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<!--Weave of '2/erl' generated by 7-->
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<ul class="crumbs"><li><a href="../webs.html">★</a></li><li><a href="index.html">codegen</a></li><li><a href="index.html#2">Chapter 2: Miscellaneous Pipeline Stages</a></li><li><b>Eliminate Redundant Labels</b></li></ul><p class="purpose">To remove labels which are defined but never jumped to.</p>
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<ul class="toc"><li><a href="#SP1">§1. Pipeline stage</a></li><li><a href="#SP1_1">§1.1. Peephole optimisation</a></li></ul><hr class="tocbar">
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<p class="inwebparagraph"><a id="SP1"></a><b>§1. Pipeline stage. </b>I7 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="inwebparagraph">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 the Inter functions in a repository.
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</p>
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<pre class="display">
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<span class="reserved">void</span><span class="plain"> </span><span class="functiontext">CodeGen::Labels::create_pipeline_stage</span><span class="plain">(</span><span class="reserved">void</span><span class="plain">) {</span>
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<span class="functiontext">CodeGen::Stage::new</span><span class="plain">(</span><span class="identifier">I</span><span class="string">"eliminate-redundant-labels"</span><span class="plain">, </span><span class="functiontext">CodeGen::Labels::run_pipeline_stage</span><span class="plain">, </span><span class="constant">NO_STAGE_ARG</span><span class="plain">, </span><span class="identifier">FALSE</span><span class="plain">);</span>
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<span class="plain">}</span>
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<span class="reserved">int</span><span class="plain"> </span><span class="functiontext">CodeGen::Labels::run_pipeline_stage</span><span class="plain">(</span><span class="reserved">pipeline_step</span><span class="plain"> *</span><span class="identifier">step</span><span class="plain">) {</span>
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<span class="identifier">Inter::Tree::traverse</span><span class="plain">(</span><span class="identifier">step</span><span class="plain">-</span><span class="element">>repository</span><span class="plain">, </span><span class="functiontext">CodeGen::Labels::visitor</span><span class="plain">, </span><span class="identifier">NULL</span><span class="plain">, </span><span class="identifier">NULL</span><span class="plain">, 0);</span>
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<span class="reserved">return</span><span class="plain"> </span><span class="identifier">TRUE</span><span class="plain">;</span>
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<span class="plain">}</span>
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<span class="reserved">void</span><span class="plain"> </span><span class="functiontext">CodeGen::Labels::visitor</span><span class="plain">(</span><span class="identifier">inter_tree</span><span class="plain"> *</span><span class="identifier">I</span><span class="plain">, </span><span class="identifier">inter_tree_node</span><span class="plain"> *</span><span class="identifier">P</span><span class="plain">, </span><span class="reserved">void</span><span class="plain"> *</span><span class="identifier">state</span><span class="plain">) {</span>
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<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">P</span><span class="plain">-></span><span class="identifier">W</span><span class="plain">.</span><span class="identifier">data</span><span class="plain">[</span><span class="identifier">ID_IFLD</span><span class="plain">] == </span><span class="identifier">PACKAGE_IST</span><span class="plain">) {</span>
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<span class="identifier">inter_package</span><span class="plain"> *</span><span class="identifier">pack</span><span class="plain"> = </span><span class="identifier">Inter::Package::defined_by_frame</span><span class="plain">(</span><span class="identifier">P</span><span class="plain">);</span>
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<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">Inter::Packages::is_codelike</span><span class="plain">(</span><span class="identifier">pack</span><span class="plain">)) </span><<span class="cwebmacro">Perform peephole optimisation on this block</span> <span class="cwebmacronumber">1.1</span>><span class="plain">;</span>
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<span class="plain">}</span>
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<span class="plain">}</span>
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</pre>
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<p class="inwebparagraph"></p>
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<p class="endnote">The function CodeGen::Labels::create_pipeline_stage is used in 1/stg (<a href="1-stg.html#SP2">§2</a>).</p>
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<p class="endnote">The function CodeGen::Labels::run_pipeline_stage appears nowhere else.</p>
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<p class="endnote">The function CodeGen::Labels::visitor appears nowhere else.</p>
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<p class="inwebparagraph"><a id="SP1_1"></a><b>§1.1. Peephole optimisation. </b>We can now forget about the larger package structure, and just look through
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the peephole at a single function in Inter. It has its own symbols table, for
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local variables and label names, and also has a tree of code to define what
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it does.
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</p>
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<p class="macrodefinition"><code class="display">
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<<span class="cwebmacrodefn">Perform peephole optimisation on this block</span> <span class="cwebmacronumber">1.1</span>> =
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</code></p>
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<pre class="displaydefn">
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<span class="identifier">inter_symbols_table</span><span class="plain"> *</span><span class="identifier">local_symbols</span><span class="plain"> = </span><span class="identifier">Inter::Packages::scope</span><span class="plain">(</span><span class="identifier">pack</span><span class="plain">);</span>
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<<span class="cwebmacro">Mark all the labels for this function as being unused</span> <span class="cwebmacronumber">1.1.1</span>><span class="plain">;</span>
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<<span class="cwebmacro">Look through the function for mentions of labels, marking those as used</span> <span class="cwebmacronumber">1.1.2</span>><span class="plain">;</span>
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<<span class="cwebmacro">Remove the label declarations for any that are still marked unused</span> <span class="cwebmacronumber">1.1.3</span>><span class="plain">;</span>
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</pre>
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<p class="inwebparagraph"></p>
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<p class="endnote">This code is used in <a href="#SP1">§1</a>.</p>
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<p class="inwebparagraph"><a id="SP1_1_1"></a><b>§1.1.1. </b>The symbol flag <code class="display"><span class="extract">USED_MARK_BIT</span></code> 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="macrodefinition"><code class="display">
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<<span class="cwebmacrodefn">Mark all the labels for this function as being unused</span> <span class="cwebmacronumber">1.1.1</span>> =
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</code></p>
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<pre class="displaydefn">
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<span class="identifier">LOOP_OVER_SYMBOLS_TABLE</span><span class="plain">(</span><span class="identifier">S</span><span class="plain">, </span><span class="identifier">local_symbols</span><span class="plain">)</span>
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<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">Inter::Symbols::is_label</span><span class="plain">(</span><span class="identifier">S</span><span class="plain">))</span>
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<span class="identifier">Inter::Symbols::clear_flag</span><span class="plain">(</span><span class="identifier">S</span><span class="plain">, </span><span class="identifier">USED_MARK_BIT</span><span class="plain">);</span>
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</pre>
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<p class="inwebparagraph"></p>
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<p class="endnote">This code is used in <a href="#SP1_1">§1.1</a>.</p>
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<p class="inwebparagraph"><a id="SP1_1_2"></a><b>§1.1.2. </b><code class="display">
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<<span class="cwebmacrodefn">Look through the function for mentions of labels, marking those as used</span> <span class="cwebmacronumber">1.1.2</span>> =
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</code></p>
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<pre class="displaydefn">
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<span class="identifier">inter_tree_node</span><span class="plain"> *</span><span class="identifier">D</span><span class="plain"> = </span><span class="identifier">Inter::Packages::definition</span><span class="plain">(</span><span class="identifier">pack</span><span class="plain">);</span>
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<span class="functiontext">CodeGen::Labels::traverse_code_tree</span><span class="plain">(</span><span class="identifier">D</span><span class="plain">);</span>
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</pre>
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<p class="inwebparagraph"></p>
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<p class="endnote">This code is used in <a href="#SP1_1">§1.1</a>.</p>
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<p class="inwebparagraph"><a id="SP1_1_3"></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 <code class="display"><span class="extract">.Example</span></code> 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="macrodefinition"><code class="display">
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<<span class="cwebmacrodefn">Remove the label declarations for any that are still marked unused</span> <span class="cwebmacronumber">1.1.3</span>> =
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</code></p>
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<pre class="displaydefn">
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<span class="identifier">LOOP_OVER_SYMBOLS_TABLE</span><span class="plain">(</span><span class="identifier">S</span><span class="plain">, </span><span class="identifier">local_symbols</span><span class="plain">)</span>
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<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">Inter::Symbols::is_label</span><span class="plain">(</span><span class="identifier">S</span><span class="plain">))</span>
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<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">Inter::Symbols::get_flag</span><span class="plain">(</span><span class="identifier">S</span><span class="plain">, </span><span class="identifier">USED_MARK_BIT</span><span class="plain">) == </span><span class="identifier">FALSE</span><span class="plain">) {</span>
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<span class="identifier">Inter::Symbols::strike_definition</span><span class="plain">(</span><span class="identifier">S</span><span class="plain">);</span>
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<span class="identifier">Inter::Symbols::remove_from_table</span><span class="plain">(</span><span class="identifier">S</span><span class="plain">);</span>
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<span class="plain">}</span>
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</pre>
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<p class="inwebparagraph"></p>
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<p class="endnote">This code is used in <a href="#SP1_1">§1.1</a>.</p>
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<p class="inwebparagraph"><a id="SP2"></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="display">
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<span class="reserved">void</span><span class="plain"> </span><span class="functiontext">CodeGen::Labels::traverse_code_tree</span><span class="plain">(</span><span class="identifier">inter_tree_node</span><span class="plain"> *</span><span class="identifier">P</span><span class="plain">) {</span>
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<span class="identifier">LOOP_THROUGH_INTER_CHILDREN</span><span class="plain">(</span><span class="identifier">F</span><span class="plain">, </span><span class="identifier">P</span><span class="plain">) {</span>
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<<span class="cwebmacro">Examine a line of code in the function</span> <span class="cwebmacronumber">2.1</span>><span class="plain">;</span>
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<span class="functiontext">CodeGen::Labels::traverse_code_tree</span><span class="plain">(</span><span class="identifier">F</span><span class="plain">);</span>
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<span class="plain">}</span>
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<span class="plain">}</span>
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</pre>
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<p class="inwebparagraph"></p>
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<p class="endnote">The function CodeGen::Labels::traverse_code_tree is used in <a href="#SP1_1_2">§1.1.2</a>.</p>
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<p class="inwebparagraph"><a id="SP2_1"></a><b>§2.1. </b>If a label is used, there will be line reading <code class="display"><span class="extract">lab Example</span></code> or similar.
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We look for such lines.
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</p>
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<p class="macrodefinition"><code class="display">
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<<span class="cwebmacrodefn">Examine a line of code in the function</span> <span class="cwebmacronumber">2.1</span>> =
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</code></p>
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<pre class="displaydefn">
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<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">F</span><span class="plain">-></span><span class="identifier">W</span><span class="plain">.</span><span class="identifier">data</span><span class="plain">[</span><span class="identifier">ID_IFLD</span><span class="plain">] == </span><span class="identifier">LAB_IST</span><span class="plain">) {</span>
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<span class="identifier">inter_symbol</span><span class="plain"> *</span><span class="identifier">lab</span><span class="plain"> = </span><span class="identifier">Inter::Lab::label_symbol</span><span class="plain">(</span><span class="identifier">F</span><span class="plain">);</span>
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<span class="identifier">Inter::Symbols::set_flag</span><span class="plain">(</span><span class="identifier">lab</span><span class="plain">, </span><span class="identifier">USED_MARK_BIT</span><span class="plain">);</span>
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<span class="plain">}</span>
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</pre>
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<p class="inwebparagraph"></p>
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<p class="endnote">This code is used in <a href="#SP2">§2</a>.</p>
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<hr class="tocbar">
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<ul class="toc"><li><a href="2-erm.html">Back to 'Eliminate Redundant Matter'</a></li><li><i>(This section ends Chapter 2: Miscellaneous Pipeline Stages.)</i></li></ul><hr class="tocbar">
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<!--End of weave-->
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