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<!DOCTYPE html>
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<a class="link subtopic" href="statements-&-expressions.html#defer"> Defer</a>
<a class="link subtopic" href="statements-&-expressions.html#goto"> Goto</a>
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<a class="link" href="structs.html">Structs</a>
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<a class="link subtopic" href="structs.html#noinit-structs"> [noinit] structs</a>
<a class="link subtopic" href="structs.html#methods"> Methods</a>
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<a class="link" href="functions-2.html">Functions 2</a>
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<a class="link subtopic" href="functions-2.html#variable-number-of-arguments"> Variable number of arguments</a>
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<a class="link subtopic" href="builtin-functions.html#println"> println</a>
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<a class="link" href="sizeof-and-__offsetof.html">sizeof and __offsetof</a>
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<a class="link" href="limited-operator-overloading.html">Limited operator overloading</a>
<a class="link subtopic" href="limited-operator-overloading.html#implicitly-generated-overloads"> Implicitly generated overloads</a>
<a class="link subtopic" href="limited-operator-overloading.html#restriction"> Restriction</a>
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<a class="link active" href="performance-tuning.html">Performance tuning</a>
<a class="link subtopic" href="performance-tuning.html#tuning-operations-details"> Tuning operations details</a>
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<a class="link" href="atomics.html">Atomics</a>
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<a class="link" href="cross-compilation.html">Cross compilation</a>
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<a class="link" href="debugging.html">Debugging</a>
<a class="link subtopic" href="debugging.html#c-backend-binaries-(default)"> C Backend binaries (Default)</a>
<a class="link subtopic" href="debugging.html#native-backend-binaries"> Native Backend binaries</a>
<a class="link subtopic" href="debugging.html#javascript-backend"> Javascript Backend</a>
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<a class="link" href="v-and-c.html">V and C</a>
<a class="link subtopic" href="v-and-c.html#calling-c-from-v"> Calling C from V</a>
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<a class="link subtopic" href="v-and-c.html#passing-c-compilation-flags"> Passing C compilation flags</a>
<a class="link subtopic" href="v-and-c.html#pkgconfig"> pkgconfig</a>
<a class="link subtopic" href="v-and-c.html#including-c-code"> Including C code</a>
<a class="link subtopic" href="v-and-c.html#c-types"> C types</a>
<a class="link subtopic" href="v-and-c.html#c-declarations"> C Declarations</a>
<a class="link subtopic" href="v-and-c.html#export-to-shared-library"> Export to shared library</a>
<a class="link subtopic" href="v-and-c.html#translating-c-to-v"> Translating C to V</a>
<a class="link subtopic" href="v-and-c.html#working-around-c-issues"> Working around C issues</a>
<div class="items">
<a class="link" href="other-v-features.html">Other V Features</a>
<a class="link subtopic" href="other-v-features.html#inline-assembly"> Inline assembly</a>
<a class="link subtopic" href="other-v-features.html#hot-code-reloading"> Hot code reloading</a>
<a class="link subtopic" href="other-v-features.html#cross-platform-shell-scripts-in-v"> Cross-platform shell scripts in V</a>
<a class="link subtopic" href="other-v-features.html#vsh-scripts-with-no-extension"> Vsh scripts with no extension</a>
<div class="items">
<a class="link" href="appendix-i-keywords.html">Appendix I: Keywords</a>
<div class="items">
<a class="link" href="appendix-ii-operators.html">Appendix II: Operators</a>
</div>
</section>
</div>
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<article>
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<div class="content">
<div class="content-container">
<main>
<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>-skip-unused</code></td>
<td>Performance, Compile time, Size</td>
<td>Potential instability</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>
<li>When the prediction can be wrong, as it might cause a performance penalty due to branch
misprediction.</li>
</ul>
<h3 id="-skip-unused"><code>-skip-unused</code> <a href="performance-tuning.html#-skip-unused" class="header-anchor" aria-hidden="true">#</a></h3>
<p>This flag tells the V compiler to omit code that is not needed in the final executable to run your
program correctly. This will remove unneeded <code>const</code> arrays allocations and unused functions
from the code in the generated executable.</p>
<p>This flag will be on by default in the future when its implementation will be stabilized and all
severe bugs will be found.</p>
<p><strong>When to Use</strong></p>
<ul>
<li>For production builds where you want to reduce the executable size and improve runtime
performance.</li>
</ul>
<p><strong>When to Avoid</strong></p>
<ul>
<li>Where it doesn't work for you.</li>
</ul>
<h3 id="-fast-math"><code>-fast-math</code> <a href="performance-tuning.html#-fast-math" class="header-anchor" aria-hidden="true">#</a></h3>
<p>This flag enables optimizations that disregard strict compliance with the IEEE standard for
floating-point arithmetic. While this could lead to faster code, it may produce incorrect or
less accurate mathematical results.</p>
<p>The full specter of math operations that <code>-fast-math</code> affects can be found
<a href="https://clang.llvm.org/docs/UsersManual.html#cmdoption-ffast-math">here</a>.</p>
<p><strong>When to Use</strong></p>
<ul>
<li>In applications where performance is more critical than precision, like certain graphics
rendering tasks.</li>
</ul>
<p><strong>When to Avoid</strong></p>
<ul>
<li>In applications requiring strict mathematical accuracy, such as scientific simulations or
financial calculations.</li>
</ul>
<h3 id="-d-no_segfault_handler"><code>-d no_segfault_handler</code> <a href="performance-tuning.html#-d-no_segfault_handler" class="header-anchor" aria-hidden="true">#</a></h3>
<p>Using this flag omits the segfault handler, reducing the executable size and potentially improving
compile time. However, in the case of a segmentation fault, the output will not contain stack trace
information, making debugging more challenging.</p>
<p><strong>When to Use</strong></p>
<ul>
<li>In small, well-tested utilities where a stack trace is not essential for debugging.</li>
</ul>
<p><strong>When to Avoid</strong></p>
<ul>
<li>In large-scale, complex applications where robust debugging is required.</li>
</ul>
<h3 id="-cflags--march=native"><code>-cflags -march=native</code> <a href="#-cflags--march=native" class="header-anchor" aria-hidden="true">#</a></h3>
<p>This flag directs the C compiler to generate instructions optimized for the host CPU. This can
improve performance but will produce an executable incompatible with other/older CPUs.</p>
<p><strong>When to Use</strong></p>
<ul>
<li>When the software is intended to run only on the build machine or in a controlled environment
with identical hardware.</li>
</ul>
<p><strong>When to Avoid</strong></p>
<ul>
<li>When distributing the software to users with potentially older CPUs.</li>
</ul>
<h3 id="-compress"><code>-compress</code> <a href="performance-tuning.html#-compress" class="header-anchor" aria-hidden="true">#</a></h3>
<p>This flag executes <code>upx</code> to compress the resultant executable, reducing its size by around 50%-70%.
The executable will be uncompressed at runtime, so it will take a bit more time to start.
It will also take extra RAM initially, as the compressed version of the app will be loaded into
memory, and then expanded to another chunk of memory.
Debugging such an application can be a bit harder, if you do not account for it.
Some antivirus programs also use heuristics, that trigger more often for compressed applications.</p>
<p><strong>When to Use</strong></p>
<ul>
<li>For really tiny environments, where the size of the executable on the file system,
or when deploying is important (docker containers, rescue disks etc).</li>
</ul>
<p><strong>When to Avoid</strong></p>
<ul>
<li>When you need to debug the application</li>
<li>When the app's startup time is extremely important (where 1-2ms can be meaningful for you)</li>
<li>When you can not afford to allocate more memory during application startup</li>
<li>When you are deploying an app to users with antivirus software that could misidentify your
app as malicious, just because it decompresses its code at runtime.</li>
</ul>
<h3 id="pgo-(profile-guided-optimization)">PGO (Profile-Guided Optimization) <a href="performance-tuning.html#pgo-(profile-guided-optimization)" class="header-anchor" aria-hidden="true">#</a></h3>
<p>PGO allows the compiler to optimize code based on its behavior during sample runs. This can improve
performance and reduce the size of the output executable, but it adds complexity to the build
process.</p>
<p><strong>When to Use</strong></p>
<ul>
<li>For performance-critical applications where the added build complexity is justifiable.</li>
</ul>
<p><strong>When to Avoid</strong></p>
<ul>
<li>For small, short-lived, or rapidly-changing projects where the added build complexity isn't
justified.</li>
</ul>
<p><strong>PGO with Clang</strong></p>
<p>This is an example bash script you can use to optimize your CLI V program without user interactions.
In most cases, you will need to change this script to make it suitable for your particular program.</p>
<pre><code class="language-bash">#!/usr/bin/env bash
# Get the full path to the current directory
CUR_DIR=$(pwd)
# Remove existing PGO data
rm -f *.profraw
rm -f default.profdata
# Initial build with PGO instrumentation
v -cc clang -skip-unused -prod -cflags -fprofile-generate -o pgo_gen .
# Run the instrumented executable 10 times
for i in {1..10}; do
./pgo_gen
done
# Merge the collected data
llvm-profdata merge -o default.profdata *.profraw
# Compile the optimized version using the PGO data
v -cc clang -skip-unused -prod -cflags "-fprofile-use=${CUR_DIR}/default.profdata" -o optimized_program .
# Remove PGO data and instrumented executable
rm *.profraw
rm pgo_gen
</code></pre>
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<time datetime="2024-09-21T06:13:15.036Z">Last updated:
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