riscv32imc-unknown-none-elf with --target-feature=+d: cannot link files with different floating-point ABI

[jneem]

I tried this minimal no-std code:

#![no_std]
#![no_main]

use core::panic::PanicInfo;

#[panic_handler]
fn panic(_panic: &PanicInfo<'_>) -> ! {
    loop {}
}

Compiling it with rustc src/main.rs --target riscv32imc-unknown-none-elf -C target-feature=+d gives the linker error rust-lld: error: main.main.2f941e1c5654685d-cgu.0.rcgu.o: cannot link object files with different floating-point ABI from /tmp/rustcnFzdMx/symbols.o.

I was expecting it to successfully compile a binary that uses the double-precision standard extension to the riscv ABI.

I see that this PR changed the behavior so that the float part of the ABI comes from llvm_abiname, and only the RVC part comes from target-feature. Is this the intended behavior? I read on reddit (I know, it isn't canon) that target-feature is the say we're supposed to use the standard riscv extensions.

Meta

rustc --version --verbose:

rustc 1.76.0-nightly (2c1b65ee1 2023-11-11)
binary: rustc
commit-hash: 2c1b65ee1431f8d3fe2142e821eb13c623bbf8a0
commit-date: 2023-11-11
host: x86_64-unknown-linux-gnu
release: 1.76.0-nightly
LLVM version: 17.0.4

[bjorn3]

The standard library for riscv32imc-unknown-none-elf is compiled without the d target feature, but needs to be linked in. You have to recompile the standard library with the d target feature enabled using eg cargo build -Zbuild-std=core --target riscv32imc-unknown-none-elf

Edit: Yeah, llvm_abiname needs to be changed too. You would need to use a custom target. For example (untested):

{
  "arch": "riscv32",
  "atomic-cas": false,
  "cpu": "generic-rv32",
  "crt-objects-fallback": "false",
  "data-layout": "e-m:e-p:32:32-i64:64-n32-S128",
  "eh-frame-header": false,
  "emit-debug-gdb-scripts": false,
  "features": "+m,+c,+d",
  "linker": "rust-lld",
  "linker-flavor": "gnu-lld",
  "llvm-target": "riscv32",
  "llvm-abiname": "ilp32d",
  "max-atomic-width": 0,
  "panic-strategy": "abort",
  "relocation-model": "static",
  "target-pointer-width": "32"
}

You can put this in a .json file and then pass the path to it as --target argument.

[jneem]

Thanks for the quick response! I guess this is already addressed in your edit, but just for completeness I did try adding build-std in .cargo/config.toml, and cargo build still gives the same linking error.

And thanks for the example target! I can confirm that this allows the build to complete, and the resulting binary has the double-float ABI. I still think that the compiler should "just work" when the +d feature is added, in the same way that it "just works" when +c is added.

My (non-working) .cargo/config.toml:

[build]
rustflags = [ "-C", "target-feature=+d" ]

target = "riscv32imc-unknown-none-elf"

[unstable]
build-std = ["core"]

[bjorn3]

+c and +m don't affect the ABI while +f and +d do. On x86 target features affect the abi without the linker safeguard that you hit here and that gives a lot of pain which we are currently trying to fix: https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/Pre-RFC.20discussion.3A.20Forbidding.20SIMD.20types.20w.2Fo.20features and https://hackmd.io/e4bYAMh2RWG2yKZHivmF9Q

[jneem]

Makes sense, thanks for the links!

[RalfJung]

Yeah, riscv32imc-unknown-none-elf is a softfloat target. You can't just turn a softfloat target into a hardfloat target with -Ctarget-faeture, the difference between those targets is too big. If you need a hardfloat variant of this target, the only solution is a new target.

We should ideally already error about this on the Rust side so you don't just get a linker error; that is being worked on in the threads linked above.