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library/compiler-builtins/README.md
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# `compiler-builtins`
> Porting `compiler-rt` intrinsics to Rust
This crate provides external symbols that the compiler expects to be available when
building Rust projects, typically software routines for basic operations that do not
have hardware support. It is largely a port of LLVM's [`compiler-rt`].
See [rust-lang/rust#35437][0].
It is distributed as part of Rust's sysroot.
[0]: https://github.com/rust-lang/rust/issues/35437
## When and how to use this crate?
If you are working with a target that doesn't have binary releases of std
available via rustup (this probably means you are building the core crate
yourself) and need compiler-rt intrinsics (i.e. you are probably getting linker
errors when building an executable: `undefined reference to __aeabi_memcpy`),
you can use this crate to get those intrinsics and solve the linker errors. To
do that, add this crate somewhere in the dependency graph of the crate you are
building:
```toml
# Cargo.toml
[dependencies]
compiler_builtins = { git = "https://github.com/rust-lang/compiler-builtins" }
```
```rust
extern crate compiler_builtins;
// ...
```
If you still get an "undefined reference to $INTRINSIC" error after that change,
that means that we haven't ported `$INTRINSIC` to Rust yet! Please open [an
issue] with the name of the intrinsic and the LLVM triple (e.g.
thumbv7m-none-eabi) of the target you are using. That way we can prioritize
porting that particular intrinsic.
If you've got a C compiler available for your target then while we implement
this intrinsic you can temporarily enable a fallback to the actual compiler-rt
implementation as well for unimplemented intrinsics:
```toml
[dependencies.compiler_builtins]
git = "https://github.com/rust-lang/compiler-builtins"
features = ["c"]
```
[an issue]: https://github.com/rust-lang/compiler-builtins/issues
[`compiler-rt`]: https://github.com/llvm/llvm-project/tree/1b1dc505057322f4fa1110ef4f53c44347f52986/compiler-rt
## Contributing