href="#tests">Test Programs</a></li>
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<li><a href="#exercise">Exercise</a></li>
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<li><a href="#todo">Things Remaining To Be Done</a></li>
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</ol></li>
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</ol>
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<div class="doc_author">
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<p>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a></p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_section"><a name="abstract">Abstract</a></div>
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<div class="doc_text">
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<p>This document is another way to learn about LLVM. Unlike the
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<a href="LangRef.html">LLVM Reference Manual</a> or
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<a href="ProgrammersManual.html">LLVM Programmer's Manual</a>, here we learn
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about LLVM through the experience of creating a simple programming language
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named Stacker. Stacker was invented specifically as a demonstration of
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LLVM. The emphasis in this document is not on describing the
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intricacies of LLVM itself but on how to use it to build your own
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compiler system.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_section"> <a name="introduction">Introduction</a> </div>
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<div class="doc_text">
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<p>Amongst other things, LLVM is a platform for compiler writers.
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Because of its exceptionally clean and small IR (intermediate
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representation), compiler writing with LLVM is much easier than with
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other system. As proof, I wrote the entire compiler (language definition,
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lexer, parser, code generator, etc.) in about <em>four days</em>!
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That's important to know because it shows how quickly you can get a new
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language running when using LLVM. Furthermore, this was the <em >first</em>
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language the author ever created using LLVM. The learning curve is
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included in that four days.</p>
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<p>The language described here, Stacker, is Forth-like. Programs
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are simple collections of word definitions, and the only thing definitions
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can do is manipulate a stack or generate I/O. Stacker is not a "real"
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programming language; it's very simple. Although it is computationally
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complete, you wouldn't use it for your next big project. However,
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the fact that it is complete, it's simple, and it <em>doesn't</em> have
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a C-like syntax make it useful for demonstration purposes. It shows
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that LLVM could be applied to a wide variety of languages.</p>
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<p>The basic notions behind stacker is very simple. There's a stack of
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integers (or character pointers) that the program manipulates. Pretty
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much the only thing the program can do is manipulate the stack and do
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some limited I/O operations. The language provides you with several
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built-in words that manipulate the stack in interesting ways. To get
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your feet wet, here's how you write the traditional "Hello, World"
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program in Stacker:</p>
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<p><code>: hello_world "Hello, World!" >s DROP CR ;<br>
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: MAIN hello_world ;<br></code></p>
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<p>This has two "definitions" (Stacker manipulates words, not
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functions and words have definitions): <code>MAIN</code> and <code>
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hello_world</code>. The <code>MAIN</code> definition is standard; it
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tells Stacker where to start. Here, <code>MAIN</code> is defined to
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simply invoke the word <code>hello_world</code>. The
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<code>hello_world</code> definition tells stacker to push the
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<code>"Hello, World!"</code> string on to the stack, print it out
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(<code>>s</code>), pop it off the stack (<code>DROP</code>), and
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finally print a carriage return (<code>CR</code>). Although
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<code>hello_world</code> uses the stack, its net effect is null. Well
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written Stacker definitions have that characteristic. </p>
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<p>Exercise for the reader: how could you make this a one line program?</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_section"><a name="lessons"></a>Lessons I Learned About LLVM</div>
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<div class="doc_text">
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<p>Stacker was written for two purposes: </p>
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<ol>
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<li>to get the author over the learning curve, and</li>
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<li>to provide a simple example of how to write a compiler using LLVM.</li>
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</ol>
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<p>During the development of Stacker, many lessons about LLVM were
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learned. Those lessons are described in the following subsections.<p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection"><a name="value"></a>Everything's a Value!</div>
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<div class="doc_text">
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<p>Although I knew that LLVM uses a Single Static Assignment (SSA) format,
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it wasn't obvious to me how prevalent this idea was in LLVM until I really
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started using it. Reading the <a href="ProgrammersManual.html">
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Programmer's Manual</a> and <a href="LangRef.html">Language Reference</a>,
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I noted that most of the important LLVM IR (Intermediate Representation) C++
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classes were derived from the Value class. The full power of that simple
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design only became fully understood once I started constructing executable
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expressions for Stacker.</p>
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<p>This really makes your programming go faster. Think about compiling code
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for the following C/C++ expression: <code>(a|b)*((x+1)/(y+1))</code>. Assuming
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the values are on the stack in the order a, b, x, y, this could be
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expressed in stacker as: <code>1 + SWAP 1 + / ROT2 OR *</code>.
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You could write a function using LLVM that computes this expression like
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this: </p>
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<div class="doc_code"><pre>
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Value*
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expression(BasicBlock* bb, Value* a, Value* b, Value* x, Value* y )
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{
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ConstantInt* one = ConstantInt::get(Type::IntTy, 1);
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BinaryOperator* or1 = BinaryOperator::createOr(a, b, "", bb);
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BinaryOperator* add1 = BinaryOperator::createAdd(x, one, "", bb);
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BinaryOperator* add2 = BinaryOperator::createAdd(y, one, "", bb);
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BinaryOperator* div1 = BinaryOperator::createDiv(add1, add2, "", bb);
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BinaryOperator* mult1 = BinaryOperator::createMul(or1, div1, "", bb);
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return mult1;
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}
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</pre></div>
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<p>"Okay, big deal," you say? It is a big deal. Here's why. Note that I didn't
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have to tell this function which kinds of Values are being passed in. They could be
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<code>Instruction</code>s, <code>Constant</code>s, <code>GlobalVariable</code>s, or
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any of the other subclasses of <code>Value</code> that LLVM supports.
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Furthermore, if you specify Values that are incorrect for this sequence of
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operations, LLVM will either notice right away (at compilation time) or the LLVM
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Verifier will pick up the inconsistency when the compiler runs. In either case
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LLVM prevents you from making a type error that gets passed through to the
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generated program. This <em>really</em> helps you write a compiler that
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always generates correct code!<p>
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<p>The second point is that we don't have to worry about branching, registers,
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stack variables, saving partial results, etc. The instructions we create
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<em>are</em> the values we use. Note that all that was created in the above
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code is a Constant value and five operators. Each of the instructions <em>is</em>
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the resulting value of that instruction. This saves a lot of time.</p>
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<p>The lesson is this: <em>SSA form is very powerful: there is no difference
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between a value and the instruction that created it.</em> This is fully
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enforced by the LLVM IR. Use it to your best advantage.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection"><a name="terminate"></a>Terminate Those Blocks!</div>
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<div class="doc_text">
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<p>I had to learn about terminating blocks the hard way: using the debugger
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to figure out what the LLVM verifier was trying to tell me and begging for
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help on the LLVMdev mailing list. I hope you avoid this experience.</p>
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<p>Emblazon this rule in your mind:</p>
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<ul>
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<li><em>All</em> <code>BasicBlock</code>s in your compiler <b>must</b> be
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terminated with a terminating instruction (branch, return, etc.).
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</li>
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</ul>
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<p>Terminating instructions are a semantic requirement of the LLVM IR. There
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is no facility for implicitly chaining together blocks placed into a function
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in the order they occur. Indeed, in the general case, blocks will not be
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added to the function in the order of execution because of the recursive
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way compilers are written.</p>
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<p>Furthermore, if you don't terminate your blocks, your compiler code will
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compile just fine. You won't find out about the problem until you're running
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the compiler and the module you just created fails on the LLVM Verifier.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection"><a name="blocks"></a>Concrete Blocks</div>
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<div class="doc_text">
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<p>After a little initial fumbling around, I quickly caught on to how blocks
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should be constructed. In general, here's what I learned:
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<ol>
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<li><em>Create your blocks early.</em> While writing your compiler, you
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will encounter several situations where you know apriori that you will
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need several blocks. For example, if-then-else, switch, while, and for
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statements in C/C++ all need multiple blocks for expression in LVVM.
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The rule is, create them early.</li>
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<li><em>Terminate your blocks early.</em> This just reduces the chances
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that you forget to terminate your blocks which is required (go
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<a href="#terminate">here</a> for more).
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<li><em>Use getTerminator() for instruction insertion.</em> I noticed early on
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that many of the constructors for the Instruction classes take an optional
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<code>insert_before</code> argument. At first, I thought this was a mistake
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because clearly the normal mode of inserting instructions would be one at
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a time <em>after</em> some other instruction, not <em>before</em>. However,
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if you hold on to your terminating instruction (or use the handy dandy
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<code>getTerminator()</code> method on a <code>BasicBlock</code>), it can
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always be used as the <code>insert_before</code> argument to your instruction
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constructors. This causes the instruction to automatically be inserted in
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the RightPlace™ place, just before the terminating instruction. The
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nice thing about this design is that you can pass blocks around and insert
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new instructions into them without ever knowing what instructions came
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before. This makes for some very clean compiler design.</li>
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</ol>
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<p>The foregoing is such an important principal, its worth making an idiom:</p>
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<pre>
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BasicBlock* bb = new BasicBlock();
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bb->getInstList().push_back( new Branch( ... ) );
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new Instruction(..., bb->getTerminator() );
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</pre>
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<p>To make this clear, consider the typical if-then-else statement
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(see StackerCompiler::handle_if() method). We can set this up
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in a single function using LLVM in the following way: </p>
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<pre>
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using namespace llvm;
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BasicBlock*
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MyCompiler::handle_if( BasicBlock* bb, SetCondInst* condition )
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{
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// Create the blocks to contain code in the structure of if/then/else
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BasicBlock* then_bb = new BasicBlock();
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BasicBlock* else_bb = new BasicBlock();
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BasicBlock* exit_bb = new BasicBlock();
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// Insert the branch instruction for the "if"
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bb->getInstList().push_back( new BranchInst( then_bb, else_bb, condition ) );
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// Set up the terminating instructions
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then->getInstList().push_back( new BranchInst( exit_bb ) );
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else->getInstList().push_back( new BranchInst( exit_bb ) );
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// Fill in the then part .. details excised for brevity
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this->fill_in( then_bb );
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// Fill in the else part .. details excised for brevity
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this->fill_in( else_bb );
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// Return a block to the caller that can be filled in with the code
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// that follows the if/then/else construct.
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return exit_bb;
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}
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</pre>
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<p>Presumably in the foregoing, the calls to the "fill_in" method would add
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the instructions for the "then" and "else" parts. They would use the third part
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of the idiom almost exclusively (inserting new instructions before the
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terminator). Furthermore, they could even recurse back to <code>handle_if</code>
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should they encounter another if/then/else statement, and it will just work.</p>
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<p>Note how cleanly this all works out. In particular, the push_back methods on
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the <code>BasicBlock</code>'s instruction list. These are lists of type
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<code>Instruction</code> (which is also of type <code>Value</code>). To create
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the "if" branch we merely instantiate a <code>BranchInst</code> that takes as
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arguments the blocks to branch to and the condition to branch on. The
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<code>BasicBlock</code> objects act like branch labels! This new
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<code>BranchInst</code> terminates the <code>BasicBlock</code> provided
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as an argument. To give the caller a way to keep inserting after calling
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<code>handle_if</code>, we create an <code>exit_bb</code> block which is
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returned
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to the caller. Note that the <code>exit_bb</code> block is used as the
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terminator for both the <code>then_bb</code> and the <code>else_bb</code>
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blocks. This guarantees that no matter what else <code>handle_if</code>
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or <code>fill_in</code> does, they end up at the <code>exit_bb</code> block.
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</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection"><a name="push_back"></a>push_back Is Your Friend</div>
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<div class="doc_text">
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<p>
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One of the first things I noticed is the frequent use of the "push_back"
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method on the various lists. This is so common that it is worth mentioning.
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The "push_back" inserts a value into an STL list, vector, array, etc. at the
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end. The method might have also been named "insert_tail" or "append".
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Although I've used STL quite frequently, my use of push_back wasn't very
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high in other programs. In LLVM, you'll use it all the time.
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</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection"><a name="gep"></a>The Wily GetElementPtrInst</div>
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<div class="doc_text">
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<p>
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It took a little getting used to and several rounds of postings to the LLVM
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mailing list to wrap my head around this instruction correctly. Even though I had
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read the Language Reference and Programmer's Manual a couple times each, I still
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missed a few <em>very</em> key points:
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</p>
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<ul>
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<li>GetElementPtrInst gives you back a Value for the last thing indexed.</li>
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<li>All global variables in LLVM are <em>pointers</em>.</li>
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<li>Pointers must also be dereferenced with the GetElementPtrInst
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instruction.</li>
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</ul>
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<p>This means that when you look up an element in the global variable (assuming
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it's a struct or array), you <em>must</em> deference the pointer first! For many
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things, this leads to the idiom:
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</p>
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<pre>
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std::vector<Value*> index_vector;
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index_vector.push_back( ConstantInt::get( Type::LongTy, 0 );
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// ... push other indices ...
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GetElementPtrInst* gep = new GetElementPtrInst( ptr, index_vector );
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</pre>
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<p>For example, suppose we have a global variable whose type is [24 x int]. The
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variable itself represents a <em>pointer</em> to that array. To subscript the
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array, we need two indices, not just one. The first index (0) dereferences the
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pointer. The second index subscripts the array. If you're a "C" programmer, this
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will run against your grain because you'll naturally think of the global array
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variable and the address of its first element as the same. That tripped me up
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for a while until I realized that they really do differ .. by <em>type</em>.
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Remember that LLVM is strongly typed. Everything has a type.
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The "type" of the global variable is [24 x int]*. That is, it's
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a pointer to an array of 24 ints. When you dereference that global variable with
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a single (0) index, you now have a "[24 x int]" type. Although
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the pointer value of the dereferenced global and the address of the zero'th element
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in the array will be the same, they differ in their type. The zero'th element has
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type "int" while the pointer value has type "[24 x int]".</p>
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<p>Get this one aspect of LLVM right in your head, and you'll save yourself
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a lot of compiler writing headaches down the road.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection"><a name="linkage"></a>Getting Linkage Types Right</div>
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<div class="doc_text">
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<p>Linkage types in LLVM can be a little confusing, especially if your compiler
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writing mind has affixed firm concepts to particular words like "weak",
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"external", "global", "linkonce", etc. LLVM does <em>not</em> use the precise
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definitions of, say, ELF or GCC, even though they share common terms. To be fair,
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the concepts are related and similar but not precisely the same. This can lead
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you to think you know what a linkage type represents but in fact it is slightly
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different. I recommend you read the
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<a href="LangRef.html#linkage"> Language Reference on this topic</a> very
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carefully. Then, read it again.<p>
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<p>Here are some handy tips that I discovered along the way:</p>
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<ul>
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<li><em>Uninitialized means external.</em> That is, the symbol is declared in the current
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module and can be used by that module, but it is not defined by that module.</li>
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<li><em>Setting an initializer changes a global' linkage type.</em> Setting an
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initializer changes a global's linkage type from whatever it was to a normal,
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defined global (not external). You'll need to call the setLinkage() method to
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reset it if you specify the initializer after the GlobalValue has been constructed.
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This is important for LinkOnce and Weak linkage types.</li>
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<li><em>Appending linkage can keep track of things.</em> Appending linkage can
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be used to keep track of compilation information at runtime. It could be used,
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for example, to build a full table of all the C++ virtual tables or hold the
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C++ RTTI data, or whatever. Appending linkage can only be applied to arrays.
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All arrays with the same name in each module are concatenated together at link
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time.</li>
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</ul>
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</div>
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<!-- ======================================================================= -->
|
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<div class="doc_subsection"><a name="constants"></a>Constants Are Easier Than That!</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
Constants in LLVM took a little getting used to until I discovered a few utility
|
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functions in the LLVM IR that make things easier. Here's what I learned: </p>
|
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<ul>
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<li>Constants are Values like anything else and can be operands of instructions</li>
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<li>Integer constants, frequently needed, can be created using the static "get"
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methods of the ConstantInt class. The nice thing about these is that you can
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"get" any kind of integer quickly.</li>
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<li>There's a special method on Constant class which allows you to get the null
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constant for <em>any</em> type. This is really handy for initializing large
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arrays or structures, etc.</li>
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</ul>
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</div>
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<!-- ======================================================================= -->
|
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<div class="doc_section"> <a name="lexicon">The Stacker Lexicon</a></div>
|
|
<div class="doc_text"><p>This section describes the Stacker language</p></div>
|
|
<div class="doc_subsection"><a name="stack"></a>The Stack</div>
|
|
<div class="doc_text">
|
|
<p>Stacker definitions define what they do to the global stack. Before
|
|
proceeding, a few words about the stack are in order. The stack is simply
|
|
a global array of 32-bit integers or pointers. A global index keeps track
|
|
of the location of the top of the stack. All of this is hidden from the
|
|
programmer, but it needs to be noted because it is the foundation of the
|
|
conceptual programming model for Stacker. When you write a definition,
|
|
you are, essentially, saying how you want that definition to manipulate
|
|
the global stack.</p>
|
|
<p>Manipulating the stack can be quite hazardous. There is no distinction
|
|
given and no checking for the various types of values that can be placed
|
|
on the stack. Automatic coercion between types is performed. In many
|
|
cases, this is useful. For example, a boolean value placed on the stack
|
|
can be interpreted as an integer with good results. However, using a
|
|
word that interprets that boolean value as a pointer to a string to
|
|
print out will almost always yield a crash. Stacker simply leaves it
|
|
to the programmer to get it right without any interference or hindering
|
|
on interpretation of the stack values. You've been warned. :) </p>
|
|
</div>
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection"> <a name="punctuation"></a>Punctuation</div>
|
|
<div class="doc_text">
|
|
<p>Punctuation in Stacker is very simple. The colon and semi-colon
|
|
characters are used to introduce and terminate a definition
|
|
(respectively). Except for <em>FORWARD</em> declarations, definitions
|
|
are all you can specify in Stacker. Definitions are read left to right.
|
|
Immediately after the colon comes the name of the word being defined.
|
|
The remaining words in the definition specify what the word does. The definition
|
|
is terminated by a semi-colon.</p>
|
|
<p>So, your typical definition will have the form:</p>
|
|
<pre><code>: name ... ;</code></pre>
|
|
<p>The <code>name</code> is up to you but it must start with a letter and contain
|
|
only letters, numbers, and underscore. Names are case sensitive and must not be
|
|
the same as the name of a built-in word. The <code>...</code> is replaced by
|
|
the stack manipulating words that you wish to define <code>name</code> as. <p>
|
|
</div>
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection"><a name="comments"></a>Comments</div>
|
|
<div class="doc_text">
|
|
<p>Stacker supports two types of comments. A hash mark (#) starts a comment
|
|
that extends to the end of the line. It is identical to the kind of comments
|
|
commonly used in shell scripts. A pair of parentheses also surround a comment.
|
|
In both cases, the content of the comment is ignored by the Stacker compiler. The
|
|
following does nothing in Stacker.
|
|
</p>
|
|
<pre><code>
|
|
# This is a comment to end of line
|
|
( This is an enclosed comment )
|
|
</code></pre>
|
|
<p>See the <a href="#example">example</a> program to see comments in use in
|
|
a real program.</p>
|
|
</div>
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection"><a name="literals"></a>Literals</div>
|
|
<div class="doc_text">
|
|
<p>There are three kinds of literal values in Stacker: Integers, Strings,
|
|
and Booleans. In each case, the stack operation is to simply push the
|
|
value on to the stack. So, for example:<br/>
|
|
<code> 42 " is the answer." TRUE </code><br/>
|
|
will push three values on to the stack: the integer 42, the
|
|
string " is the answer.", and the boolean TRUE.</p>
|
|
</div>
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection"><a name="words"></a>Words</div>
|
|
<div class="doc_text">
|
|
<p>Each definition in Stacker is composed of a set of words. Words are
|
|
read and executed in order from left to right. There is very little
|
|
checking in Stacker to make sure you're doing the right thing with
|
|
the stack. It is assumed that the programmer knows how the stack
|
|
transformation he applies will affect the program.</p>
|
|
<p>Words in a definition come in two flavors: built-in and programmer
|
|
defined. Simply mentioning the name of a previously defined or declared
|
|
programmer-defined word causes that word's stack actions to be invoked. It
|
|
is somewhat like a function call in other languages. The built-in
|
|
words have various effects, described <a href="#builtins">below</a>.</p>
|
|
<p>Sometimes you need to call a word before it is defined. For this, you can
|
|
use the <code>FORWARD</code> declaration. It looks like this:</p>
|
|
<p><code>FORWARD name ;</code></p>
|
|
<p>This simply states to Stacker that "name" is the name of a definition
|
|
that is defined elsewhere. Generally it means the definition can be found
|
|
"forward" in the file. But, it doesn't have to be in the current compilation
|
|
unit. Anything declared with <code>FORWARD</code> is an external symbol for
|
|
linking.</p>
|
|
</div>
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection"><a name="style"></a>Standard Style</div>
|
|
<div class="doc_text">
|
|
<p>TODO</p>
|
|
</div>
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection"><a name="builtins"></a>Built In Words</div>
|
|
<div class="doc_text">
|
|
<p>The built-in words of the Stacker language are put in several groups
|
|
depending on what they do. The groups are as follows:</p>
|
|
<ol>
|
|
<li><em>Logical</em>: These words provide the logical operations for
|
|
comparing stack operands.<br/>The words are: < > <= >=
|
|
= <> true false.</li>
|
|
<li><em>Bitwise</em>: These words perform bitwise computations on
|
|
their operands. <br/> The words are: << >> XOR AND NOT</li>
|
|
<li><em>Arithmetic</em>: These words perform arithmetic computations on
|
|
their operands. <br/> The words are: ABS NEG + - * / MOD */ ++ -- MIN MAX</li>
|
|
<li><em>Stack</em>These words manipulate the stack directly by moving
|
|
its elements around.<br/> The words are: DROP DROP2 NIP NIP2 DUP DUP2
|
|
SWAP SWAP2 OVER OVER2 ROT ROT2 RROT RROT2 TUCK TUCK2 PICK SELECT ROLL</li>
|
|
<li><em>Memory</em>These words allocate, free, and manipulate memory
|
|
areas outside the stack.<br/>The words are: MALLOC FREE GET PUT</li>
|
|
<li><em>Control</em>: These words alter the normal left to right flow
|
|
of execution.<br/>The words are: IF ELSE ENDIF WHILE END RETURN EXIT RECURSE</li>
|
|
<li><em>I/O</em>: These words perform output on the standard output
|
|
and input on the standard input. No other I/O is possible in Stacker.
|
|
<br/>The words are: SPACE TAB CR >s >d >c <s <d <c.</li>
|
|
</ol>
|
|
<p>While you may be familiar with many of these operations from other
|
|
programming languages, a careful review of their semantics is important
|
|
for correct programming in Stacker. Of most importance is the effect
|
|
that each of these built-in words has on the global stack. The effect is
|
|
not always intuitive. To better describe the effects, we'll borrow from Forth the idiom of
|
|
describing the effect on the stack with:</p>
|
|
<p><code> BEFORE -- AFTER </code></p>
|
|
<p>That is, to the left of the -- is a representation of the stack before
|
|
the operation. To the right of the -- is a representation of the stack
|
|
after the operation. In the table below that describes the operation of
|
|
each of the built in words, we will denote the elements of the stack
|
|
using the following construction:</p>
|
|
<ol>
|
|
<li><em>b</em> - a boolean truth value</li>
|
|
<li><em>w</em> - a normal integer valued word.</li>
|
|
<li><em>s</em> - a pointer to a string value</li>
|
|
<li><em>p</em> - a pointer to a malloc'd memory block</li>
|
|
</ol>
|
|
</div>
|
|
<div class="doc_text" >
|
|
<table>
|
|
<tr><th colspan="4">Definition Of Operation Of Built In Words</th></tr>
|
|
<tr><th colspan="4"><b>LOGICAL OPERATIONS</b></th></tr>
|
|
<tr>
|
|
<td>Word</td>
|
|
<td>Name</td>
|
|
<td>Operation</td>
|
|
<td>Description</td>
|
|
</tr>
|
|
<tr>
|
|
<td><</td>
|
|
<td>LT</td>
|
|
<td>w1 w2 -- b</td>
|
|
<td>Two values (w1 and w2) are popped off the stack and
|
|
compared. If w1 is less than w2, TRUE is pushed back on
|
|
the stack, otherwise FALSE is pushed back on the stack.</td>
|
|
</tr>
|
|
<tr><td>></td>
|
|
<td>GT</td>
|
|
<td>w1 w2 -- b</td>
|
|
<td>Two values (w1 and w2) are popped off the stack and
|
|
compared. If w1 is greater than w2, TRUE is pushed back on
|
|
the stack, otherwise FALSE is pushed back on the stack.</td>
|
|
</tr>
|
|
<tr><td>>=</td>
|
|
<td>GE</td>
|
|
<td>w1 w2 -- b</td>
|
|
<td>Two values (w1 and w2) are popped off the stack and
|
|
compared. If w1 is greater than or equal to w2, TRUE is
|
|
pushed back on the stack, otherwise FALSE is pushed back
|
|
on the stack.</td>
|
|
</tr>
|
|
<tr><td><=</td>
|
|
<td>LE</td>
|
|
<td>w1 w2 -- b</td>
|
|
<td>Two values (w1 and w2) are popped off the stack and
|
|
compared. If w1 is less than or equal to w2, TRUE is
|
|
pushed back on the stack, otherwise FALSE is pushed back
|
|
on the stack.</td>
|
|
</tr>
|
|
<tr><td>=</td>
|
|
<td>EQ</td>
|
|
<td>w1 w2 -- b</td>
|
|
<td>Two values (w1 and w2) are popped off the stack and
|
|
compared. If w1 is equal to w2, TRUE is
|
|
pushed back on the stack, otherwise FALSE is pushed back
|
|
</td>
|
|
</tr>
|
|
<tr><td><></td>
|
|
<td>NE</td>
|
|
<td>w1 w2 -- b</td>
|
|
<td>Two values (w1 and w2) are popped off the stack and
|
|
compared. If w1 is equal to w2, TRUE is
|
|
pushed back on the stack, otherwise FALSE is pushed back
|
|
</td>
|
|
</tr>
|
|
<tr><td>FALSE</td>
|
|
<td>FALSE</td>
|
|
<td> -- b</td>
|
|
<td>The boolean value FALSE (0) is pushed on to the stack.</td>
|
|
</tr>
|
|
<tr><td>TRUE</td>
|
|
<td>TRUE</td>
|
|
<td> -- b</td>
|
|
<td>The boolean value TRUE (-1) is pushed on to the stack.</td>
|
|
</tr>
|
|
<tr><th colspan="4"><b>BITWISE OPERATORS</b></th></tr>
|
|
<tr>
|
|
<td>Word</td>
|
|
<td>Name</td>
|
|
<td>Operation</td>
|
|
<td>Description</td>
|
|
</tr>
|
|
<tr><td><<</td>
|
|
<td>SHL</td>
|
|
<td>w1 w2 -- w1<<w2</td>
|
|
<td>Two values (w1 and w2) are popped off the stack. The w2
|
|
operand is shifted left by the number of bits given by the
|
|
w1 operand. The result is pushed back to the stack.</td>
|
|
</tr>
|
|
<tr><td>>></td>
|
|
<td>SHR</td>
|
|
<td>w1 w2 -- w1>>w2</td>
|
|
<td>Two values (w1 and w2) are popped off the stack. The w2
|
|
operand is shifted right by the number of bits given by the
|
|
w1 operand. The result is pushed back to the stack.</td>
|
|
</tr>
|
|
<tr><td>OR</td>
|
|
<td>OR</td>
|
|
<td>w1 w2 -- w2|w1</td>
|
|
<td>Two values (w1 and w2) are popped off the stack. The values
|
|
are bitwise OR'd together and pushed back on the stack. This is
|
|
not a logical OR. The sequence 1 2 OR yields 3 not 1.</td>
|
|
</tr>
|
|
<tr><td>AND</td>
|
|
<td>AND</td>
|
|
<td>w1 w2 -- w2&w1</td>
|
|
<td>Two values (w1 and w2) are popped off the stack. The values
|
|
are bitwise AND'd together and pushed back on the stack. This is
|
|
not a logical AND. The sequence 1 2 AND yields 0 not 1.</td>
|
|
</tr>
|
|
<tr><td>XOR</td>
|
|
<td>XOR</td>
|
|
<td>w1 w2 -- w2^w1</td>
|
|
<td>Two values (w1 and w2) are popped off the stack. The values
|
|
are bitwise exclusive OR'd together and pushed back on the stack.
|
|
For example, The sequence 1 3 XOR yields 2.</td>
|
|
</tr>
|
|
<tr><th colspan="4"><b>ARITHMETIC OPERATORS</b></th></tr>
|
|
<tr>
|
|
<td>Word</td>
|
|
<td>Name</td>
|
|
<td>Operation</td>
|
|
<td>Description</td>
|
|
</tr>
|
|
<tr><td>ABS</td>
|
|
<td>ABS</td>
|
|
<td>w -- |w|</td>
|
|
<td>One value s popped off the stack; its absolute value is computed
|
|
and then pushed on to the stack. If w1 is -1 then w2 is 1. If w1 is
|
|
1 then w2 is also 1.</td>
|
|
</tr>
|
|
<tr><td>NEG</td>
|
|
<td>NEG</td>
|
|
<td>w -- -w</td>
|
|
<td>One value is popped off the stack which is negated and then
|
|
pushed back on to the stack. If w1 is -1 then w2 is 1. If w1 is
|
|
1 then w2 is -1.</td>
|
|
</tr>
|
|
<tr><td> + </td>
|
|
<td>ADD</td>
|
|
<td>w1 w2 -- w2+w1</td>
|
|
<td>Two values are popped off the stack. Their sum is pushed back
|
|
on to the stack</td>
|
|
</tr>
|
|
<tr><td> - </td>
|
|
<td>SUB</td>
|
|
<td>w1 w2 -- w2-w1</td>
|
|
<td>Two values are popped off the stack. Their difference is pushed back
|
|
on to the stack</td>
|
|
</tr>
|
|
<tr><td> * </td>
|
|
<td>MUL</td>
|
|
<td>w1 w2 -- w2*w1</td>
|
|
<td>Two values are popped off the stack. Their product is pushed back
|
|
on to the stack</td>
|
|
</tr>
|
|
<tr><td> / </td>
|
|
<td>DIV</td>
|
|
<td>w1 w2 -- w2/w1</td>
|
|
<td>Two values are popped off the stack. Their quotient is pushed back
|
|
on to the stack</td>
|
|
</tr>
|
|
<tr><td>MOD</td>
|
|
<td>MOD</td>
|
|
<td>w1 w2 -- w2%w1</td>
|
|
<td>Two values are popped off the stack. Their remainder after division
|
|
of w1 by w2 is pushed back on to the stack</td>
|
|
</tr>
|
|
<tr><td> */ </td>
|
|
<td>STAR_SLAH</td>
|
|
<td>w1 w2 w3 -- (w3*w2)/w1</td>
|
|
<td>Three values are popped off the stack. The product of w1 and w2 is
|
|
divided by w3. The result is pushed back on to the stack.</td>
|
|
</tr>
|
|
<tr><td> ++ </td>
|
|
<td>INCR</td>
|
|
<td>w -- w+1</td>
|
|
<td>One value is popped off the stack. It is incremented by one and then
|
|
pushed back on to the stack.</td>
|
|
</tr>
|
|
<tr><td> -- </td>
|
|
<td>DECR</td>
|
|
<td>w -- w-1</td>
|
|
<td>One value is popped off the stack. It is decremented by one and then
|
|
pushed back on to the stack.</td>
|
|
</tr>
|
|
<tr><td>MIN</td>
|
|
<td>MIN</td>
|
|
<td>w1 w2 -- (w2<w1?w2:w1)</td>
|
|
<td>Two values are popped off the stack. The larger one is pushed back
|
|
on to the stack.</td>
|
|
</tr>
|
|
<tr><td>MAX</td>
|
|
<td>MAX</td>
|
|
<td>w1 w2 -- (w2>w1?w2:w1)</td>
|
|
<td>Two values are popped off the stack. The larger value is pushed back
|
|
on to the stack.</td>
|
|
</tr>
|
|
<tr><th colspan="4"><b>STACK MANIPULATION OPERATORS</b></th></tr>
|
|
<tr>
|
|
<td>Word</td>
|
|
<td>Name</td>
|
|
<td>Operation</td>
|
|
<td>Description</td>
|
|
</tr>
|
|
<tr><td>DROP</td>
|
|
<td>DROP</td>
|
|
<td>w -- </td>
|
|
<td>One value is popped off the stack.</td>
|
|
</tr>
|
|
<tr><td>DROP2</td>
|
|
<td>DROP2</td>
|
|
<td>w1 w2 -- </td>
|
|
<td>Two values are popped off the stack.</td>
|
|
</tr>
|
|
<tr><td>NIP</td>
|
|
<td>NIP</td>
|
|
<td>w1 w2 -- w2</td>
|
|
<td>The second value on the stack is removed from the stack. That is,
|
|
a value is popped off the stack and retained. Then a second value is
|
|
popped and the retained value is pushed.</td>
|
|
</tr>
|
|
<tr><td>NIP2</td>
|
|
<td>NIP2</td>
|
|
<td>w1 w2 w3 w4 -- w3 w4</td>
|
|
<td>The third and fourth values on the stack are removed from it. That is,
|
|
two values are popped and retained. Then two more values are popped and
|
|
the two retained values are pushed back on.</td>
|
|
</tr>
|
|
<tr><td>DUP</td>
|
|
<td>DUP</td>
|
|
<td>w1 -- w1 w1</td>
|
|
<td>One value is popped off the stack. That value is then pushed on to
|
|
the stack twice to duplicate the top stack vaue.</td>
|
|
</tr>
|
|
<tr><td>DUP2</td>
|
|
<td>DUP2</td>
|
|
<td>w1 w2 -- w1 w2 w1 w2</td>
|
|
<td>The top two values on the stack are duplicated. That is, two vaues
|
|
are popped off the stack. They are alternately pushed back on the
|
|
stack twice each.</td>
|
|
</tr>
|
|
<tr><td>SWAP</td>
|
|
<td>SWAP</td>
|
|
<td>w1 w2 -- w2 w1</td>
|
|
<td>The top two stack items are reversed in their order. That is, two
|
|
values are popped off the stack and pushed back on to the stack in
|
|
the opposite order they were popped.</td>
|
|
</tr>
|
|
<tr><td>SWAP2</td>
|
|
<td>SWAP2</td>
|
|
<td>w1 w2 w3 w4 -- w3 w4 w2 w1</td>
|
|
<td>The top four stack items are swapped in pairs. That is, two values
|
|
are popped and retained. Then, two more values are popped and retained.
|
|
The values are pushed back on to the stack in the reverse order but
|
|
in pairs.</td>
|
|
</tr>
|
|
<tr><td>OVER</td>
|
|
<td>OVER</td>
|
|
<td>w1 w2-- w1 w2 w1</td>
|
|
<td>Two values are popped from the stack. They are pushed back
|
|
on to the stack in the order w1 w2 w1. This seems to cause the
|
|
top stack element to be duplicated "over" the next value.</td>
|
|
</tr>
|
|
<tr><td>OVER2</td>
|
|
<td>OVER2</td>
|
|
<td>w1 w2 w3 w4 -- w1 w2 w3 w4 w1 w2</td>
|
|
<td>The third and fourth values on the stack are replicated on to the
|
|
top of the stack</td>
|
|
</tr>
|
|
<tr><td>ROT</td>
|
|
<td>ROT</td>
|
|
<td>w1 w2 w3 -- w2 w3 w1</td>
|
|
<td>The top three values are rotated. That is, three value are popped
|
|
off the stack. They are pushed back on to the stack in the order
|
|
w1 w3 w2.</td>
|
|
</tr>
|
|
<tr><td>ROT2</td>
|
|
<td>ROT2</td>
|
|
<td>w1 w2 w3 w4 w5 w6 -- w3 w4 w5 w6 w1 w2</td>
|
|
<td>Like ROT but the rotation is done using three pairs instead of
|
|
three singles.</td>
|
|
</tr>
|
|
<tr><td>RROT</td>
|
|
<td>RROT</td>
|
|
<td>w1 w2 w3 -- w3 w1 w2</td>
|
|
<td>Reverse rotation. Like ROT, but it rotates the other way around.
|
|
Essentially, the third element on the stack is moved to the top
|
|
of the stack.</td>
|
|
</tr>
|
|
<tr><td>RROT2</td>
|
|
<td>RROT2</td>
|
|
<td>w1 w2 w3 w4 w5 w6 -- w3 w4 w5 w6 w1 w2</td>
|
|
<td>Double reverse rotation. Like RROT but the rotation is done using
|
|
three pairs instead of three singles. The fifth and sixth stack
|
|
elements are moved to the first and second positions</td>
|
|
</tr>
|
|
<tr><td>TUCK</td>
|
|
<td>TUCK</td>
|
|
<td>w1 w2 -- w2 w1 w2</td>
|
|
<td>Similar to OVER except that the second operand is being
|
|
replicated. Essentially, the first operand is being "tucked"
|
|
in between two instances of the second operand. Logically, two
|
|
values are popped off the stack. They are placed back on the
|
|
stack in the order w2 w1 w2.</td>
|
|
</tr>
|
|
<tr><td>TUCK2</td>
|
|
<td>TUCK2</td>
|
|
<td>w1 w2 w3 w4 -- w3 w4 w1 w2 w3 w4</td>
|
|
<td>Like TUCK but a pair of elements is tucked over two pairs.
|
|
That is, the top two elements of the stack are duplicated and
|
|
inserted into the stack at the fifth and positions.</td>
|
|
</tr>
|
|
<tr><td>PICK</td>
|
|
<td>PICK</td>
|
|
<td>x0 ... Xn n -- x0 ... Xn x0</td>
|
|
<td>The top of the stack is used as an index into the remainder of
|
|
the stack. The element at the nth position replaces the index
|
|
(top of stack). This is useful for cycling through a set of
|
|
values. Note that indexing is zero based. So, if n=0 then you
|
|
get the second item on the stack. If n=1 you get the third, etc.
|
|
Note also that the index is replaced by the n'th value. </td>
|
|
</tr>
|
|
<tr><td>SELECT</td>
|
|
<td>SELECT</td>
|
|
<td>m n X0..Xm Xm+1 .. Xn -- Xm</td>
|
|
<td>This is like PICK but the list is removed and you need to specify
|
|
both the index and the size of the list. Careful with this one,
|
|
the wrong value for n can blow away a huge amount of the stack.</td>
|
|
</tr>
|
|
<tr><td>ROLL</td>
|
|
<td>ROLL</td>
|
|
<td>x0 x1 .. xn n -- x1 .. xn x0</td>
|
|
<td><b>Not Implemented</b>. This one has been left as an exercise to
|
|
the student. See <a href="#exercise">Exercise</a>. ROLL requires
|
|
a value, "n", to be on the top of the stack. This value specifies how
|
|
far into the stack to "roll". The n'th value is <em>moved</em> (not
|
|
copied) from its location and replaces the "n" value on the top of the
|
|
stack. In this way, all the values between "n" and x0 roll up the stack.
|
|
The operation of ROLL is a generalized ROT. The "n" value specifies
|
|
how much to rotate. That is, ROLL with n=1 is the same as ROT and
|
|
ROLL with n=2 is the same as ROT2.</td>
|
|
</tr>
|
|
<tr><th colspan="4"><b>MEMORY OPERATORS</b></th></tr>
|
|
<tr>
|
|
<td>Word</td>
|
|
<td>Name</td>
|
|
<td>Operation</td>
|
|
<td>Description</td>
|
|
</tr>
|
|
<tr><td>MALLOC</td>
|
|
<td>MALLOC</td>
|
|
<td>w1 -- p</td>
|
|
<td>One value is popped off the stack. The value is used as the size
|
|
of a memory block to allocate. The size is in bytes, not words.
|
|
The memory allocation is completed and the address of the memory
|
|
block is pushed on to the stack.</td>
|
|
</tr>
|
|
<tr><td>FREE</td>
|
|
<td>FREE</td>
|
|
<td>p -- </td>
|
|
<td>One pointer value is popped off the stack. The value should be
|
|
the address of a memory block created by the MALLOC operation. The
|
|
associated memory block is freed. Nothing is pushed back on the
|
|
stack. Many bugs can be created by attempting to FREE something
|
|
that isn't a pointer to a MALLOC allocated memory block. Make
|
|
sure you know what's on the stack. One way to do this is with
|
|
the following idiom:<br/>
|
|
<code>64 MALLOC DUP DUP (use ptr) DUP (use ptr) ... FREE</code>
|
|
<br/>This ensures that an extra copy of the pointer is placed on
|
|
the stack (for the FREE at the end) and that every use of the
|
|
pointer is preceded by a DUP to retain the copy for FREE.</td>
|
|
</tr>
|
|
<tr><td>GET</td>
|
|
<td>GET</td>
|
|
<td>w1 p -- w2 p</td>
|
|
<td>An integer index and a pointer to a memory block are popped of
|
|
the block. The index is used to index one byte from the memory
|
|
block. That byte value is retained, the pointer is pushed again
|
|
and the retained value is pushed. Note that the pointer value
|
|
s essentially retained in its position so this doesn't count
|
|
as a "use ptr" in the FREE idiom.</td>
|
|
</tr>
|
|
<tr><td>PUT</td>
|
|
<td>PUT</td>
|
|
<td>w1 w2 p -- p </td>
|
|
<td>An integer value is popped of the stack. This is the value to
|
|
be put into a memory block. Another integer value is popped of
|
|
the stack. This is the indexed byte in the memory block. A
|
|
pointer to the memory block is popped off the stack. The
|
|
first value (w1) is then converted to a byte and written
|
|
to the element of the memory block(p) at the index given
|
|
by the second value (w2). The pointer to the memory block is
|
|
pushed back on the stack so this doesn't count as a "use ptr"
|
|
in the FREE idiom.</td>
|
|
</tr>
|
|
<tr><th colspan="4"><b>CONTROL FLOW OPERATORS</b></th></tr>
|
|
<tr>
|
|
<td>Word</td>
|
|
<td>Name</td>
|
|
<td>Operation</td>
|
|
<td>Description</td>
|
|
</tr>
|
|
<tr><td>RETURN</td>
|
|
<td>RETURN</td>
|
|
<td> -- </td>
|
|
<td>The currently executing definition returns immediately to its caller.
|
|
Note that there is an implicit <code>RETURN</code> at the end of each
|
|
definition, logically located at the semi-colon. The sequence
|
|
<code>RETURN ;</code> is valid but redundant.</td>
|
|
</tr>
|
|
<tr><td>EXIT</td>
|
|
<td>EXIT</td>
|
|
<td>w1 -- </td>
|
|
<td>A return value for the program is popped off the stack. The program is
|
|
then immediately terminated. This is normally an abnormal exit from the
|
|
program. For a normal exit (when <code>MAIN</code> finishes), the exit
|
|
code will always be zero in accordance with UNIX conventions.</td>
|
|
</tr>
|
|
<tr><td>RECURSE</td>
|
|
<td>RECURSE</td>
|
|
<td> -- </td>
|
|
<td>The currently executed definition is called again. This operation is
|
|
needed since the definition of a word doesn't exist until the semi colon
|
|
is reacher. Attempting something like:<br/>
|
|
<code> : recurser recurser ; </code><br/> will yield and error saying that
|
|
"recurser" is not defined yet. To accomplish the same thing, change this
|
|
to:<br/>
|
|
<code> : recurser RECURSE ; </code></td>
|
|
</tr>
|
|
<tr><td>IF (words...) ENDIF</td>
|
|
<td>IF (words...) ENDIF</td>
|
|
<td>b -- </td>
|
|
<td>A boolean value is popped of the stack. If it is non-zero then the "words..."
|
|
are executed. Otherwise, execution continues immediately following the ENDIF.</td>
|
|
</tr>
|
|
<tr><td>IF (words...) ELSE (words...) ENDIF</td>
|
|
<td>IF (words...) ELSE (words...) ENDIF</td>
|
|
<td>b -- </td>
|
|
<td>A boolean value is popped of the stack. If it is non-zero then the "words..."
|
|
between IF and ELSE are executed. Otherwise the words between ELSE and ENDIF are
|
|
executed. In either case, after the (words....) have executed, execution continues
|
|
immediately following the ENDIF. </td>
|
|
</tr>
|
|
<tr><td>WHILE word END</td>
|
|
<td>WHILE word END</td>
|
|
<td>b -- b </td>
|
|
<td>The boolean value on the top of the stack is examined (not popped). If
|
|
it is non-zero then the "word" between WHILE and END is executed.
|
|
Execution then begins again at the WHILE where the boolean on the top of
|
|
the stack is examined again. The stack is not modified by the WHILE...END
|
|
loop, only examined. It is imperative that the "word" in the body of the
|
|
loop ensure that the top of the stack contains the next boolean to examine
|
|
when it completes. Note that since booleans and integers can be coerced
|
|
you can use the following "for loop" idiom:<br/>
|
|
<code>(push count) WHILE word -- END</code><br/>
|
|
For example:<br/>
|
|
<code>10 WHILE >d -- END</code><br/>
|
|
This will print the numbers from 10 down to 1. 10 is pushed on the
|
|
stack. Since that is non-zero, the while loop is entered. The top of
|
|
the stack (10) is printed out with >d. The top of the stack is
|
|
decremented, yielding 9 and control is transfered back to the WHILE
|
|
keyword. The process starts all over again and repeats until
|
|
the top of stack is decremented to 0 at which point the WHILE test
|
|
fails and control is transfered to the word after the END.
|
|
</td>
|
|
</tr>
|
|
<tr><th colspan="4"><b>INPUT & OUTPUT OPERATORS</b></th></tr>
|
|
<tr>
|
|
<td>Word</td>
|
|
<td>Name</td>
|
|
<td>Operation</td>
|
|
<td>Description</td>
|
|
</tr>
|
|
<tr><td>SPACE</td>
|
|
<td>SPACE</td>
|
|
<td> -- </td>
|
|
<td>A space character is put out. There is no stack effect.</td>
|
|
</tr>
|
|
<tr><td>TAB</td>
|
|
<td>TAB</td>
|
|
<td> -- </td>
|
|
<td>A tab character is put out. There is no stack effect.</td>
|
|
</tr>
|
|
<tr><td>CR</td>
|
|
<td>CR</td>
|
|
<td> -- </td>
|
|
<td>A carriage return character is put out. There is no stack effect.</td>
|
|
</tr>
|
|
<tr><td>>s</td>
|
|
<td>OUT_STR</td>
|
|
<td> -- </td>
|
|
<td>A string pointer is popped from the stack. It is put out.</td>
|
|
</tr>
|
|
<tr><td>>d</td>
|
|
<td>OUT_STR</td>
|
|
<td> -- </td>
|
|
<td>A value is popped from the stack. It is put out as a decimal
|
|
integer.</td>
|
|
</tr>
|
|
<tr><td>>c</td>
|
|
<td>OUT_CHR</td>
|
|
<td> -- </td>
|
|
<td>A value is popped from the stack. It is put out as an ASCII
|
|
character.</td>
|
|
</tr>
|
|
<tr><td><s</td>
|
|
<td>IN_STR</td>
|
|
<td> -- s </td>
|
|
<td>A string is read from the input via the scanf(3) format string " %as".
|
|
The resulting string is pushed on to the stack.</td>
|
|
</tr>
|
|
<tr><td><d</td>
|
|
<td>IN_STR</td>
|
|
<td> -- w </td>
|
|
<td>An integer is read from the input via the scanf(3) format string " %d".
|
|
The resulting value is pushed on to the stack</td>
|
|
</tr>
|
|
<tr><td><c</td>
|
|
<td>IN_CHR</td>
|
|
<td> -- w </td>
|
|
<td>A single character is read from the input via the scanf(3) format string
|
|
" %c". The value is converted to an integer and pushed on to the stack.</td>
|
|
</tr>
|
|
<tr><td>DUMP</td>
|
|
<td>DUMP</td>
|
|
<td> -- </td>
|
|
<td>The stack contents are dumped to standard output. This is useful for
|
|
debugging your definitions. Put DUMP at the beginning and end of a definition
|
|
to see instantly the net effect of the definition.</td>
|
|
</tr>
|
|
</table>
|
|
|
|
</div>
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_section"> <a name="example">Prime: A Complete Example</a></div>
|
|
<div class="doc_text">
|
|
<p>The following fully documented program highlights many features of both
|
|
the Stacker language and what is possible with LLVM. The program has two modes
|
|
of operation. If you provide numeric arguments to the program, it checks to see
|
|
if those arguments are prime numbers and prints out the results. Without any
|
|
arguments, the program prints out any prime numbers it finds between 1 and one
|
|
million (there's a lot of them!). The source code comments below tell the
|
|
remainder of the story.
|
|
</p>
|
|
</div>
|
|
<div class="doc_text">
|
|
<pre><code>
|
|
################################################################################
|
|
#
|
|
# Brute force prime number generator
|
|
#
|
|
# This program is written in classic Stacker style, that being the style of a
|
|
# stack. Start at the bottom and read your way up !
|
|
#
|
|
# Reid Spencer - Nov 2003
|
|
################################################################################
|
|
# Utility definitions
|
|
################################################################################
|
|
: print >d CR ;
|
|
: it_is_a_prime TRUE ;
|
|
: it_is_not_a_prime FALSE ;
|
|
: continue_loop TRUE ;
|
|
: exit_loop FALSE;
|
|
|
|
################################################################################
|
|
# This definition tries an actual division of a candidate prime number. It
|
|
# determines whether the division loop on this candidate should continue or
|
|
# not.
|
|
# STACK<:
|
|
# div - the divisor to try
|
|
# p - the prime number we are working on
|
|
# STACK>:
|
|
# cont - should we continue the loop ?
|
|
# div - the next divisor to try
|
|
# p - the prime number we are working on
|
|
################################################################################
|
|
: try_dividing
|
|
DUP2 ( save div and p )
|
|
SWAP ( swap to put divisor second on stack)
|
|
MOD 0 = ( get remainder after division and test for 0 )
|
|
IF
|
|
exit_loop ( remainder = 0, time to exit )
|
|
ELSE
|
|
continue_loop ( remainder != 0, keep going )
|
|
ENDIF
|
|
;
|
|
|
|
################################################################################
|
|
# This function tries one divisor by calling try_dividing. But, before doing
|
|
# that it checks to see if the value is 1. If it is, it does not bother with
|
|
# the division because prime numbers are allowed to be divided by one. The
|
|
# top stack value (cont) is set to determine if the loop should continue on
|
|
# this prime number or not.
|
|
# STACK<:
|
|
# cont - should we continue the loop (ignored)?
|
|
# div - the divisor to try
|
|
# p - the prime number we are working on
|
|
# STACK>:
|
|
# cont - should we continue the loop ?
|
|
# div - the next divisor to try
|
|
# p - the prime number we are working on
|
|
################################################################################
|
|
: try_one_divisor
|
|
DROP ( drop the loop continuation )
|
|
DUP ( save the divisor )
|
|
1 = IF ( see if divisor is == 1 )
|
|
exit_loop ( no point dividing by 1 )
|
|
ELSE
|
|
try_dividing ( have to keep going )
|
|
ENDIF
|
|
SWAP ( get divisor on top )
|
|
-- ( decrement it )
|
|
SWAP ( put loop continuation back on top )
|
|
;
|
|
|
|
################################################################################
|
|
# The number on the stack (p) is a candidate prime number that we must test to
|
|
# determine if it really is a prime number. To do this, we divide it by every
|
|
# number from one p-1 to 1. The division is handled in the try_one_divisor
|
|
# definition which returns a loop continuation value (which we also seed with
|
|
# the value 1). After the loop, we check the divisor. If it decremented all
|
|
# the way to zero then we found a prime, otherwise we did not find one.
|
|
# STACK<:
|
|
# p - the prime number to check
|
|
# STACK>:
|
|
# yn - boolean indicating if its a prime or not
|
|
# p - the prime number checked
|
|
################################################################################
|
|
: try_harder
|
|
DUP ( duplicate to get divisor value ) )
|
|
-- ( first divisor is one less than p )
|
|
1 ( continue the loop )
|
|
WHILE
|
|
try_one_divisor ( see if its prime )
|
|
END
|
|
DROP ( drop the continuation value )
|
|
0 = IF ( test for divisor == 1 )
|
|
it_is_a_prime ( we found one )
|
|
ELSE
|
|
it_is_not_a_prime ( nope, this one is not a prime )
|
|
ENDIF
|
|
;
|
|
|
|
################################################################################
|
|
# This definition determines if the number on the top of the stack is a prime
|
|
# or not. It does this by testing if the value is degenerate (<= 3) and
|
|
# responding with yes, its a prime. Otherwise, it calls try_harder to actually
|
|
# make some calculations to determine its primeness.
|
|
# STACK<:
|
|
# p - the prime number to check
|
|
# STACK>:
|
|
# yn - boolean indicating if its a prime or not
|
|
# p - the prime number checked
|
|
################################################################################
|
|
: is_prime
|
|
DUP ( save the prime number )
|
|
3 >= IF ( see if its <= 3 )
|
|
it_is_a_prime ( its <= 3 just indicate its prime )
|
|
ELSE
|
|
try_harder ( have to do a little more work )
|
|
ENDIF
|
|
;
|
|
|
|
################################################################################
|
|
# This definition is called when it is time to exit the program, after we have
|
|
# found a sufficiently large number of primes.
|
|
# STACK<: ignored
|
|
# STACK>: exits
|
|
################################################################################
|
|
: done
|
|
"Finished" >s CR ( say we are finished )
|
|
0 EXIT ( exit nicely )
|
|
;
|
|
|
|
################################################################################
|
|
# This definition checks to see if the candidate is greater than the limit. If
|
|
# it is, it terminates the program by calling done. Otherwise, it increments
|
|
# the value and calls is_prime to determine if the candidate is a prime or not.
|
|
# If it is a prime, it prints it. Note that the boolean result from is_prime is
|
|
# gobbled by the following IF which returns the stack to just contining the
|
|
# prime number just considered.
|
|
# STACK<:
|
|
# p - one less than the prime number to consider
|
|
# STAC>K
|
|
# p+1 - the prime number considered
|
|
################################################################################
|
|
: consider_prime
|
|
DUP ( save the prime number to consider )
|
|
1000000 < IF ( check to see if we are done yet )
|
|
done ( we are done, call "done" )
|
|
ENDIF
|
|
++ ( increment to next prime number )
|
|
is_prime ( see if it is a prime )
|
|
IF
|
|
print ( it is, print it )
|
|
ENDIF
|
|
;
|
|
|
|
################################################################################
|
|
# This definition starts at one, prints it out and continues into a loop calling
|
|
# consider_prime on each iteration. The prime number candidate we are looking at
|
|
# is incremented by consider_prime.
|
|
# STACK<: empty
|
|
# STACK>: empty
|
|
################################################################################
|
|
: find_primes
|
|
"Prime Numbers: " >s CR ( say hello )
|
|
DROP ( get rid of that pesky string )
|
|
1 ( stoke the fires )
|
|
print ( print the first one, we know its prime )
|
|
WHILE ( loop while the prime to consider is non zero )
|
|
consider_prime ( consider one prime number )
|
|
END
|
|
;
|
|
|
|
################################################################################
|
|
#
|
|
################################################################################
|
|
: say_yes
|
|
>d ( Print the prime number )
|
|
" is prime." ( push string to output )
|
|
>s ( output it )
|
|
CR ( print carriage return )
|
|
DROP ( pop string )
|
|
;
|
|
|
|
: say_no
|
|
>d ( Print the prime number )
|
|
" is NOT prime." ( push string to put out )
|
|
>s ( put out the string )
|
|
CR ( print carriage return )
|
|
DROP ( pop string )
|
|
;
|
|
|
|
################################################################################
|
|
# This definition processes a single command line argument and determines if it
|
|
# is a prime number or not.
|
|
# STACK<:
|
|
# n - number of arguments
|
|
# arg1 - the prime numbers to examine
|
|
# STACK>:
|
|
# n-1 - one less than number of arguments
|
|
# arg2 - we processed one argument
|
|
################################################################################
|
|
: do_one_argument
|
|
-- ( decrement loop counter )
|
|
SWAP ( get the argument value )
|
|
is_prime IF ( determine if its prime )
|
|
say_yes ( uhuh )
|
|
ELSE
|
|
say_no ( nope )
|
|
ENDIF
|
|
DROP ( done with that argument )
|
|
;
|
|
|
|
################################################################################
|
|
# The MAIN program just prints a banner and processes its arguments.
|
|
# STACK<:
|
|
# n - number of arguments
|
|
# ... - the arguments
|
|
################################################################################
|
|
: process_arguments
|
|
WHILE ( while there are more arguments )
|
|
do_one_argument ( process one argument )
|
|
END
|
|
;
|
|
|
|
################################################################################
|
|
# The MAIN program just prints a banner and processes its arguments.
|
|
# STACK<: arguments
|
|
################################################################################
|
|
: MAIN
|
|
NIP ( get rid of the program name )
|
|
-- ( reduce number of arguments )
|
|
DUP ( save the arg counter )
|
|
1 <= IF ( See if we got an argument )
|
|
process_arguments ( tell user if they are prime )
|
|
ELSE
|
|
find_primes ( see how many we can find )
|
|
ENDIF
|
|
0 ( push return code )
|
|
;
|
|
</code>
|
|
</pre>
|
|
</div>
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_section"> <a name="internal">Internals</a></div>
|
|
<div class="doc_text">
|
|
<p><b>This section is under construction.</b>
|
|
<p>In the mean time, you can always read the code! It has comments!</p>
|
|
</div>
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection"> <a name="directory">Directory Structure</a></div>
|
|
<div class="doc_text">
|
|
<p>The source code, test programs, and sample programs can all be found
|
|
under the LLVM "projects" directory. You will need to obtain the LLVM sources
|
|
to find it (either via anonymous CVS or a tarball. See the
|
|
<a href="GettingStarted.html">Getting Started</a> document).</p>
|
|
<p>Under the "projects" directory there is a directory named "Stacker". That
|
|
directory contains everything, as follows:</p>
|
|
<ul>
|
|
<li><em>lib</em> - contains most of the source code
|
|
<ul>
|
|
<li><em>lib/compiler</em> - contains the compiler library
|
|
<li><em>lib/runtime</em> - contains the runtime library
|
|
</ul></li>
|
|
<li><em>test</em> - contains the test programs</li>
|
|
<li><em>tools</em> - contains the Stacker compiler main program, stkrc
|
|
<ul>
|
|
<li><em>lib/stkrc</em> - contains the Stacker compiler main program
|
|
</ul</li>
|
|
<li><em>sample</em> - contains the sample programs</li>
|
|
</ul>
|
|
</div>
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection"><a name="lexer"></a>The Lexer</div>
|
|
<div class="doc_text">
|
|
<p>See projects/Stacker/lib/compiler/Lexer.l</p>
|
|
</div>
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection"><a name="parser"></a>The Parser</div>
|
|
<div class="doc_text">
|
|
<p>See projects/Stacker/lib/compiler/StackerParser.y</p>
|
|
</div>
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection"><a name="compiler"></a>The Compiler</div>
|
|
<div class="doc_text">
|
|
<p>See projects/Stacker/lib/compiler/StackerCompiler.cpp</p>
|
|
</div>
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection"><a name="runtime"></a>The Runtime</div>
|
|
<div class="doc_text">
|
|
<p>See projects/Stacker/lib/runtime/stacker_rt.c</p>
|
|
</div>
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection"><a name="driver"></a>Compiler Driver</div>
|
|
<div class="doc_text">
|
|
<p>See projects/Stacker/tools/stkrc/stkrc.cpp</p>
|
|
</div>
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection"><a name="tests"></a>Test Programs</div>
|
|
<div class="doc_text">
|
|
<p>See projects/Stacker/test/*.st</p>
|
|
</div>
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection"> <a name="exercise">Exercise</a></div>
|
|
<div class="doc_text">
|
|
<p>As you may have noted from a careful inspection of the Built-In word
|
|
definitions, the ROLL word is not implemented. This word was left out of
|
|
Stacker on purpose so that it can be an exercise for the student. The exercise
|
|
is to implement the ROLL functionality (in your own workspace) and build a test
|
|
program for it. If you can implement ROLL, you understand Stacker and probably
|
|
a fair amount about LLVM since this is one of the more complicated Stacker
|
|
operations. The work will almost be completely limited to the
|
|
<a href="#compiler">compiler</a>.
|
|
<p>The ROLL word is already recognized by both the lexer and parser but ignored
|
|
by the compiler. That means you don't have to futz around with figuring out how
|
|
to get the keyword recognized. It already is. The part of the compiler that
|
|
you need to implement is the <code>ROLL</code> case in the
|
|
<code>StackerCompiler::handle_word(int)</code> method.</p> See the
|
|
implementations of PICK and SELECT in the same method to get some hints about
|
|
how to complete this exercise.<p>
|
|
<p>Good luck!</p>
|
|
</div>
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection"><a name="todo">Things Remaining To Be Done</a></div>
|
|
<div class="doc_text">
|
|
<p>The initial implementation of Stacker has several deficiencies. If you're
|
|
interested, here are some things that could be implemented better:</p>
|
|
<ol>
|
|
<li>Write an LLVM pass to compute the correct stack depth needed by the
|
|
program. Currently the stack is set to a fixed number which means programs
|
|
with large numbers of definitions might fail.</li>
|
|
<li>Write an LLVM pass to optimize the use of the global stack. The code
|
|
emitted currently is somewhat wasteful. It gets cleaned up a lot by existing
|
|
passes but more could be done.</li>
|
|
<li>Make the compiler driver use the LLVM linking facilities (with IPO)
|
|
before depending on GCC to do the final link.</li>
|
|
<li>Clean up parsing. It doesn't handle errors very well.</li>
|
|
<li>Rearrange the StackerCompiler.cpp code to make better use of inserting
|
|
instructions before a block's terminating instruction. I didn't figure this
|
|
technique out until I was nearly done with LLVM. As it is, its a bad example
|
|
of how to insert instructions!</li>
|
|
<li>Provide for I/O to arbitrary files instead of just stdin/stdout.</li>
|
|
<li>Write additional built-in words; with inspiration from FORTH</li>
|
|
<li>Write additional sample Stacker programs.</li>
|
|
<li>Add your own compiler writing experiences and tips in the
|
|
<a href="#lessons">Lessons I Learned About LLVM</a> section.</li>
|
|
</ol>
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
<hr>
|
|
<address>
|
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src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
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|
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<a href="mailto:rspencer@x10sys.com">Reid Spencer</a><br>
|
|
<a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
|
|
Last modified: $Date$
|
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</address>
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