Status

This project has benn SUPERSEDED by cargo-sysroot, a tool to cross compile the standard crates in a "sysroot".

This project won't receive further updates or bug fixes.

The main reason of the deprecation is that the approach used here of making a crate explicitly depend on a standard crate like std via a Cargo dependency is, IMO, not the right one -- check cargo-sysroot for an explanation and an alternative approach.

-- @japaric, 2016/02/05

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std-with-cargo

Cross compile the std crate using cargo.

What is this?

A collection of patches for the rust-lang/rust repo that will make the std crate cross compilable with cargo.

Why use cargo to cross compile the std crate?

Two reasons:

Easier cross compilation

One of the usual (1) requirements for cross compiling a Rust program is a cross compiled std crate. Right now, the only (?) way to get that is to build it yourself using the Rust build system (configure --target=$TRIPLE && make) but this is time consuming and wasteful -- the build system will bootstrap a native Rust toolchain (this takes 30+ mins) and then use that toolchain to cross compile the std crate and co.

A much faster way is to use the Rust toolchain that you already have installed, instead of bootstrapping a new one, along with cargo, this can bring down the build time to less than one minute (with full optimizations enabled).

(1) Unless you are targeting a bare metal (no OS) device, in that case you probably only want a cross compiled core crate, but theses patches can also help with that use case.

Making Rust binaries smaller

Rust binaries are huge, the size of "hello world" cross compiled for MIPS (with -C lto -O -C link-args=-s) is 309KB. If you are targeting a router with only 8MB of flash, where half of that has already been taken by the OS, those sizes are discouraging.

-C prefer-dynamic doesn't really help, that may make the binary smaller than 10KB, but then you would need to install libstd.so, which is ~4.8MB (!), in the target device, so you end up requiring way more memory than in the statically linked case.

But, why are Rust binaries so large? If you inspect them using nm --size-sort -S, you'll find out that a good chunk of the binary is jemalloc, and RUST_BACKTRACE support. To quantify how much is a "good chunk", I tested disabling jemalloc, that brought the size of "hello world" down to 146KB, and then with a small modification to the std crate I also disabled backtrace support, and the size of "hello world" went down to 73KB, that's a ~75% reduction in size.

How does cargo fit in all this? The changes I mentioned above were implemented using cargo features, and both jemalloc and backtrace support are now optional features. This means that you could, for example, enable backtrace support during development phase, and disable it for release to get smaller binaries; or pick between malloc (smaller footprint) and jemalloc (better performance) as the allocator for your program.

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Another interesting idea to explore is dynamically linking to libjemalloc and libbacktrace, rather than statically, like all Rust programs do today. This would reduce each binary by 200KB+, while keeping both jemalloc and backtrace support enabled, at the cost of having to install libjemalloc.so and libbacktrace.so in the target device. This option is not provided by the Rust build system, but seems doable with cargo.

Update: I've added a jemalloc_dynamic cargo feature that enables jemalloc but using dynamic linking instead of static linking. Some numbers: hello world is 73KB with jemalloc and backtrace support disabled, enabling jemalloc increases the size to 241KB, OTOH enabling jemalloc_dynamic only increases the size to 77KB at the cost of having to install libjemalloc.so.2, which is 351KB (or 271KB stripped), in the target device.

Dependencies

To cross compile the std crate you'll need:

• Rust nightly channel
• The usual stuff for cross compilation: toolchain and cross compiled C libraries
• LLVM, in particular both llvm-mc and llc should be in your PATH

How to use

If you have a rust repo lying around, you could simply apply the patches provided by this repository, but be sure to first checkout the repo to match the version of your installed rustc.

$ cd /path/to/rust

$ rustc -Vv
rustc 1.2.0-nightly (613e57b44 2015-06-01) (built 2015-06-02)
binary: rustc
commit-hash: 613e57b448c88591b6076a8cea9799f1f3876687
commit-date: 2015-06-01
build-date: 2015-06-02
host: x86_64-unknown-linux-gnu
release: 1.2.0-nightly

$ git checkout 613e57b448c88591b6076a8cea9799f1f3876687

$ patch -p1 < /path/to/some.patch

Otherwise, you can use the fetch-and-patch-rust.sh script, it will download a snapshot of rust-lang/rust that matches your installed rustc and apply the patches.

Once you have a patched rust repo, modify the Cargo.toml of the program that will be cross compiled to depend on the patched std crate:

# hello world example
$ cargo new --bin hello

$ cd hello

$ cat Cargo.toml
(..)
[dependencies.std]
path = "/path/to/patched/rust/src/libstd"
# optionally enable jemalloc and backtrace support
features = ["jemalloc", "backtrace"]
# or enable jemalloc in dynamically linked flavor
#features = ["jemalloc_dynamic"]

# don't forget to enable LTO for smaller binaries
[profile.release]
lto = true

Additionally, you'll need to set these two environment variables to tell cargo which compiler and archiver to use:

export AR_mips_unknown_linux_gnu=mips-openwrt-linux-ar
export CC_mips_unknown_linux_gnu=mips-openwrt-linux-gcc

Note to the each variable is suffixed by the target triple, in this case I'm targeting a mips-unknown-linux-gnu device.

With all that set, you can call cargo build --target=$TRIPLE like you normally would:

$ cargo build --target=mips-unknown-linux-gnu --release
   Compiling core v0.0.0 (file:///home/japaric/tmp/rust/src/hello)
   Compiling rustc_bitflags v0.0.0 (file:///home/japaric/tmp/rust/src/hello)
   Compiling gcc v0.3.6
   Compiling std v0.0.0 (file:///home/japaric/tmp/rust/src/hello)
   Compiling libc v0.0.0 (file:///home/japaric/tmp/rust/src/hello)
   Compiling rustc_unicode v0.0.0 (file:///home/japaric/tmp/rust/src/hello)
   Compiling rand v0.0.0 (file:///home/japaric/tmp/rust/src/hello)
   Compiling alloc v0.0.0 (file:///home/japaric/tmp/rust/src/hello)
   Compiling collections v0.0.0 (file:///home/japaric/tmp/rust/src/hello)
   Compiling hello v0.1.0 (file:///home/japaric/tmp/rust/src/hello)

$ file target/mips-unknown-linux-gnu/release/hello
target/mips-unknown-linux-gnu/release/hello: ELF 32-bit MSB shared object, MIPS, MIPS32 rel2 version 1 (SYSV), dynamically linked, interpreter /lib/ld-uClibc.so.0, not stripped

What do the patches do?

The cargo-ify.patch adds a bunch of Cargo.tomls with information about the dependencies between crates. It also adds build.rss to handle cross compilation of C dependencies like jemalloc, backtrace, compiler-rt, etc.

The remove-mno-compact-eh-flag.patch only matters if you are targeting the mips-unknown-linux-gnu triple, it removes a compilation flag that's not recognized by gcc.

The optional-backtrace.patch modifies Rust source code to allow disabling backtrace support via a cargo feature.

The optional-jemalloc.patch modifies Rust source code to allow picking between malloc and jemalloc via a cargo feature.

Known limitations

Huge binaries in debug profile

Building a crate with debug information results in a massive binary (several MBs), this is because the debug information of the std crate and all its dependencies are included in the binary. This doesn't occur with native compilation because the crates included in the Rust distribution are build without debug information. I'm unsure of how to dealt with this, we may need to add a new option to cargo that will force it to compile dependencies without debug information.

For now a workaround is to disable debug information in the debug profile:

// Cargo.toml
[profile.debug]
debug = false

This reduces the binary size, but the downside is that you won't be able to debug the program using gdb.

Can't compile the std natively

Unimplemented, but it should work in principle.

TODO

• Currently, the alloc crate is always compiled without jemalloc support. Add cargo features that will let us pick between no jemalloc, static jemalloc (default) and dynamic jemalloc. These cargo features will likely need to be "bubbled up" in the collections and std crates.

• Currently, the std crate has a cargo feature for backtrace support, but the feature is disabled by default -- that default should be reversed. It may also be worthwhile to add another feature to dynamically link to libbacktrace.

License

The scripts and patches in this repository are licensed under the MIT license.

See LICENSE-MIT for more details.