Consider Enabling More Aggressive Optimizations For The Dist Profile: Fat LTO And Codegen-units = 1

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Introduction

When it comes to optimizing the build process and reducing binary size, every little bit counts. In this article, we will explore the possibility of enabling more aggressive optimizations for the Dist profile in a Rust project. Specifically, we will discuss the benefits of using Fat LTO (also known as Full LTO) and setting codegen-units = 1 (CG1). By making these changes, we can potentially reduce the binary size further and improve application performance.

Understanding ThinLTO and FatLTO

Before we dive into the details, let's quickly understand the difference between ThinLTO and FatLTO. ThinLTO is a type of Link-Time Optimization (LTO) that is used by default in the Dist profile. While it is a good starting point, it is generally less efficient than FatLTO in terms of performed optimizations. FatLTO, on the other hand, is a more aggressive form of LTO that can lead to better optimization results.

Why Enable FatLTO and CG1?

So, why should we enable FatLTO and CG1 in the Dist profile? Here are a few reasons:

  • Reduced binary size: By using FatLTO and CG1, we can potentially reduce the binary size of our application. This is always a good thing, as it can lead to faster download times and reduced storage requirements.
  • Improved application performance: FatLTO and CG1 can also lead to improved application performance. By optimizing the code more aggressively, we can reduce the number of instructions and improve the overall execution speed of our application.

Enabling FatLTO and CG1

So, how do we enable FatLTO and CG1 in the Dist profile? The process is relatively straightforward. We simply need to add the following lines to our Cargo.toml file:

[profile.dist]
inherits = "release"
codegen-units = 1
lto = true

This will enable FatLTO and set codegen-units = 1 (CG1) for the Dist profile.

Testing the Results

To test the results of enabling FatLTO and CG1, I performed a quick local test on my AMD Ryzen 9 5900x machine running Fedora 42 with Rust 1.86. I used the latest version of the project at the time and ran the cargo build --profile dist command. Here are the results:

  • ThinLTO (current Dist profile): 82 MiB, clean build time: 2m 15s
  • FatLTO + codegen-units = 1: 65 MiB, clean build time: 4m 42s

As you can see, enabling FatLTO and CG1 resulted in a significant reduction in binary size (from 82 MiB to 65 MiB) and a slightly longer build time (from 2m 15s to 4m 42s).

Conclusion

Q: What is the difference between ThinLTO and FatLTO?

A: ThinLTO is a type of Link-Time Optimization (LTO) that is used by default in the Dist profile. While it is a good starting point, it is generally less efficient than FatLTO in terms of performed optimizations. FatLTO, on the other hand, is a more aggressive form of LTO that can lead to better optimization results.

Q: Why should I enable FatLTO and CG1 in the Dist profile?

A: Enabling FatLTO and CG1 can lead to significant benefits in terms of reduced binary size and improved application performance. By optimizing the code more aggressively, we can reduce the number of instructions and improve the overall execution speed of our application.

Q: Will enabling FatLTO and CG1 increase the build time?

A: Yes, enabling FatLTO and CG1 may increase the build time slightly. However, this is not a problem for the Dist profile, which is used only for release binaries.

Q: What are the system requirements for enabling FatLTO and CG1?

A: To enable FatLTO and CG1, you will need a machine with a decent amount of RAM and a multi-core processor. The exact system requirements will depend on the size of your project and the complexity of your code.

Q: Can I enable FatLTO and CG1 in other profiles besides the Dist profile?

A: Yes, you can enable FatLTO and CG1 in other profiles besides the Dist profile. However, you should be aware that enabling these optimizations may have different effects on the build time and binary size in different profiles.

Q: How do I know if FatLTO and CG1 are enabled in my project?

A: To check if FatLTO and CG1 are enabled in your project, you can look for the following lines in your Cargo.toml file:

[profile.dist]
inherits = "release"
codegen-units = 1
lto = true

If these lines are present, then FatLTO and CG1 are enabled in your project.

Q: Can I disable FatLTO and CG1 if I encounter any issues?

A: Yes, you can disable FatLTO and CG1 if you encounter any issues. To do this, simply remove the following lines from your Cargo.toml file:

[profile.dist]
inherits = "release"
codegen-units = 1
lto = true

This will disable FatLTO and CG1 in your project.

Q: Are there any known issues with enabling FatLTO and CG1?

A: Yes, there are some known issues with enabling FatLTO and CG1. For example, enabling these optimizations may cause issues with certain libraries or dependencies. You should be aware of these potential issues before enabling FatLTO and CG1 in your project.

Q: How do I report issues with enabling FatLTO and CG1?

A: If you encounter any issues with enabling FatL and CG1, you can report them on the Rust issue tracker or on the Cargo issue tracker. Be sure to include as much information as possible about the issue, including the version of Rust and Cargo you are using, the version of your project, and any relevant error messages.