Size optimization of Rust binaries

This repository shows some examples of binary size optimization in Rust as part of our Newsletter see Rust-Trends.com

Rust optimizes for speed of execution and compilation instead of binary size. But in some cases optimization for size is preferred, e.g. when you have limited space (Embedded Systems) or when hackers want to stay unnoticed on a system, i.e. larger binaries are more suspicious and easier to detect.

Build with custom profiles

When executing cargo build the Rust binary is built in Debug mode. Debug mode comes with overhead to provide extra information to the IDE (debuggers). This overhead is stripped when you build with the release flag. cargo build --release.

To minimize binary size, we build with custom profiles that inherit from this release mode. Note that custom profiles are stable since Rust 1.57.

Ran below on a MacBook Pro (13-inch, 2018) Intel

$ cargo build --release
    Compiling size-optimization-rust-binaries v0.1.0
    Finished release [optimized] target(s) in 26.27s

$ cargo build --profile=size-opt1
    Compiling size-optimization-rust-binaries v0.1.0 
    Finished size-opt1 [optimized] target(s) in 22.77s

$ cargo build --profile=size-opt2
    Compiling size-optimization-rust-binaries v0.1.0
    Finished size-opt2 [optimized] target(s) in 25.31s

$ cargo build --profile=size-opt3
    Compiling size-optimization-rust-binaries v0.1.0
    Finished size-opt3 [optimized] target(s) in 25.71s

$ du -h target/release/size-optimization-rust-binaries
1.2M    target/release/size-optimization-rust-binaries

$ du -h target/size-opt1 size-optimization-rust-binaries
804K    target/size-opt1/size-optimization-rust-binaries

$ du -h target/size-opt2 size-optimization-rust-binaries
668K    target/size-opt2/size-optimization-rust-binaries

$ du -h target/size-opt3 size-optimization-rust-binaries
652K    target/size-opt3/size-optimization-rust-binaries

Note the reduction in size and the difference in compile time.

The custom profiles explained

There are three custom profiles, you can also find them in the Cargo.toml file.

size-opt1

[profile.size-opt1]
inherits = "release"
strip = true # strip symbols from the binary
opt-level = 'z' # Optimize for size

strip strips the symbols from the binary. With symbols, also called function symbols, a debugger can show what part of code belongs to what function. Easy when you are debugging but not necessary in a release. The Cargo Book

The optimization level opt-level is default set to 3 in release mode and includes all speed optimizations. Changing it to z sets it to optimize for size instead. For more information see opt-level in The Cargo Book

size-opt2

[profile.size-opt2]
inherits = "release"
strip = true # strip symbols from the binary
opt-level = 'z' # Optimize for size
lto = true # Link Time Optimization (LTO)

Link Time Optimization will instruct the compiler toolchain before linking the final binary, to look one last time for possible optimizations. The rustc Book

size-opt3

[profile.size-opt3]
inherits = "release"
strip = true # strip symbols from the binary
opt-level = 'z' # Optimize for size
lto = true # Link Time Optimization (LTO)
codegen-units = 1 # Parallel Code Generation Units

Code generation units greater than 1 specify that the compiler toolchain is allowed to process our code in parallel. This most of the time leaves us with unused optimization potential, in this case for size, since it will look at our code in parallel. By default when not specified it is set to 16, which increases compilation speed.

Links

• For more info look at the min-sized-rust - GitHub repository. Do not forget to look at the links at the bottom of their README.md file for more reading material.

• Blog post about Optimizing Rust Binary Size

• Most programs are built on top of other crates, if you want to analyze them use cargo-bloat. You can install it with $ cargo install cargo-bloat and run it with cargo bloat in your project folder.

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