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<a class="nav-entry-link" href="v-and-c.html">V and C</a>
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<a class="nav-entry-link" href="v-and-c.html#calling-c-from-v"> Calling C from V</a>
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<a class="nav-entry-link" href="v-and-c.html#calling-v-from-c"> Calling V from C</a>
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<a class="nav-entry-link" href="v-and-c.html#passing-c-compilation-flags"> Passing C compilation flags</a>
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<a class="nav-entry-link" href="v-and-c.html##pkgconfig"> #pkgconfig</a>
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<a class="nav-entry-link" href="v-and-c.html#including-c-code"> Including C code</a>
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<a class="nav-entry-link" href="v-and-c.html#c-types"> C types</a>
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<a class="nav-entry-link" href="v-and-c.html#c-declarations"> C Declarations</a>
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<a class="nav-entry-link" href="v-and-c.html#translating-c-to-v"> Translating C to V</a>
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<a class="nav-entry-link" href="v-and-c.html#working-around-c-issues"> Working around C issues</a>
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<a class="nav-entry-link" href="other-v-features.html#inline-assembly"> Inline assembly</a>
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<a class="nav-entry-link" href="other-v-features.html#hot-code-reloading"> Hot code reloading</a>
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<a class="nav-entry-link" href="other-v-features.html#cross-platform-shell-scripts-in-v"> Cross-platform shell scripts in V</a>
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<a class="nav-entry-link" href="other-v-features.html#vsh-scripts-with-no-extension"> Vsh scripts with no extension</a>
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<a class="nav-entry-link" href="appendix-i-keywords.html">Appendix I: Keywords</a>
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<a class="nav-entry-link" href="appendix-ii-operators.html">Appendix II: Operators</a>
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<a class="nav-entry-link" href="other-online-resources.html#v-contributing-guide"> V contributing guide</a>
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<a class="nav-entry-link" href="other-online-resources.html#v-language-documentation"> V language documentation</a>
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<a class="nav-entry-link" href="other-online-resources.html#v-online-playground"> V online playground</a>
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<a class="nav-entry-link" href="other-online-resources.html#awesome-v"> Awesome V</a>
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<a class="nav-entry-link" href="other-online-resources.html#the-v-language-discord"> The V language Discord</a>
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<h1 class="main">Performance tuning</h1>
<p>When compiled with <code>-prod</code>, V's generated C code usually performs well. However, in specialized
scenarios, additional compiler flags and attributes can further optimize the executable for
performance, memory usage, or size.</p>
<blockquote class="tip">
<p>[!NOTE]
These are <em>rarely</em> needed, and should not be used unless you
<em>profile your code</em>, and then see that there are significant benefits for them.
To cite GCC's documentation: "Programmers are notoriously bad at predicting
how their programs actually perform".</p>
</blockquote>
<table>
<thead>
<tr>
<th>Tuning Operation</th>
<th>Benefits</th>
<th>Drawbacks</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>@[inline]</code></td>
<td>Performance</td>
<td>Increased executable size</td>
</tr>
<tr>
<td><code>@[direct_array_access]</code></td>
<td>Performance</td>
<td>Safety risks</td>
</tr>
<tr>
<td><code>@[packed]</code></td>
<td>Memory usage</td>
<td>Potential performance loss</td>
</tr>
<tr>
<td><code>@[minify]</code></td>
<td>Performance, Memory usage</td>
<td>May break binary serialization/reflection</td>
</tr>
<tr>
<td><code>_likely_/_unlikely_</code></td>
<td>Performance</td>
<td>Risk of negative performance impact</td>
</tr>
<tr>
<td><code>-fast-math</code></td>
<td>Performance</td>
<td>Risk of incorrect mathematical operations results</td>
</tr>
<tr>
<td><code>-d no_segfault_handler</code></td>
<td>Compile time, Size</td>
<td>Loss of segfault trace</td>
</tr>
<tr>
<td><code>-cflags -march=native</code></td>
<td>Performance</td>
<td>Risk of reduced CPU compatibility</td>
</tr>
<tr>
<td><code>-compress</code></td>
<td>Size</td>
<td>Harder to debug, extra dependency <code>upx</code></td>
</tr>
<tr>
<td><code>PGO</code></td>
<td>Performance, Size</td>
<td>Usage complexity</td>
</tr>
</tbody>
</table>
<h2 id="tuning-operations-details">Tuning operations details <a href="performance-tuning.html#tuning-operations-details" class="header-anchor" aria-hidden="true">#</a></h2>
<h3 id="@inline"><code>@[inline]</code> <a href="performance-tuning.html#@inline" class="header-anchor" aria-hidden="true">#</a></h3>
<p>You can tag functions with <code>@[inline]</code>, so the C compiler will try to inline them, which in some
cases, may be beneficial for performance, but may impact the size of your executable.</p>
<p><strong>When to Use</strong></p>
<ul>
<li>Functions that are called frequently in performance-critical loops.</li>
</ul>
<p><strong>When to Avoid</strong></p>
<ul>
<li>Large functions, as it might cause code bloat and actually decrease performance.</li>
<li>Large functions in <code>if</code> expressions - may have negative impact on instructions cache.</li>
</ul>
<h3 id="@direct_array_access"><code>@[direct_array_access]</code> <a href="performance-tuning.html#@direct_array_access" class="header-anchor" aria-hidden="true">#</a></h3>
<p>In functions tagged with <code>@[direct_array_access]</code> the compiler will translate array operations
directly into C array operations - omitting bounds checking. This may save a lot of time in a
function that iterates over an array but at the cost of making the function unsafe - unless the
boundaries will be checked by the user.</p>
<p><strong>When to Use</strong></p>
<ul>
<li>In tight loops that access array elements, where bounds have been manually verified or you are
sure that the access index will be valid.</li>
</ul>
<p><strong>When to Avoid</strong></p>
<ul>
<li>Everywhere else.</li>
</ul>
<h3 id="@packed"><code>@[packed]</code> <a href="performance-tuning.html#@packed" class="header-anchor" aria-hidden="true">#</a></h3>
<p>The <code>@[packed]</code> attribute can be applied to a structure to create an unaligned memory layout,
which decreases the overall memory footprint of the structure. Using the <code>@[packed]</code> attribute
may negatively impact performance or even be prohibited on certain CPU architectures.</p>
<p><strong>When to Use</strong></p>
<ul>
<li>When memory usage is more critical than performance, e.g., in embedded systems.</li>
</ul>
<p><strong>When to Avoid</strong></p>
<ul>
<li>On CPU architectures that do not support unaligned memory access or when high-speed memory access
is needed.</li>