riscv: unable to use atomics with linker-plugin-lto

[xobs]

The following code compiles fine without linker-plugin-lto, but fails to compile when it is enabled:

#![no_std]
#![no_main]

use core::sync::atomic::{AtomicUsize, Ordering};
static MM_HANDLE_COUNT: AtomicUsize = AtomicUsize::new(0);

#[no_mangle]
pub extern "C" fn _start() -> ! {
    let _count = MM_HANDLE_COUNT.fetch_add(1, Ordering::SeqCst);
    loop {}
}

use core::panic::PanicInfo;
#[panic_handler]
fn handle_panic(_arg: &PanicInfo) -> ! {
    loop {}
}

#[no_mangle]
pub extern "C" fn abort() -> ! {
    loop {}
}

This code should compile cleanly with:

$ rustc +nightly main.rs --target riscv64gc-unknown-none-elf
$

However, with the lto plugin, it doesn't compile:

$ rustc +nightly -C linker-plugin-lto main.rs --target riscv64gc-unknown-none-elf error: linking with `rust-lld` failed: exit code: 1
  |
  = note: "rust-lld" "-flavor" "gnu" "-plugin-opt=O0" "-plugin-opt=mcpu=generic-rv64" "-L" "/home/user/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/riscv64gc-unknown-none-elf/lib" "main.main.7rcbfp3g-cgu.0.rcgu.o" "main.main.7rcbfp3g-cgu.1.rcgu.o" "main.main.7rcbfp3g-cgu.2.rcgu.o" "-o" "main" "--gc-sections" "-L" "/home/user/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/riscv64gc-unknown-none-elf/lib" "-Bstatic" "/home/user/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/riscv64gc-unknown-none-elf/lib/librustc_std_workspace_core-a43e904133f39ebe.rlib" "/home/user/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/riscv64gc-unknown-none-elf/lib/libcore-de86a0d6e9117873.rlib" "/home/user/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/riscv64gc-unknown-none-elf/lib/libcompiler_builtins-980292810b049237.rlib" "-Bdynamic"
  = note: rust-lld: error: undefined symbol: __atomic_fetch_add_8
          >>> referenced by main.7rcbfp3g-cgu.2
          >>>               lto.tmp:(core::sync::atomic::AtomicUsize::fetch_add::h6aa949cc8cd90341)


error: aborting due to previous error
$

Meta

rustc --version --verbose:

rustc 1.43.0-nightly (4ad624882 2020-03-03)
binary: rustc
commit-hash: 4ad62488258972bdb0e2df225d100f99ef58dbad
commit-date: 2020-03-03
host: x86_64-unknown-linux-gnu
release: 1.43.0-nightly
LLVM version: 9.0

This occurs on both riscv64gc-unknown-none-elf and riscv32imac-unknown-none-elf.

[xobs]

This may be a bug in rust-lld caused by it not passing (or supporting passing?) the "features" flags to the linker: rust-lang/compiler-builtins#346 (comment)

[xobs]

Note that passing "-plugin-opt=--mattr=+a" seems to fix the issue. It seems like compiling with -C linker-plugin-lto should also pass the target's features to lld, via -plugin-opt=--mattr=.