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performance-tuning.html
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<!DOCTYPE html>
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<div class="nav-entry is-group-header">
<a class="nav-entry-link" href="appendix-i-keywords.html">Appendix I: Keywords</a>
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</div>
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<div class="nav-entry is-group-header">
<a class="nav-entry-link" href="appendix-ii-operators.html">Appendix II: Operators</a>
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</div>
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<a class="nav-entry-link" href="other-online-resources.html">Other online resources</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-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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<article>
<div class="container">
<div class="content">
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<main class="v-doc">
<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>
</ul>
<h3 id="@aligned"><code>@[aligned]</code> <a href="performance-tuning.html#@aligned" class="header-anchor" aria-hidden="true">#</a></h3>
<p>The <code>@[aligned]</code> attribute can be applied to a structure or union to specify a minimum alignment
(in bytes) for variables of that type. Using the <code>@[aligned]</code> attribute you can only <em>increase</em>
the default alignment. Use <code>@[packed]</code> if you want to <em>decrease</em> it. The alignment of any struct
or union, should be at least a perfect multiple of the lowest common multiple of the alignments of
all of the members of the struct or union.</p>
<p>Example:</p>
<div class="language-v">// Each u16 in the `data` field below, takes 2 bytes, and we have 3 of them = 6 bytes.
// The smallest power of 2, bigger than 6 is 8, i.e. with `@[aligned]`, the alignment
// for the entire struct U16s, will be 8:
@[aligned]
struct U16s {
data [3]u16
}
</div>
<p><strong>When to Use</strong></p>
<ul>
<li>Only if the instances of your types, will be used in performance critical sections, or with
specialised machine instructions, that do require a specific alignment to work.</li>
</ul>
<p><strong>When to Avoid</strong></p>
<ul>
<li>On CPU architectures, that do not support unaligned memory access. If you are not working on
performance critical algorithms, you do not really need it, since the proper minimum alignment
is CPU specific, and the compiler already usually will choose a good default for you.</li>
</ul>
<blockquote class="tip">
<p>[!NOTE]
You can leave out the alignment factor, i.e. use just <code>@[aligned]</code>, in which case the compiler
will align a type to the maximum useful alignment for the target machine you are compiling for,
i.e. the alignment will be the largest alignment which is ever used for any data type on the
target machine. Doing this can often make copy operations more efficient, because the compiler
can choose whatever instructions copy the biggest chunks of memory, when performing copies to or
from the variables which have types that you have aligned this way.</p>
</blockquote>
<p>See also <a href="https://people.freebsd.org/~lstewart/articles/cpumemory.pdf">"What Every Programmer Should Know About Memory", by Ulrich Drepper</a> .</p>
<h3 id="@minify"><code>@[minify]</code> <a href="performance-tuning.html#@minify" class="header-anchor" aria-hidden="true">#</a></h3>
<p>The <code>@[minify]</code> attribute can be added to a struct, allowing the compiler to reorder the fields in
a way that minimizes internal gaps while maintaining alignment. Using the <code>@[minify]</code> attribute may
cause issues with binary serialization or reflection. Be mindful of these potential side effects
when using this attribute.</p>
<p><strong>When to Use</strong></p>
<ul>
<li>When you want to minimize memory usage and you're not using binary serialization or reflection.</li>
</ul>
<p><strong>When to Avoid</strong></p>
<ul>
<li>When using binary serialization or reflection, as it may cause unexpected behavior.</li>
</ul>
<h3 id="likelyunlikely"><code>_likely_/_unlikely_</code> <a href="#likelyunlikely" class="header-anchor" aria-hidden="true">#</a></h3>
<p><code>if _likely_(bool expression) {</code> - hints to the C compiler, that the passed boolean expression is
very likely to be true, so it can generate assembly code, with less chance of branch misprediction.
In the JS backend, that does nothing.</p>
<p><code>if _unlikely_(bool expression) {</code> is similar to <code>_likely_(x)</code>, but it hints that the boolean
expression is highly improbable. In the JS backend, that does nothing.</p>
<p><strong>When to Use</strong></p>
<ul>
<li>In conditional statements where one branch is clearly more frequently executed than the other.</li>
</ul>
<p><strong>When to Avoid</strong></p>
<ul>