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    <p>"Unrolling a loop" normally means replacing a repetition with a sequence of actions. The loop:</p> <pre><code>for (int i = 0; i &lt; 4; ++i) { a[i] = b[i] + c[i]; } </code></pre> <p>would unroll into the equivalent:</p> <pre><code>a[0] = b[0] + c[0]; a[1] = b[1] + c[1]; a[2] = b[2] + c[2]; a[3] = b[3] + c[3]; </code></pre> <p>It appears to me that whoever was being quoted by Wikipedia was using the phrase in a somewhat metaphorical sense. So, in that sense...</p> <p>Your sample would normally be invoked inside a interpreter that is walking a tree of AST nodes, which might look something like this:</p> <pre><code> ASSIGN | +--+---+ | | REF MINUS | | x +--+---+ | | VAR PLUS | | a +--+--+ | | VAR CONST | | b 3 </code></pre> <p>and the <code>interpret</code> function would have additional options:</p> <pre><code>int interpret(node) { switch(node) { case PLUS: return interpret(child(0))+interpret(child(1)); case MINUS: return interpret(child(0))-interpret(child(1)); case ASSIGN: return set(child(0), interpret(child(1)); case VAR: return fetch(child(0)); case CONST: return value(child(0)); ... } } </code></pre> <p>If you walk the AST with that <code>interpet</code> function (actually performing the operations), you're interpreting. But if the function <em>records</em> the actions to be performed, rather than <em>executing</em> them, you're compiling. In pseudocode (actually, pseudocode <em>twice</em>, as I'm assuming a hypothetical stack machine as the compilation target):</p> <pre><code>string compile(node) { switch(node) { case PLUS: return(compile(child(0))) + compile(child(1)) + ADD); case MINUS: return(compile(child(0))) + compile(child(1)) + SUB); case ASSIGN: return(PUSHA(child(0))) + compile(child(1)) + STORE); case REF: return(PUSHA(child(0))); case VAR: return(PUSHA(child(0)) + FETCH); case CONST: return(PUSHLIT + value(child(0))); ... } } </code></pre> <p>Invoking <code>compile</code> on that AST (ignoring any pseudocode typos ;-) would spit out something like:</p> <pre><code>PUSHA x PUSHA a FETCH PUSHA b FETCH PUSHLIT 3 ADD SUB STORE </code></pre> <p>FWIW, I'd tend to think of that as unrolling the AST, rather than unrolling the interpreter, but won't criticize somebody else's metaphor without reading it in context.</p>
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    1. POHow do I unroll (compile) an interpreter loop?
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    1. USjoel.neely
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    1. POHow do I unroll (compile) an interpreter loop?
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    1. CORecursion is a fancy loop.
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    2. COYou could perhaps mention what kind of code you are producing here (stack machine vs. other forms of intermediate language). Otherwise, good answer.
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      1. USKonrad Rudolph
    3. CO@Konrad: Thanks for the suggestion. I edited to incorporate it.
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