Minimizing Rust Binary Size
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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.
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
Remove Jemalloc
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;
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
Note: 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"
libstd
with build-std
Optimize
Note: See also Xargo, the predecessor to
build-std
. Xargo is currently in maintenance status.
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
$ cargo +nightly build -Z build-std=std,panic_abort --target x86_64-apple-darwin --release
On macOS, the final stripped binary size is reduced to 51KB.
panic
String Formatting with panic_immediate_abort
Remove
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.
core::fmt
with #![no_main]
and Careful Usage of libstd
Remove
Example project is located in the
no_main
folder.
This section was contributed in part by @vi
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.
libstd
with #![no_std]
Removing
Example project is 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 strip
ed 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
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
It should be noted that there have been times that UPX-packed binaries have flagged heuristic-based anti-virus 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 Docker containers that run Rust binaries.
- mini-docker-rust
- rust-musl-builder
- muslrust
- docker-slim - Minify Docker images
References
- Why is a Rust executable large? - 2016
- Freestanding Rust Binary - 2018
- Tiny Rocket - 2018
- Formatting is Unreasonably Expensive for Embedded Rust - 2019
- Tiny Windows executable in Rust - 2019
- Making a really tiny WebAssembly graphics demos - 2019
- Reducing the size of the Rust GStreamer plugin - 2020
- Tighten rustβs belt: shrinking embedded Rust binaries - 2022
- Avoiding allocations in Rust to shrink Wasm modules - 2022
- A very small Rust binary indeed - 2022
min-sized-rust-windows
- Windows-specific tricks to reduce binary size