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<!--Weave of 'Eliminate Redundant Operations 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 Operations Stage</b></li></ul></div>
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<p class="purpose">To remove logical or arithmetic operations which neither do anything, nor have side-effects.</p>
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<p class="commentary firstcommentary"><a id="SP1" class="paragraph-anchor"></a><b>§1. </b>This stage removes redundant operations, replacing each of the following
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with just <span class="extract"><span class="extract-syntax">x</span></span>. This is useful mainly for the first of these cases, because
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<a href="../imperative-module/2-cc.html" class="internal">Compile Conditions (in imperative)</a> has a tendency to make redundant <span class="extract"><span class="extract-syntax">OR_BIP</span></span>
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operations. The other cases occur much more rarely, but we might as well
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handle them too.
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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"> x || false</span>
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<span class="plain-syntax"> x && true</span>
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<span class="plain-syntax"> x + 0</span>
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<span class="plain-syntax"> 0 + x</span>
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<span class="plain-syntax"> x - 0</span>
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<span class="plain-syntax"> x * 1</span>
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<span class="plain-syntax"> 1 * x</span>
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<span class="plain-syntax"> x / 1</span>
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</pre>
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<p class="commentary">We could also perform constant-folding here (e.g., replacing <span class="extract"><span class="extract-syntax">2+3</span></span> with <span class="extract"><span class="extract-syntax">5</span></span>),
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but we would need to be careful about word size on the VM, and there's not much
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gain because the next compiler after us (e.g. Inform 6) will perform its own
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constant-folding anyway.
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</p>
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<p class="commentary">We do not replace <span class="extract"><span class="extract-syntax">x * 0</span></span> with <span class="extract"><span class="extract-syntax">0</span></span>, nor <span class="extract"><span class="extract-syntax">x && false</span></span> with <span class="extract"><span class="extract-syntax">false</span></span>, because then
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any intended side-effects of evaluating <span class="extract"><span class="extract-syntax">x</span></span> would be lost.
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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">EliminateRedundantOperationsStage::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">EliminateRedundantOperationsStage::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-operations"</span><span class="plain-syntax">,</span>
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<span class="plain-syntax"> </span><a href="6-eros.html#SP1" class="function-link"><span class="function-syntax">EliminateRedundantOperationsStage::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_operations_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">EliminateRedundantOperationsStage::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_operations_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-eros.html#SP1" class="function-link"><span class="function-syntax">EliminateRedundantOperationsStage::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="constant-syntax">0</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_operations_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 operation(s) removed\n"</span><span class="plain-syntax">, </span><span class="identifier-syntax">redundant_operations_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">EliminateRedundantOperationsStage::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="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">P</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="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="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-eros.html#SP2" class="function-link"><span class="function-syntax">EliminateRedundantOperationsStage::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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<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" class="paragraph-anchor"></a><b>§2. </b><span class="extract"><span class="extract-syntax">iden[0]</span></span> and <span class="extract"><span class="extract-syntax">iden[1]</span></span> hold left and right identity elements for these binary
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operations:
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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">EliminateRedundantOperationsStage::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">EliminateRedundantOperationsStage::traverse_code_tree</span></span>:<br/><a href="6-eros.html#SP1">§1</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">PROTECTED_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><a href="6-eros.html#SP2" class="function-link"><span class="function-syntax">EliminateRedundantOperationsStage::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><span class="identifier-syntax">PROTECTED_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="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="identifier-syntax">iden</span><span class="plain-syntax">[2] = { -1, -1 };</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">Inode::is</span><span class="plain-syntax">(</span><span class="identifier-syntax">F</span><span class="plain-syntax">, </span><span class="identifier-syntax">INV_IST</span><span class="plain-syntax">)) &&</span>
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<span class="plain-syntax"> (</span><span class="identifier-syntax">InvInstruction::method</span><span class="plain-syntax">(</span><span class="identifier-syntax">F</span><span class="plain-syntax">) == </span><span class="identifier-syntax">PRIMITIVE_INVMETH</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">prim</span><span class="plain-syntax"> = </span><span class="identifier-syntax">InvInstruction::primitive</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="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">Primitives::to_BIP</span><span class="plain-syntax">(</span><span class="identifier-syntax">P</span><span class="plain-syntax">-></span><span class="element-syntax">tree</span><span class="plain-syntax">, </span><span class="identifier-syntax">prim</span><span class="plain-syntax">) == </span><span class="identifier-syntax">OR_BIP</span><span class="plain-syntax">) { </span><span class="identifier-syntax">iden</span><span class="plain-syntax">[1] = </span><span class="constant-syntax">0</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">Primitives::to_BIP</span><span class="plain-syntax">(</span><span class="identifier-syntax">P</span><span class="plain-syntax">-></span><span class="element-syntax">tree</span><span class="plain-syntax">, </span><span class="identifier-syntax">prim</span><span class="plain-syntax">) == </span><span class="identifier-syntax">AND_BIP</span><span class="plain-syntax">) { </span><span class="identifier-syntax">iden</span><span class="plain-syntax">[1] = </span><span class="constant-syntax">1</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">Primitives::to_BIP</span><span class="plain-syntax">(</span><span class="identifier-syntax">P</span><span class="plain-syntax">-></span><span class="element-syntax">tree</span><span class="plain-syntax">, </span><span class="identifier-syntax">prim</span><span class="plain-syntax">) == </span><span class="identifier-syntax">PLUS_BIP</span><span class="plain-syntax">) { </span><span class="identifier-syntax">iden</span><span class="plain-syntax">[0] = </span><span class="constant-syntax">0</span><span class="plain-syntax">; </span><span class="identifier-syntax">iden</span><span class="plain-syntax">[1] = </span><span class="constant-syntax">0</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">Primitives::to_BIP</span><span class="plain-syntax">(</span><span class="identifier-syntax">P</span><span class="plain-syntax">-></span><span class="element-syntax">tree</span><span class="plain-syntax">, </span><span class="identifier-syntax">prim</span><span class="plain-syntax">) == </span><span class="identifier-syntax">MINUS_BIP</span><span class="plain-syntax">) { </span><span class="identifier-syntax">iden</span><span class="plain-syntax">[1] = </span><span class="constant-syntax">0</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">Primitives::to_BIP</span><span class="plain-syntax">(</span><span class="identifier-syntax">P</span><span class="plain-syntax">-></span><span class="element-syntax">tree</span><span class="plain-syntax">, </span><span class="identifier-syntax">prim</span><span class="plain-syntax">) == </span><span class="identifier-syntax">TIMES_BIP</span><span class="plain-syntax">) { </span><span class="identifier-syntax">iden</span><span class="plain-syntax">[0] = </span><span class="constant-syntax">1</span><span class="plain-syntax">; </span><span class="identifier-syntax">iden</span><span class="plain-syntax">[1] = </span><span class="constant-syntax">1</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">Primitives::to_BIP</span><span class="plain-syntax">(</span><span class="identifier-syntax">P</span><span class="plain-syntax">-></span><span class="element-syntax">tree</span><span class="plain-syntax">, </span><span class="identifier-syntax">prim</span><span class="plain-syntax">) == </span><span class="identifier-syntax">DIVIDE_BIP</span><span class="plain-syntax">) { </span><span class="identifier-syntax">iden</span><span class="plain-syntax">[1] = </span><span class="constant-syntax">1</span><span class="plain-syntax">; }</span>
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<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">iden</span><span class="plain-syntax">[0] >= </span><span class="constant-syntax">0</span><span class="plain-syntax">) || (</span><span class="identifier-syntax">iden</span><span class="plain-syntax">[1] >= </span><span class="constant-syntax">0</span><span class="plain-syntax">)) </span><span class="named-paragraph-container code-font"><a href="6-eros.html#SP2_1" class="named-paragraph-link"><span class="named-paragraph">An elimination candidate</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>
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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><span class="named-paragraph-container code-font"><span class="named-paragraph-defn">An elimination candidate</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="identifier-syntax">inter_tree_node</span><span class="plain-syntax"> *</span><span class="identifier-syntax">operands</span><span class="plain-syntax">[2];</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">operands</span><span class="plain-syntax">[0] = </span><span class="identifier-syntax">InterTree::first_child</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">operands</span><span class="plain-syntax">[1] = </span><span class="identifier-syntax">InterTree::second_child</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="reserved-syntax">if</span><span class="plain-syntax"> ((</span><span class="identifier-syntax">operands</span><span class="plain-syntax">[0]) && (</span><span class="identifier-syntax">operands</span><span class="plain-syntax">[1])) {</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">for</span><span class="plain-syntax"> (</span><span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="identifier-syntax">i</span><span class="plain-syntax"> = </span><span class="constant-syntax">0</span><span class="plain-syntax">; </span><span class="identifier-syntax">i</span><span class="plain-syntax"> < </span><span class="constant-syntax">2</span><span class="plain-syntax">; </span><span class="identifier-syntax">i</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">iden</span><span class="plain-syntax">[</span><span class="identifier-syntax">i</span><span class="plain-syntax">] >= </span><span class="constant-syntax">0</span><span class="plain-syntax">) && (</span><span class="identifier-syntax">Inode::is</span><span class="plain-syntax">(</span><span class="identifier-syntax">operands</span><span class="plain-syntax">[</span><span class="identifier-syntax">i</span><span class="plain-syntax">], </span><span class="identifier-syntax">VAL_IST</span><span class="plain-syntax">))) {</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">inter_pair</span><span class="plain-syntax"> </span><span class="identifier-syntax">val</span><span class="plain-syntax"> = </span><span class="identifier-syntax">ValInstruction::value</span><span class="plain-syntax">(</span><span class="identifier-syntax">operands</span><span class="plain-syntax">[</span><span class="identifier-syntax">i</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">InterValuePairs::is_number</span><span class="plain-syntax">(</span><span class="identifier-syntax">val</span><span class="plain-syntax">)) &&</span>
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<span class="plain-syntax"> (</span><span class="identifier-syntax">InterValuePairs::to_number</span><span class="plain-syntax">(</span><span class="identifier-syntax">val</span><span class="plain-syntax">) == (</span><span class="identifier-syntax">inter_ti</span><span class="plain-syntax">) </span><span class="identifier-syntax">iden</span><span class="plain-syntax">[</span><span class="identifier-syntax">i</span><span class="plain-syntax">])) {</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">redundant_operations_removed</span><span class="plain-syntax">++;</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">NodePlacement::remove</span><span class="plain-syntax">(</span><span class="identifier-syntax">operands</span><span class="plain-syntax">[</span><span class="identifier-syntax">i</span><span class="plain-syntax">]);</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">NodePlacement::move_to</span><span class="plain-syntax">(</span><span class="identifier-syntax">operands</span><span class="plain-syntax">[1-</span><span class="identifier-syntax">i</span><span class="plain-syntax">], </span><span class="identifier-syntax">InterBookmark::immediately_after</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">NodePlacement::set_levels</span><span class="plain-syntax">(</span><span class="identifier-syntax">operands</span><span class="plain-syntax">[1-</span><span class="identifier-syntax">i</span><span class="plain-syntax">], </span><span class="identifier-syntax">Inode::get_level</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">NodePlacement::remove</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="reserved-syntax">break</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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<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-eros.html#SP2">§2</a>.</li></ul>
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