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Seth Pellegrino 897c7bb23b feat: riscv-interrupt-{m,s} calling conventions

Similar to prior support added for the mips430, avr, and x86 targets
this change implements the rough equivalent of clang's
[`__attribute__((interrupt))`][clang-attr] for riscv targets, enabling
e.g.

```rust
static mut CNT: usize = 0;

pub extern "riscv-interrupt-m" fn isr_m() {
    unsafe {
        CNT += 1;
    }
}
```

to produce highly effective assembly like:

```asm
pub extern "riscv-interrupt-m" fn isr_m() {
420003a0:       1141                    addi    sp,sp,-16
    unsafe {
        CNT += 1;
420003a2:       c62a                    sw      a0,12(sp)
420003a4:       c42e                    sw      a1,8(sp)
420003a6:       3fc80537                lui     a0,0x3fc80
420003aa:       63c52583                lw      a1,1596(a0) # 3fc8063c <_ZN12esp_riscv_rt3CNT17hcec3e3a214887d53E.0>
420003ae:       0585                    addi    a1,a1,1
420003b0:       62b52e23                sw      a1,1596(a0)
    }
}
420003b4:       4532                    lw      a0,12(sp)
420003b6:       45a2                    lw      a1,8(sp)
420003b8:       0141                    addi    sp,sp,16
420003ba:       30200073                mret
```

(disassembly via `riscv64-unknown-elf-objdump -C -S --disassemble ./esp32c3-hal/target/riscv32imc-unknown-none-elf/release/examples/gpio_interrupt`)

This outcome is superior to hand-coded interrupt routines which, lacking
visibility into any non-assembly body of the interrupt handler, have to
be very conservative and save the [entire CPU state to the stack
frame][full-frame-save]. By instead asking LLVM to only save the
registers that it uses, we defer the decision to the tool with the best
context: it can more accurately account for the cost of spills if it
knows that every additional register used is already at the cost of an
implicit spill.

At the LLVM level, this is apparently [implemented by] marking every
register as "[callee-save]," matching the semantics of an interrupt
handler nicely (it has to leave the CPU state just as it found it after
its `{m|s}ret`).

This approach is not suitable for every interrupt handler, as it makes
no attempt to e.g. save the state in a user-accessible stack frame. For
a full discussion of those challenges and tradeoffs, please refer to
[the interrupt calling conventions RFC][rfc].

Inside rustc, this implementation differs from prior art because LLVM
does not expose the "all-saved" function flavor as a calling convention
directly, instead preferring to use an attribute that allows for
differentiating between "machine-mode" and "superivsor-mode" interrupts.

Finally, some effort has been made to guide those who may not yet be
aware of the differences between machine-mode and supervisor-mode
interrupts as to why no `riscv-interrupt` calling convention is exposed
through rustc, and similarly for why `riscv-interrupt-u` makes no
appearance (as it would complicate future LLVM upgrades).

[clang-attr]: https://clang.llvm.org/docs/AttributeReference.html#interrupt-risc-v
[full-frame-save]: 9281af2ecf/src/lib.rs (L440-L469)
[implemented by]: b7fb2a3fec/llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp (L61-L67)
[callee-save]: 973f1fe7a8/llvm/lib/Target/RISCV/RISCVCallingConv.td (L30-L37)
[rfc]: https://github.com/rust-lang/rfcs/pull/3246

2023-08-08 18:09:56 -07:00

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2023-08-08 18:09:56 -07:00

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Readme.md

Cranelift codegen backend for rust

The goal of this project is to create an alternative codegen backend for the rust compiler based on Cranelift. This has the potential to improve compilation times in debug mode. If your project doesn't use any of the things listed under "Not yet supported", it should work fine. If not please open an issue.

Building and testing

$ git clone https://github.com/bjorn3/rustc_codegen_cranelift
$ cd rustc_codegen_cranelift
$ ./y.sh prepare
$ ./y.sh build

To run the test suite replace the last command with:

$ ./test.sh

For more docs on how to build and test see build_system/usage.txt or the help message of ./y.sh.

Precompiled builds

Alternatively you can download a pre built version from the releases page. Extract the dist directory in the archive anywhere you want. If you want to use cargo clif build instead of having to specify the full path to the cargo-clif executable, you can add the bin subdirectory of the extracted dist directory to your PATH. (tutorial for Windows, and for Linux/MacOS).

Usage

rustc_codegen_cranelift can be used as a near-drop-in replacement for cargo build or cargo run for existing projects.

Assuming $cg_clif_dir is the directory you cloned this repo into and you followed the instructions (y.sh prepare and y.sh build or test.sh).

In the directory with your project (where you can do the usual cargo build), run:

$ $cg_clif_dir/dist/cargo-clif build

This will build your project with rustc_codegen_cranelift instead of the usual LLVM backend.

For additional ways to use rustc_codegen_cranelift like the JIT mode see usage.md.

Building and testing with changes in rustc code

This is useful when changing code in rustc_codegen_cranelift as part of changing main Rust repository. This can happen, for example, when you are implementing a new compiler intrinsic.

Instruction below uses $RustCheckoutDir as substitute for any folder where you cloned Rust repository.

You need to do this steps to successfully compile and use the cranelift backend with your changes in rustc code:

cd $RustCheckoutDir
Run python x.py setup and choose option for compiler (b).
Build compiler and necessary tools: python x.py build --stage=2 compiler library/std src/tools/rustdoc src/tools/rustfmt
- (Optional) You can also build cargo by adding src/tools/cargo to previous command.
Copy exectutable files from ./build/host/stage2-tools/<your hostname triple>/release to ./build/host/stage2/bin/. Note that you would need to do this every time you rebuilt rust repository.
Copy cargo from another toolchain: cp $(rustup which cargo) .build/<your hostname triple>/stage2/bin/cargo
- Another option is to build it at step 3 and copy with other executables at step 4.
Link your new rustc to toolchain: rustup toolchain link stage2 ./build/host/stage2/.
(Windows only) compile the build system: rustc +stage2 -O build_system/main.rs -o y.exe.
You need to prefix every ./y.sh (or y if you built build_system/main.rs as y) command by rustup run stage2 to make cg_clif use your local changes in rustc.

rustup run stage2 ./y.sh prepare
rustup run stage2 ./y.sh build
(Optional) run tests: rustup run stage2 ./y.sh test

Now you can use your cg_clif build to compile other Rust programs, e.g. you can open any Rust crate and run commands like $RustCheckoutDir/compiler/rustc_codegen_cranelift/dist/cargo-clif build --release.

Configuration

See the documentation on the BackendConfig struct in config.rs for all configuration options.

Not yet supported

Inline assembly (no cranelift support)
- On UNIX there is support for invoking an external assembler for global_asm! and asm!.
SIMD (tracked here, std::simd fully works, std::arch is partially supported)
Unwinding on panics (no cranelift support, -Cpanic=abort is enabled by default)

License

Licensed under either of

Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you shall be dual licensed as above, without any additional terms or conditions.