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<title>Eliminate Redundant Labels Stage</title>
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<h1><a href="../index.html">
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<!--Weave of 'Eliminate Redundant Labels Stage' generated by Inweb-->
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<div class="breadcrumbs">
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<ul class="crumbs"><li><a href="../index.html">Home</a></li><li><a href="../intern.html">Inter Modules</a></li><li><a href="index.html">pipeline</a></li><li><a href="index.html#6">Chapter 6: Optimisation</a></li><li><b>Eliminate Redundant Labels Stage</b></li></ul></div>
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<p class="purpose">To remove labels which are defined but never jumped to.</p>
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<p class="commentary firstcommentary"><a id="SP1" class="paragraph-anchor"></a><b>§1. </b><a href="../inform7/index.html" class="internal">inform7</a> tends to produce a lot of labels when compiling complicated text
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substitutions, but many — around 2000 in a typical run — are never branched to,
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either by a jump invocation or by assembly language.
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</p>
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<p class="commentary">These spurious labels cause no real problem except untidiness, but removing
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them provides a simple example of how peephole optimisation can be performed
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on an Inter tree.
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</p>
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<pre class="displayed-code all-displayed-code code-font">
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<span class="reserved-syntax">void</span><span class="plain-syntax"> </span><span class="function-syntax">EliminateRedundantLabelsStage::create_pipeline_stage</span><button class="popup" onclick="togglePopup('usagePopup1')"><span class="comment-syntax">?</span><span class="popuptext" id="usagePopup1">Usage of <span class="code-font"><span class="function-syntax">EliminateRedundantLabelsStage::create_pipeline_stage</span></span>:<br/>Parsing Pipelines - <a href="2-pp.html#SP10">§10</a></span></button><span class="plain-syntax">(</span><span class="reserved-syntax">void</span><span class="plain-syntax">) {</span>
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<span class="plain-syntax"> </span><a href="2-pp.html#SP3" class="function-link"><span class="function-syntax">ParsingPipelines::new_stage</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">I</span><span class="string-syntax">"eliminate-redundant-labels"</span><span class="plain-syntax">,</span>
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<span class="plain-syntax"> </span><a href="6-erls.html#SP1" class="function-link"><span class="function-syntax">EliminateRedundantLabelsStage::run</span></a><span class="plain-syntax">, </span><span class="constant-syntax">NO_STAGE_ARG</span><span class="plain-syntax">, </span><span class="identifier-syntax">FALSE</span><span class="plain-syntax">);</span>
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<span class="plain-syntax">}</span>
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<span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="identifier-syntax">redundant_labels_removed</span><span class="plain-syntax"> = </span><span class="constant-syntax">0</span><span class="plain-syntax">;</span>
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<span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="function-syntax">EliminateRedundantLabelsStage::run</span><span class="plain-syntax">(</span><span class="reserved-syntax">pipeline_step</span><span class="plain-syntax"> *</span><span class="identifier-syntax">step</span><span class="plain-syntax">) {</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">redundant_labels_removed</span><span class="plain-syntax"> = </span><span class="constant-syntax">0</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">InterTree::traverse</span><span class="plain-syntax">(</span><span class="identifier-syntax">step</span><span class="plain-syntax">-></span><span class="element-syntax">ephemera</span><span class="plain-syntax">.</span><span class="element-syntax">tree</span><span class="plain-syntax">,</span>
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<span class="plain-syntax"> </span><a href="6-erls.html#SP1" class="function-link"><span class="function-syntax">EliminateRedundantLabelsStage::visitor</span></a><span class="plain-syntax">, </span><span class="identifier-syntax">NULL</span><span class="plain-syntax">, </span><span class="identifier-syntax">NULL</span><span class="plain-syntax">, </span><span class="identifier-syntax">PACKAGE_IST</span><span class="plain-syntax">);</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">redundant_labels_removed</span><span class="plain-syntax"> > </span><span class="constant-syntax">0</span><span class="plain-syntax">)</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">LOG</span><span class="plain-syntax">(</span><span class="string-syntax">"%d redundant label(s) removed\n"</span><span class="plain-syntax">, </span><span class="identifier-syntax">redundant_labels_removed</span><span class="plain-syntax">);</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">TRUE</span><span class="plain-syntax">;</span>
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<span class="plain-syntax">}</span>
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<span class="reserved-syntax">void</span><span class="plain-syntax"> </span><span class="function-syntax">EliminateRedundantLabelsStage::visitor</span><span class="plain-syntax">(</span><span class="identifier-syntax">inter_tree</span><span class="plain-syntax"> *</span><span class="identifier-syntax">I</span><span class="plain-syntax">, </span><span class="identifier-syntax">inter_tree_node</span><span class="plain-syntax"> *</span><span class="identifier-syntax">P</span><span class="plain-syntax">, </span><span class="reserved-syntax">void</span><span class="plain-syntax"> *</span><span class="identifier-syntax">state</span><span class="plain-syntax">) {</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">inter_package</span><span class="plain-syntax"> *</span><span class="identifier-syntax">pack</span><span class="plain-syntax"> = </span><span class="identifier-syntax">PackageInstruction::at_this_head</span><span class="plain-syntax">(</span><span class="identifier-syntax">P</span><span class="plain-syntax">);</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">InterPackage::is_a_function_body</span><span class="plain-syntax">(</span><span class="identifier-syntax">pack</span><span class="plain-syntax">))</span>
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<span class="plain-syntax"> </span><span class="named-paragraph-container code-font"><a href="6-erls.html#SP1_1" class="named-paragraph-link"><span class="named-paragraph">Perform peephole optimisation on this function body</span><span class="named-paragraph-number">1.1</span></a></span><span class="plain-syntax">;</span>
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<span class="plain-syntax">}</span>
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</pre>
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<p class="commentary firstcommentary"><a id="SP1_1" class="paragraph-anchor"></a><b>§1.1. </b><span class="named-paragraph-container code-font"><span class="named-paragraph-defn">Perform peephole optimisation on this function body</span><span class="named-paragraph-number">1.1</span></span><span class="comment-syntax"> =</span>
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</p>
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<pre class="displayed-code all-displayed-code code-font">
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<span class="plain-syntax"> </span><span class="identifier-syntax">inter_symbols_table</span><span class="plain-syntax"> *</span><span class="identifier-syntax">local_symbols</span><span class="plain-syntax"> = </span><span class="identifier-syntax">InterPackage::scope</span><span class="plain-syntax">(</span><span class="identifier-syntax">pack</span><span class="plain-syntax">);</span>
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<span class="plain-syntax"> </span><span class="named-paragraph-container code-font"><a href="6-erls.html#SP1_1_1" class="named-paragraph-link"><span class="named-paragraph">Mark all the labels for this function as being unused</span><span class="named-paragraph-number">1.1.1</span></a></span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="named-paragraph-container code-font"><a href="6-erls.html#SP1_1_2" class="named-paragraph-link"><span class="named-paragraph">Look through the function for mentions of labels, marking those as used</span><span class="named-paragraph-number">1.1.2</span></a></span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="named-paragraph-container code-font"><a href="6-erls.html#SP1_1_3" class="named-paragraph-link"><span class="named-paragraph">Remove the label declarations for any that are still marked unused</span><span class="named-paragraph-number">1.1.3</span></a></span><span class="plain-syntax">;</span>
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</pre>
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<ul class="endnotetexts"><li>This code is used in <a href="6-erls.html#SP1">§1</a>.</li></ul>
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<p class="commentary firstcommentary"><a id="SP1_1_1" class="paragraph-anchor"></a><b>§1.1.1. </b>The symbol flag <span class="extract"><span class="extract-syntax">USED_MARK_ISYMF</span></span> is free for us to use, but its value for
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any given symbol is undefined when we begin. We'll clear it for all labels.
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</p>
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<p class="commentary"><span class="named-paragraph-container code-font"><span class="named-paragraph-defn">Mark all the labels for this function as being unused</span><span class="named-paragraph-number">1.1.1</span></span><span class="comment-syntax"> =</span>
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</p>
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<pre class="displayed-code all-displayed-code code-font">
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<span class="plain-syntax"> </span><span class="identifier-syntax">LOOP_OVER_SYMBOLS_TABLE</span><span class="plain-syntax">(</span><span class="identifier-syntax">S</span><span class="plain-syntax">, </span><span class="identifier-syntax">local_symbols</span><span class="plain-syntax">)</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">InterSymbol::is_label</span><span class="plain-syntax">(</span><span class="identifier-syntax">S</span><span class="plain-syntax">))</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">InterSymbol::clear_flag</span><span class="plain-syntax">(</span><span class="identifier-syntax">S</span><span class="plain-syntax">, </span><span class="identifier-syntax">USED_MARK_ISYMF</span><span class="plain-syntax">);</span>
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</pre>
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<ul class="endnotetexts"><li>This code is used in <a href="6-erls.html#SP1_1">§1.1</a>.</li></ul>
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<p class="commentary firstcommentary"><a id="SP1_1_2" class="paragraph-anchor"></a><b>§1.1.2. </b><span class="named-paragraph-container code-font"><span class="named-paragraph-defn">Look through the function for mentions of labels, marking those as used</span><span class="named-paragraph-number">1.1.2</span></span><span class="comment-syntax"> =</span>
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</p>
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<pre class="displayed-code all-displayed-code code-font">
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<span class="plain-syntax"> </span><span class="identifier-syntax">inter_tree_node</span><span class="plain-syntax"> *</span><span class="identifier-syntax">D</span><span class="plain-syntax"> = </span><span class="identifier-syntax">InterPackage::head</span><span class="plain-syntax">(</span><span class="identifier-syntax">pack</span><span class="plain-syntax">);</span>
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<span class="plain-syntax"> </span><a href="6-erls.html#SP2" class="function-link"><span class="function-syntax">EliminateRedundantLabelsStage::traverse_code_tree</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">D</span><span class="plain-syntax">);</span>
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</pre>
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<ul class="endnotetexts"><li>This code is used in <a href="6-erls.html#SP1_1">§1.1</a>.</li></ul>
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<p class="commentary firstcommentary"><a id="SP1_1_3" class="paragraph-anchor"></a><b>§1.1.3. </b>Anything not marked used must be unused, so we can get rid of it. We do this
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by striking its definition; the definition of a label symbol is the line
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which shows where it belongs in the function (written <span class="extract"><span class="extract-syntax">.Example</span></span> in Inter
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syntax). Striking this does two things: it removes the definition line; and
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it renders the symbol undefined. It still lives on in the function's symbols
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table, though, and (since we have made sure there are no references to it from
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anywhere) we may as well remove it.
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</p>
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<p class="commentary"><span class="named-paragraph-container code-font"><span class="named-paragraph-defn">Remove the label declarations for any that are still marked unused</span><span class="named-paragraph-number">1.1.3</span></span><span class="comment-syntax"> =</span>
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</p>
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<pre class="displayed-code all-displayed-code code-font">
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<span class="plain-syntax"> </span><span class="identifier-syntax">LOOP_OVER_SYMBOLS_TABLE</span><span class="plain-syntax">(</span><span class="identifier-syntax">S</span><span class="plain-syntax">, </span><span class="identifier-syntax">local_symbols</span><span class="plain-syntax">)</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">InterSymbol::is_label</span><span class="plain-syntax">(</span><span class="identifier-syntax">S</span><span class="plain-syntax">))</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">InterSymbol::get_flag</span><span class="plain-syntax">(</span><span class="identifier-syntax">S</span><span class="plain-syntax">, </span><span class="identifier-syntax">USED_MARK_ISYMF</span><span class="plain-syntax">) == </span><span class="identifier-syntax">FALSE</span><span class="plain-syntax">) {</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">InterSymbol::strike_definition</span><span class="plain-syntax">(</span><span class="identifier-syntax">S</span><span class="plain-syntax">);</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">InterSymbolsTable::remove_symbol</span><span class="plain-syntax">(</span><span class="identifier-syntax">S</span><span class="plain-syntax">);</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">redundant_labels_removed</span><span class="plain-syntax">++;</span>
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<span class="plain-syntax"> }</span>
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</pre>
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<ul class="endnotetexts"><li>This code is used in <a href="6-erls.html#SP1_1">§1.1</a>.</li></ul>
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<p class="commentary firstcommentary"><a id="SP2" class="paragraph-anchor"></a><b>§2. </b>The following visits every line of code in the function, in the same order
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it would be written out in a listing.
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</p>
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<pre class="displayed-code all-displayed-code code-font">
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<span class="reserved-syntax">void</span><span class="plain-syntax"> </span><span class="function-syntax">EliminateRedundantLabelsStage::traverse_code_tree</span><button class="popup" onclick="togglePopup('usagePopup2')"><span class="comment-syntax">?</span><span class="popuptext" id="usagePopup2">Usage of <span class="code-font"><span class="function-syntax">EliminateRedundantLabelsStage::traverse_code_tree</span></span>:<br/><a href="6-erls.html#SP1_1_2">§1.1.2</a></span></button><span class="plain-syntax">(</span><span class="identifier-syntax">inter_tree_node</span><span class="plain-syntax"> *</span><span class="identifier-syntax">P</span><span class="plain-syntax">) {</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">LOOP_THROUGH_INTER_CHILDREN</span><span class="plain-syntax">(</span><span class="identifier-syntax">F</span><span class="plain-syntax">, </span><span class="identifier-syntax">P</span><span class="plain-syntax">) {</span>
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<span class="plain-syntax"> </span><span class="named-paragraph-container code-font"><a href="6-erls.html#SP2_1" class="named-paragraph-link"><span class="named-paragraph">Examine a line of code in the function</span><span class="named-paragraph-number">2.1</span></a></span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><a href="6-erls.html#SP2" class="function-link"><span class="function-syntax">EliminateRedundantLabelsStage::traverse_code_tree</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">F</span><span class="plain-syntax">);</span>
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<span class="plain-syntax"> }</span>
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<span class="plain-syntax">}</span>
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</pre>
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<p class="commentary firstcommentary"><a id="SP2_1" class="paragraph-anchor"></a><b>§2.1. </b>If a label is used, there will be a line reading <span class="extract"><span class="extract-syntax">lab Example</span></span> or similar.
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We look for such lines.
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</p>
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<p class="commentary"><span class="named-paragraph-container code-font"><span class="named-paragraph-defn">Examine a line of code in the function</span><span class="named-paragraph-number">2.1</span></span><span class="comment-syntax"> =</span>
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</p>
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<pre class="displayed-code all-displayed-code code-font">
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">Inode::is</span><span class="plain-syntax">(</span><span class="identifier-syntax">F</span><span class="plain-syntax">, </span><span class="identifier-syntax">LAB_IST</span><span class="plain-syntax">)) {</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">inter_symbol</span><span class="plain-syntax"> *</span><span class="identifier-syntax">lab</span><span class="plain-syntax"> = </span><span class="identifier-syntax">LabInstruction::label_symbol</span><span class="plain-syntax">(</span><span class="identifier-syntax">F</span><span class="plain-syntax">);</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">InterSymbol::set_flag</span><span class="plain-syntax">(</span><span class="identifier-syntax">lab</span><span class="plain-syntax">, </span><span class="identifier-syntax">USED_MARK_ISYMF</span><span class="plain-syntax">);</span>
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<span class="plain-syntax"> }</span>
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</pre>
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<ul class="endnotetexts"><li>This code is used in <a href="6-erls.html#SP2">§2</a>.</li></ul>
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