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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<title>Eliminate Redundant Operations</title>
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<!--Weave of 'Eliminate Redundant Operations' generated by Inweb-->
<div class="breadcrumbs">
<ul class="crumbs"><li><a href="../index.html">Home</a></li><li><a href="../compiler.html">Inter Modules</a></li><li><a href="index.html">pipeline</a></li><li><a href="index.html#2">Chapter 2: Miscellaneous Pipeline Stages</a></li><li><b>Eliminate Redundant Operations</b></li></ul></div>
<p class="purpose">To remove logical or arithmetic operations which do nothing.</p>
<ul class="toc"><li><a href="2-ero.html#SP1">&#167;1. Pipeline stage</a></li></ul><hr class="tocbar">
<p class="commentary firstcommentary"><a id="SP1" class="paragraph-anchor"></a><b>&#167;1. Pipeline stage. </b>This stage removes redundant operations, replacing each of the following
with just <span class="extract"><span class="extract-syntax">x</span></span>. This is useful mainly because the <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> operations like these; the other
cases occur much more rarely.
</p>
<pre class="displayed-code all-displayed-code code-font">
<span class="plain-syntax"> x || false</span>
<span class="plain-syntax"> x &amp;&amp; true</span>
<span class="plain-syntax"> x + 0</span>
<span class="plain-syntax"> 0 + x</span>
<span class="plain-syntax"> x - 0</span>
<span class="plain-syntax"> x * 1</span>
<span class="plain-syntax"> 1 * x</span>
<span class="plain-syntax"> x / 1</span>
</pre>
<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>),
but we would need to be careful about word size on the VM, and there's not much
gain because the next compiler after us (e.g. Inform 6) will perform its own
constant-folding anyway.
</p>
<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 &amp;&amp; false</span></span> with <span class="extract"><span class="extract-syntax">false</span></span>, because then
any intended side-effects of evaluating <span class="extract"><span class="extract-syntax">x</span></span> would be lost.
</p>
<pre class="displayed-code all-displayed-code code-font">
<span class="reserved-syntax">void</span><span class="plain-syntax"> </span><span class="function-syntax">CodeGen::Operations::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">CodeGen::Operations::create_pipeline_stage</span></span>:<br/>Stages - <a href="1-stg.html#SP2">&#167;2</a></span></button><span class="plain-syntax">(</span><span class="reserved-syntax">void</span><span class="plain-syntax">) {</span>
<span class="plain-syntax"> </span><a href="1-stg.html#SP1" class="function-link"><span class="function-syntax">CodeGen::Stage::new</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>
<span class="plain-syntax"> </span><a href="2-ero.html#SP1" class="function-link"><span class="function-syntax">CodeGen::Operations::run_pipeline_stage</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>
<span class="plain-syntax">}</span>
<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>
<span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="function-syntax">CodeGen::Operations::run_pipeline_stage</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>
<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>
<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">-&gt;</span><span class="element-syntax">repository</span><span class="plain-syntax">, </span><a href="2-ero.html#SP1" class="function-link"><span class="function-syntax">CodeGen::Operations::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>
<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"> &gt; </span><span class="constant-syntax">0</span><span class="plain-syntax">)</span>
<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>
<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>
<span class="plain-syntax">}</span>
<span class="reserved-syntax">void</span><span class="plain-syntax"> </span><span class="function-syntax">CodeGen::Operations::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>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">P</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">W</span><span class="plain-syntax">.</span><span class="identifier-syntax">data</span><span class="plain-syntax">[</span><span class="identifier-syntax">ID_IFLD</span><span class="plain-syntax">] == </span><span class="identifier-syntax">PACKAGE_IST</span><span class="plain-syntax">) {</span>
<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">Inter::Package::defined_by_frame</span><span class="plain-syntax">(</span><span class="identifier-syntax">P</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">Inter::Packages::is_codelike</span><span class="plain-syntax">(</span><span class="identifier-syntax">pack</span><span class="plain-syntax">)) {</span>
<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">Inter::Packages::definition</span><span class="plain-syntax">(</span><span class="identifier-syntax">pack</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><a href="2-ero.html#SP2" class="function-link"><span class="function-syntax">CodeGen::Operations::traverse_code_tree</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">D</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> }</span>
<span class="plain-syntax"> }</span>
<span class="plain-syntax">}</span>
</pre>
<p class="commentary firstcommentary"><a id="SP2" class="paragraph-anchor"></a><b>&#167;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
operations:
</p>
<pre class="displayed-code all-displayed-code code-font">
<span class="reserved-syntax">void</span><span class="plain-syntax"> </span><span class="function-syntax">CodeGen::Operations::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">CodeGen::Operations::traverse_code_tree</span></span>:<br/><a href="2-ero.html#SP1">&#167;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>
<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>
<span class="plain-syntax"> </span><a href="2-ero.html#SP2" class="function-link"><span class="function-syntax">CodeGen::Operations::traverse_code_tree</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">F</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> }</span>
<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>
<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>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> ((</span><span class="identifier-syntax">F</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">W</span><span class="plain-syntax">.</span><span class="identifier-syntax">data</span><span class="plain-syntax">[</span><span class="identifier-syntax">ID_IFLD</span><span class="plain-syntax">] == </span><span class="identifier-syntax">INV_IST</span><span class="plain-syntax">) &amp;&amp;</span>
<span class="plain-syntax"> (</span><span class="identifier-syntax">F</span><span class="plain-syntax">-&gt;</span><span class="identifier-syntax">W</span><span class="plain-syntax">.</span><span class="identifier-syntax">data</span><span class="plain-syntax">[</span><span class="identifier-syntax">METHOD_INV_IFLD</span><span class="plain-syntax">] == </span><span class="identifier-syntax">INVOKED_PRIMITIVE</span><span class="plain-syntax">)) {</span>
<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">Inter::Inv::invokee</span><span class="plain-syntax">(</span><span class="identifier-syntax">F</span><span class="plain-syntax">);</span>
<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">-&gt;</span><span class="identifier-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>
<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">-&gt;</span><span class="identifier-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>
<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">-&gt;</span><span class="identifier-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>
<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">-&gt;</span><span class="identifier-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>
<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">-&gt;</span><span class="identifier-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>
<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">-&gt;</span><span class="identifier-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>
<span class="plain-syntax"> }</span>
<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] &gt;= </span><span class="constant-syntax">0</span><span class="plain-syntax">) || (</span><span class="identifier-syntax">iden</span><span class="plain-syntax">[1] &gt;= </span><span class="constant-syntax">0</span><span class="plain-syntax">)) </span><span class="named-paragraph-container code-font"><a href="2-ero.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>
<span class="plain-syntax"> }</span>
<span class="plain-syntax">}</span>
</pre>
<p class="commentary firstcommentary"><a id="SP2_1" class="paragraph-anchor"></a><b>&#167;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>
</p>
<pre class="displayed-code all-displayed-code code-font">
<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>
<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>
<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>
<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]) &amp;&amp; (</span><span class="identifier-syntax">operands</span><span class="plain-syntax">[1])) {</span>
<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"> &lt; </span><span class="constant-syntax">2</span><span class="plain-syntax">; </span><span class="identifier-syntax">i</span><span class="plain-syntax">++) {</span>
<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">] &gt;= </span><span class="constant-syntax">0</span><span class="plain-syntax">) &amp;&amp; (</span><span class="identifier-syntax">operands</span><span class="plain-syntax">[</span><span class="identifier-syntax">i</span><span class="plain-syntax">]-&gt;</span><span class="identifier-syntax">W</span><span class="plain-syntax">.</span><span class="identifier-syntax">data</span><span class="plain-syntax">[</span><span class="identifier-syntax">ID_IFLD</span><span class="plain-syntax">] == </span><span class="identifier-syntax">VAL_IST</span><span class="plain-syntax">)) {</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">inter_ti</span><span class="plain-syntax"> </span><span class="identifier-syntax">val1</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">]-&gt;</span><span class="identifier-syntax">W</span><span class="plain-syntax">.</span><span class="identifier-syntax">data</span><span class="plain-syntax">[</span><span class="identifier-syntax">VAL1_VAL_IFLD</span><span class="plain-syntax">];</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">inter_ti</span><span class="plain-syntax"> </span><span class="identifier-syntax">val2</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">]-&gt;</span><span class="identifier-syntax">W</span><span class="plain-syntax">.</span><span class="identifier-syntax">data</span><span class="plain-syntax">[</span><span class="identifier-syntax">VAL2_VAL_IFLD</span><span class="plain-syntax">];</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> ((</span><span class="identifier-syntax">val1</span><span class="plain-syntax"> == </span><span class="identifier-syntax">LITERAL_IVAL</span><span class="plain-syntax">) &amp;&amp; (</span><span class="identifier-syntax">val2</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>
<span class="plain-syntax"> </span><span class="identifier-syntax">redundant_operations_removed</span><span class="plain-syntax">++;</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">InterTree::remove_node</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="plain-syntax"> </span><span class="identifier-syntax">InterTree::place</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">IMMEDIATELY_AFTER_ICPLACEMENT</span><span class="plain-syntax">, </span><span class="identifier-syntax">F</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="identifier-syntax">InterTree::remove_node</span><span class="plain-syntax">(</span><span class="identifier-syntax">F</span><span class="plain-syntax">);</span>
<span class="plain-syntax"> </span><span class="reserved-syntax">break</span><span class="plain-syntax">;</span>
<span class="plain-syntax"> }</span>
<span class="plain-syntax"> }</span>
<span class="plain-syntax"> }</span>
<span class="plain-syntax"> }</span>
</pre>
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