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CXX with CMake build system

CXX CMake CI

This is an example repo to setup cxx with the cmake build system.

The official demo used cargo to orchestrate the two build systems and place the main function inside the rust project.

In a lot of other applications, however, we want to embed rust into a large cpp project where we don't have a chance to choose build systems. This template repo shows how to use cmake with cxx.

The cmake files do the following things:

  1. Call cargo build [--release] to build the static library and the necessary c++ header/source files
  2. Create a library from the cxx generated source and link to the rust static library
  3. Link and include the libraray to the corresponding targets

Cross-language LTO (linking time optimization)

More details in my blog.

Why?

Calling rust function from c++ (and vice versa) is not zero-overhead because the LLVM optimizer by default will not optimize across shared libraries, let alone across different languages.

LTO (linking time optimization) allows the optimizers to perform optimization during linking time (instead of compile time), thus enables cross library optimization.

How?

cmake -DENABLE_LTO=ON -DCMAKE_BUILD_TYPE=Release ..
make -j
./main

The -DENABLE_LTO=ON will compile and link both libraries with proper parameters. Note that cross language LTO between rust and c/c++ is only possible with clang toolchain, meaning that you need to have a very recent clang/lld installed.

Example output

With LTO

Points {
    x: [
        1,
        2,
        3,
    ],
    y: [
        4,
        5,
        6,
    ],
}
"cpp expert!"
Calling rust function, time elapsed: 100 ns.
Calling c++ function, time elapsed: 100 ns.

Without LTO

Points {
    x: [
        1,
        2,
        3,
    ],
    y: [
        4,
        5,
        6,
    ],
}
"cpp expert!"
Calling rust function, time elapsed: 1176600 ns.
Calling c++ function, time elapsed: 100 ns.

Credits

The cmake files are largely inspired by Using unsafe for Fun and Profit.

I learned a lot about cross language LTO from this post: Closing the gap: cross-language LTO between Rust and C/C++


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