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  <div class="section" id="design-doc-functions-operators-and-layers">
<span id="design-doc-functions-operators-and-layers"></span><h1>Design Doc: Functions, Operators, and Layers<a class="headerlink" href="#design-doc-functions-operators-and-layers" title="永久链接至标题"></a></h1>
<p>In a DL system, we can compose one or more fine grained operators into a coarse grained one.  For example, the FC layer can be composed of a multiplication operator and an add operator.</p>
<p>Historically, some fine grained operations are known as operators, and some coarse level ones are known as layers.  But we need a well-defined separation.</p>
<p>In general, operators are those very fine grained operations, e.g., mul and add. In the implementation, we can write them as C++ functions:</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">&gt;</span> <span class="n">T</span> <span class="n">add</span><span class="p">(</span><span class="n">T</span> <span class="n">x</span><span class="p">,</span> <span class="n">T</span> <span class="n">y</span><span class="p">)</span> <span class="p">{</span> <span class="k">return</span> <span class="n">x</span> <span class="o">+</span> <span class="n">y</span><span class="p">;</span> <span class="p">}</span>
<span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">&gt;</span> <span class="n">T</span> <span class="n">mul</span><span class="p">(</span><span class="n">T</span> <span class="n">x</span><span class="p">,</span> <span class="n">T</span> <span class="n">y</span><span class="p">)</span> <span class="p">{</span> <span class="k">return</span> <span class="n">x</span> <span class="o">*</span> <span class="n">y</span><span class="p">;</span> <span class="p">}</span>
</pre></div>
</div>
<p>Then we can wrap them into operators which are C++ classes and can be created from Python bindings by name.  A C macro can do this. For example, the following macro invocation</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="cp">#define MAKE_FUNCTION_OPERATOR(mul);</span>
</pre></div>
</div>
<p>generates</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">&gt;</span> <span class="k">class</span> <span class="nc">mulOp</span> <span class="o">:</span> <span class="k">public</span> <span class="n">OperatorBase</span> <span class="p">{...};</span>
<span class="n">REGISTER_OP</span><span class="p">(</span><span class="n">mulOp</span><span class="o">&lt;</span><span class="n">float32</span><span class="o">&gt;</span><span class="p">,</span> <span class="s">&quot;mul&quot;</span><span class="p">);</span>
</pre></div>
</div>
<p>so that in Python we can create operator mul by:</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="n">X1</span> <span class="o">=</span> <span class="n">Var</span><span class="p">()</span>
<span class="n">X2</span> <span class="o">=</span> <span class="n">Var</span><span class="p">()</span>
<span class="n">Y</span> <span class="o">=</span> <span class="n">Var</span><span class="p">()</span>
<span class="n">paddle</span><span class="o">.</span><span class="n">cpp</span><span class="o">.</span><span class="n">create_operator</span><span class="p">(</span><span class="s2">&quot;mul&quot;</span><span class="p">,</span> <span class="nb">input</span><span class="o">=</span><span class="p">[</span><span class="n">X1</span><span class="p">,</span> <span class="n">X2</span><span class="p">],</span> <span class="n">output</span><span class="o">=</span><span class="n">Y</span><span class="p">)</span>
</pre></div>
</div>
<p>Also, at the same time, we can compose a coarse level C++ operator class by composing functions <code class="docutils literal"><span class="pre">mul</span></code> and <code class="docutils literal"><span class="pre">add</span></code>:</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">&gt;</span>
<span class="k">class</span> <span class="nc">FCOp</span> <span class="o">:</span> <span class="k">public</span> <span class="n">OperatorBase</span> <span class="p">{</span>
 <span class="k">public</span><span class="o">:</span>
  <span class="kt">void</span> <span class="n">Run</span><span class="p">(...)</span> <span class="p">{</span>
    <span class="n">add</span><span class="p">(</span><span class="n">mul</span><span class="p">(</span><span class="n">Input</span><span class="o">&lt;</span><span class="n">T</span><span class="o">&gt;</span><span class="p">(</span><span class="s">&quot;X&quot;</span><span class="p">),</span> <span class="n">Input</span><span class="o">&lt;</span><span class="n">T</span><span class="o">&gt;</span><span class="p">(</span><span class="s">&quot;W&quot;</span><span class="p">)),</span> <span class="n">Input</span><span class="o">&lt;</span><span class="n">T</span><span class="o">&gt;</span><span class="p">(</span><span class="s">&quot;b&quot;</span><span class="p">);</span>
  <span class="p">}</span>
<span class="p">};</span>
<span class="n">REGISTER_OP</span><span class="p">(</span><span class="n">FCOp</span><span class="p">,</span> <span class="s">&quot;fc&quot;</span><span class="p">);</span>
</pre></div>
</div>
<p>We need to support such composition in Python as well.  To do so, we need a higher level Python wrapping of operator creation than <code class="docutils literal"><span class="pre">paddle.cpp.create_operator</span></code>.  This higher level operator API should be compatible with the layer API.</p>
<p>Let&#8217;s explain using an example.  Suppose that we are going to compose the FC using mul and add in Python, we&#8217;d like to have Python functions <code class="docutils literal"><span class="pre">mul</span></code> and <code class="docutils literal"><span class="pre">add</span></code> defined in module <code class="docutils literal"><span class="pre">operator</span></code>:</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">operator</span><span class="o">.</span><span class="n">mul</span><span class="p">(</span><span class="n">X1</span><span class="p">,</span> <span class="n">X2</span><span class="p">):</span>
    <span class="n">O</span> <span class="o">=</span> <span class="n">Var</span><span class="p">()</span>
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    <span class="n">paddle</span><span class="o">.</span><span class="n">cpp</span><span class="o">.</span><span class="n">create_operator</span><span class="p">(</span><span class="s2">&quot;mul&quot;</span><span class="p">,</span> <span class="nb">input</span><span class="o">=</span><span class="p">{</span><span class="n">X1</span><span class="p">,</span> <span class="n">Y1</span><span class="p">},</span> <span class="n">output</span><span class="o">=</span><span class="n">O</span><span class="p">)</span>
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    <span class="k">return</span> <span class="n">O</span>

<span class="k">def</span> <span class="nf">operator</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">X1</span><span class="p">,</span> <span class="n">X2</span><span class="p">):</span>
    <span class="n">O</span> <span class="o">=</span> <span class="n">Var</span><span class="p">()</span>
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    <span class="n">paddle</span><span class="o">.</span><span class="n">cpp</span><span class="o">.</span><span class="n">create_operator</span><span class="p">(</span><span class="s2">&quot;add&quot;</span><span class="p">,</span> <span class="nb">input</span><span class="o">=</span><span class="p">{</span><span class="n">X1</span><span class="p">,</span> <span class="n">X2</span><span class="p">},</span> <span class="n">output</span><span class="o">=</span><span class="n">O</span><span class="p">)</span>
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    <span class="k">return</span> <span class="n">O</span>
</pre></div>
</div>
<p>Above code snippets are automatically generated.  Given them, users can define</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">layer</span><span class="o">.</span><span class="n">fc</span><span class="p">(</span><span class="n">X</span><span class="p">):</span>
    <span class="n">W</span> <span class="o">=</span> <span class="n">Var</span><span class="p">()</span>
    <span class="n">b</span> <span class="o">=</span> <span class="n">Var</span><span class="p">()</span>
    <span class="k">return</span> <span class="n">operator</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">operator</span><span class="o">.</span><span class="n">mul</span><span class="p">(</span><span class="n">X</span><span class="p">,</span> <span class="n">W</span><span class="p">),</span> <span class="n">b</span><span class="p">)</span>
</pre></div>
</div>
<p>If we don&#8217;t have <code class="docutils literal"><span class="pre">operator.mul</span></code> and <code class="docutils literal"><span class="pre">operator.add</span></code>, the definiton of <code class="docutils literal"><span class="pre">layer.fc</span></code> would be complicated:</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">layer</span><span class="o">.</span><span class="n">fc</span><span class="p">(</span><span class="n">X</span><span class="p">):</span>
    <span class="n">W</span> <span class="o">=</span> <span class="n">Var</span><span class="p">()</span>
    <span class="n">b</span> <span class="o">=</span> <span class="n">Var</span><span class="p">()</span>
    <span class="n">O1</span> <span class="o">=</span> <span class="n">Var</span><span class="p">()</span>
    <span class="n">paddle</span><span class="o">.</span><span class="n">cpp</span><span class="o">.</span><span class="n">create_operator</span><span class="p">(</span><span class="s2">&quot;mul&quot;</span><span class="p">,</span> <span class="nb">input</span><span class="o">=</span><span class="p">[</span><span class="n">X</span><span class="p">,</span> <span class="n">W</span><span class="p">],</span> <span class="n">output</span><span class="o">=</span><span class="n">O1</span><span class="p">)</span>
    <span class="n">O2</span> <span class="o">=</span> <span class="n">Var</span><span class="p">()</span>
    <span class="n">paddle</span><span class="o">.</span><span class="n">cpp</span><span class="o">.</span><span class="n">create_operator</span><span class="p">(</span><span class="s2">&quot;add&quot;</span><span class="p">,</span> <span class="nb">input</span><span class="o">=</span><span class="p">[</span><span class="n">O1</span><span class="p">,</span> <span class="n">b</span><span class="p">],</span> <span class="n">output</span><span class="o">=</span><span class="n">O2</span><span class="p">)</span>
    <span class="k">return</span> <span class="n">O2</span>
</pre></div>
</div>
<p>We&#8217;d like to have Python bindings to operators in package <code class="docutils literal"><span class="pre">paddle.operator</span></code>, and Python compositions of operators in package <code class="docutils literal"><span class="pre">paddle.layer</span></code>.  So we have the following concepts in above illustrative example:</p>
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<p>| C++ functions/functors | mul          | add          |             |          |
|&#8212;&#8212;&#8212;&#8212;&#8212;&#8212;&#8212;&#8212;|&#8212;&#8212;&#8212;&#8212;&#8211;|&#8212;&#8212;&#8212;&#8212;&#8211;|&#8212;&#8212;&#8212;&#8212;-|&#8212;&#8212;&#8212;-|
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| C++ operator class     | mulOp        | addOp        | FCOp        |          |
| Python binding         | operator.mul | operator.add | operator.fc |          |
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| Python function        |              |              |             | layer.fc |</p>
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297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 
<p>This is how we differentiate layer and operators in PaddlePaddle:</p>
<ul class="simple">
<li>those defined in C++ and have a lightweighted Python wrapper in module <code class="docutils literal"><span class="pre">operators</span></code> are operators; whereas</li>
<li>those who don&#8217;t have C++ implementations but a Python implementation that compose C++ operators are known as layers.</li>
</ul>
</div>


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