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class="sidebar-link">The Contributors</a></li></ul></section></li></ul> </aside> <main class="page"> <div class="theme-default-content content__default"><h1 id="optimization"><a href="#optimization" class="header-anchor">#</a> Optimization</h1> <p>When compiling, the compiler will often modify the program to increase performance. This is permitted by the <a href="http://en.cppreference.com/w/cpp/language/as_if" target="_blank" rel="noopener noreferrer">as-if rule<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>, which allows any and all transformations that do not change observable behavior.</p> <h2 id="inline-expansion-inlining"><a href="#inline-expansion-inlining" class="header-anchor">#</a> Inline Expansion/Inlining</h2> <p>Inline expansion (also known as inlining) is compiler optimisation that replaces a call to a function with the body of that function. This saves the function call overhead, but at the cost of space, since the function may be duplicated several times.</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token comment">// source:</span>
<span class="token keyword">int</span> <span class="token function">process</span><span class="token punctuation">(</span><span class="token keyword">int</span> value<span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token keyword">return</span> <span class="token number">2</span> <span class="token operator">*</span> value<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">int</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token keyword">int</span> a<span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token keyword">return</span> <span class="token function">process</span><span class="token punctuation">(</span>a<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// program, after inlining:</span>
<span class="token keyword">int</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token keyword">int</span> a<span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token keyword">return</span> <span class="token number">2</span> <span class="token operator">*</span> a<span class="token punctuation">;</span> <span class="token comment">// the body of process() is copied into foo()</span>
<span class="token punctuation">}</span>
</code></pre></div><p>Inlining is most commonly done for small functions, where the function call overhead is significant compared to the size of the function body.</p> <h2 id="empty-base-optimization"><a href="#empty-base-optimization" class="header-anchor">#</a> Empty base optimization</h2> <p>The size of any object or member subobject is required to be at least 1 even if the type is an empty <code>class</code> type (that is, a <code>class</code> or <code>struct</code> that has no non-static data members), in order to be able to guarantee that the addresses of distinct objects of the same type are always distinct.</p> <p>However, base <code>class</code> subobjects are not so constrained, and can be completely optimized out from the object layout:</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><cassert></span></span>
<span class="token keyword">struct</span> <span class="token class-name">Base</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token comment">// empty class</span>
<span class="token keyword">struct</span> <span class="token class-name">Derived1</span> <span class="token operator">:</span> <span class="token base-clause"><span class="token class-name">Base</span></span> <span class="token punctuation">{</span>
<span class="token keyword">int</span> i<span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// the size of any object of empty class type is at least 1</span>
<span class="token function">assert</span><span class="token punctuation">(</span><span class="token keyword">sizeof</span><span class="token punctuation">(</span>Base<span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// empty base optimization applies</span>
<span class="token function">assert</span><span class="token punctuation">(</span><span class="token keyword">sizeof</span><span class="token punctuation">(</span>Derived1<span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token keyword">sizeof</span><span class="token punctuation">(</span><span class="token keyword">int</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p>Empty base optimization is commonly used by allocator-aware standard library classes (<code>std::vector</code>, <code>std::function</code>, <code>std::shared_ptr</code>, etc) to avoid occupying any additional storage for its allocator member if the allocator is stateless. This is achieved by storing one of the required data members (e.g., <code>begin</code>, <code>end</code>, or <code>capacity</code> pointer for the <code>vector</code>).</p> <p>Reference: <a href="http://en.cppreference.com/w/cpp/language/ebo" target="_blank" rel="noopener noreferrer">cppreference<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></p></div> <footer class="page-edit"><div class="edit-link"><a href="https://github.com/devtut/generate/edit/master/docs/cpp/optimization.md" target="_blank" rel="noopener noreferrer">Edit this page on GitHub</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <!----></footer> <div class="page-nav"><p class="inner"><span class="prev">
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