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Minimizing Rust Binary Size

This repository demonstrates how to minimize the size of a Rust binary.

By default, Rust optimizes for execution speed, compilation speed, and ease of debugging rather than binary size, since for the vast majority of applications this is ideal. But for situations where a developer wants to optimize for binary size instead, Rust provides mechanisms to accomplish this.

Build in Release Mode

By default, cargo build builds the Rust binary in debug mode. Debug mode disables many optimizations, which helps debuggers (and IDEs that run them) provide a better debugging experience. Debug binaries can be 30% or more larger than release binaries.

To minimize binary size, build in release mode:

$ cargo build --release

`strip` Symbols from Binary

By default on Linux and macOS, symbol information is included in the compiled .elf file. This information is not needed to properly execute the binary.

Cargo can be configured to automatically strip binaries. Modify Cargo.toml in this way:

[profile.release]
strip = true  # Automatically strip symbols from the binary.

Prior to Rust 1.59, run strip directly on the .elf file instead:

$ strip target/release/min-sized-rust

Optimize For Size

Cargo defaults its optimization level to 3 for release builds, which optimizes the binary for speed. To instruct Cargo to optimize for minimal binary size, use the z optimization level in Cargo.toml:

[profile.release]
opt-level = "z"  # Optimize for size.

[!NOTE] In some cases the "s" level may result in a smaller binary than "z", as explained in the opt-level documentation:

It is recommended to experiment with different levels to find the right balance for your project. There may be surprising results, such as ... the "s" and "z" levels not being necessarily smaller.

Enable Link Time Optimization (LTO)

By default, Cargo instructs compilation units to be compiled and optimized in isolation. LTO instructs the linker to optimize at the link stage. This can, for example, remove dead code and often times reduces binary size.

Enable LTO in Cargo.toml:

[profile.release]
lto = true

Reduce Parallel Code Generation Units to Increase Optimization

By default, Cargo specifies 16 parallel codegen units for release builds. This improves compile times, but prevents some optimizations.

Set this to 1 in Cargo.toml to allow for maximum size reduction optimizations:

[profile.release]
codegen-units = 1

Abort on Panic

[!IMPORTANT] Up to this point, the features discussed to reduce binary size did not have an impact on the behaviour of the program (only its execution speed). This feature does have an impact on behavior.

By default, when Rust code encounters a situation when it must call panic!(), it unwinds the stack and produces a helpful backtrace. The unwinding code, however, does require extra binary size. rustc can be instructed to abort immediately rather than unwind, which removes the need for this extra unwinding code.

Enable this in Cargo.toml:

[profile.release]
panic = "abort"

Remove Location Details

By default, Rust includes file, line, and column information for panic!() and [track_caller] to provide more useful traceback information. This information requires space in the binary and thus increases the size of the compiled binaries.

To remove this file, line, and column information, use the unstable rustc -Zlocation-detail flag:

$ RUSTFLAGS="-Zlocation-detail=none" cargo +nightly build --release

Remove `fmt::Debug`

With the -Zfmt-debug flag you can turn #[derive(Debug)]and {:?} formatting into no-ops. This will ruin output of dbg!(), assert!(), unwrap(), etc., and may break code that unwisely relies on the debug formatting, but it will remove derived fmt functions and their strings.

$ RUSTFLAGS="-Zfmt-debug=none" cargo +nightly build --release

Optimize `libstd` with `build-std`

[!NOTE] See also Xargo, the predecessor to build-std. Xargo is currently in maintenance status.

[!NOTE] Example project is located in the build_std folder.

Rust ships pre-built copies of the standard library (libstd) with its toolchains. This means that developers don't need to build libstd every time they build their applications. libstd is statically linked into the binary instead.

While this is very convenient there are several drawbacks if a developer is trying to aggressively optimize for size.

The prebuilt libstd is optimized for speed, not size.
It's not possible to remove portions of libstd that are not used in a particular application (e.g. LTO and panic behaviour).

This is where build-std comes in. The build-std feature is able to compile libstd with your application from the source. It does this with the rust-src component that rustup conveniently provides.

Install the appropriate toolchain and the rust-src component:

$ rustup toolchain install nightly
$ rustup component add rust-src --toolchain nightly

Build using build-std:

# Find your host's target triple.
$ rustc -vV
...
host: x86_64-apple-darwin

# Use that target triple when building with build-std.
# Add the =std,panic_abort to the option to make panic = "abort" Cargo.toml option work.
# See: https://github.com/rust-lang/wg-cargo-std-aware/issues/56
$ RUSTFLAGS="-Zlocation-detail=none -Zfmt-debug=none" cargo +nightly build \
  -Z build-std=std,panic_abort \
  -Z build-std-features="optimize_for_size" \
  --target x86_64-apple-darwin --release

The optimize_for_size flag provides a hint to libstd that it should try to use algorithms optimized for binary size. More information about it can be found in the tracking issue.

On macOS, the final stripped binary size is reduced to 51KB.

Remove `panic` String Formatting with `panic_immediate_abort`

Even if panic = "abort" is specified in Cargo.toml, rustc will still include panic strings and formatting code in final binary by default. An unstable panic_immediate_abort feature has been merged into the nightly rustc compiler to address this.

To use this, repeat the instructions above to use build-std, but also pass the following -Z build-std-features=panic_immediate_abort option.

$ cargo +nightly build -Z build-std=std,panic_abort -Z build-std-features=panic_immediate_abort \
    --target x86_64-apple-darwin --release

On macOS, the final stripped binary size is reduced to 30KB.

Remove `core::fmt` with `#![no_main]` and Careful Usage of `libstd`

[!NOTE] Example projects are located in the no_main folder.

Up until this point, we haven't restricted what utilities we used from libstd. In this section we will restrict our usage of libstd in order to reduce binary size further.

If you want an executable smaller than 20 kilobytes, Rust's string formatting code, core::fmt must be removed. panic_immediate_abort only removes some usages of this code. There is a lot of other code that uses formatting in some cases. That includes Rust's "pre-main" code in libstd.

By using a C entry point (by adding the #![no_main] attribute) , managing stdio manually, and carefully analyzing which chunks of code you or your dependencies include, you can sometimes make use of libstd while avoiding bloated core::fmt.

Expect the code to be hacky and unportable, with more unsafe{}s than usual. It feels like no_std, but with libstd.

Start with an empty executable, ensure xargo bloat --release --target=... contains no core::fmt or something about padding. Add (uncomment) a little bit. See that xargo bloat now reports drastically more. Review source code that you've just added. Probably some external crate or a new libstd function is used. Recurse into that with your review process (it requires [replace] Cargo dependencies and maybe digging in libstd), find out why it weighs more than it should. Choose alternative way or patch dependencies to avoid unnecessary features. Uncomment a bit more of your code, debug exploded size with xargo bloat and so on.

On macOS, the final stripped binary is reduced to 8KB.

Removing `libstd` with `#![no_std]`

[!NOTE] Example projects are located in the no_std folder.

Up until this point, our application was using the Rust standard library, libstd. libstd provides many convenient, well tested cross-platform APIs and data types. But if a user wants to reduce binary size to an equivalent C program size, it is possible to depend only on libc.

It's important to understand that there are many drawbacks to this approach. For one, you'll likely need to write a lot of unsafe code and lose access to a majority of Rust crates that depend on libstd. Nevertheless, it is one (albeit extreme) option to reducing binary size.

A striped binary built this way is around 8KB.

#![no_std]
#![no_main]

extern crate libc;

#[no_mangle]
pub extern "C" fn main(_argc: isize, _argv: *const *const u8) -> isize {
    // Since we are passing a C string the final null character is mandatory.
    const HELLO: &'static str = "Hello, world!\n\0";
    unsafe {
        libc::printf(HELLO.as_ptr() as *const _);
    }
    0
}

#[panic_handler]
fn my_panic(_info: &core::panic::PanicInfo) -> ! {
    loop {}
}

Compress the binary

[!NOTE] Up until this point, all size-reducing techniques were Rust-specific. This section describes a language-agnostic binary packing tool that is an option to reduce binary size further.

UPX is a powerful tool for creating a self-contained, compressed binary with no addition runtime requirements. It claims to typically reduce binary size by 50-70%, but the actual result depends on your executable.

$ upx --best --lzma target/release/min-sized-rust

[!WARNING] There have been times that UPX-packed binaries have flagged heuristic-based antivirus software because malware often uses UPX.

Tools

cargo-bloat - Find out what takes most of the space in your executable.
cargo-unused-features - Find and prune enabled but potentially unused feature flags from your project.
momo - proc_macro crate to help keeping the code footprint of generic methods in check.
Twiggy - A code size profiler for Wasm.

Containers

Sometimes it's advantageous to deploy Rust into containers (e.g. Docker). There are several great existing resources to help create minimum sized container images that run Rust binaries.

Official rust:alpine image
mini-docker-rust
muslrust
docker-slim - Minify Docker images
dive - A tool for exploring a container image and discovering ways to shrink the size of the image.

References

Organizations

wg-binary-size: Working group for improving the size of Rust programs and libraries.

Legacy Techniques

The following techniques are no longer relevant for modern Rust development, but may apply to older versions of Rust and are maintained for historical purposes.

Remove Jemalloc

[!IMPORTANT] As of Rust 1.32, jemalloc is removed by default. If using Rust 1.32 or newer, no action is needed to reduce binary size regarding this feature.

Prior to Rust 1.32, to improve performance on some platforms Rust bundled jemalloc, an allocator that often outperforms the default system allocator. Bundling jemalloc added around 200KB to the resulting binary, however.

To remove jemalloc on Rust 1.28 - Rust 1.31, add this code to the top of main.rs:

use std::alloc::System;

#[global_allocator]
static A: System = System;