compiler-rt does not define aarch64 outline atomics

[nitsky]

Zig Version

0.9.0-dev.1551+8346e011c

Steps to Reproduce

On aarch64 linux:

#include <stdatomic.h>
#include <stdio.h>

int main() {
  atomic_int atomic_counter = 0;
  atomic_counter++;
  printf("%u\n", atomic_counter);
}

1. gcc main.c
2. zig cc main.c
3. gcc -moutline-atomics main.c
4. zig cc -moutline-atomics main.c

Expected Behavior

I expect all variations to compile and run correctly.

Actual Behavior

zig cc -moutline-atomics main.c fails to compile:

ld.lld: error: undefined symbol: __aarch64_ldadd4_acq_rel

I discovered this issue while attempting to cross compile Rust to aarch64-linux with zig cc. Rust enables outline atomics by default on aarch64-linux and currently prohibits disabling them.

[andrewrk]

Thanks for the report. How is the __aarch64_ldadd4_acq_rel symbol expected to be provided? It does not appear to be included in LLVM's compiler-rt. Is that a gcc extension?

[nitsky]

@andrewrk I see that outline atomics were added to compiler-rt a year ago: https://reviews.llvm.org/D91156.

[andrewrk]

Ah, I see the symbol names are constructed with string concatenation. Missed that in my grepping. Thanks! So the next step to resolve this issue is to port more of LLVM's compiler-rt into zig's.

[nitsky]

Great, thanks. For those running into this issue with Rust, rust-lang/rust#90623 was recently merged which allows you to disable outline atomics with -C target-feature=-outline-atomics.

[irishgreencitrus]

I have this issue compiling https://github.com/mattnite/gyro on aarch64 Raspberry Pi OS, with just normal zig build

[PiotrSikora]

zig c++ seems to be completely broken when cross-compiling to aarch64-linux-*, because libc++ relies on atomics.

$ cat hello.cpp

#include <iostream>

int main() {
    std::cout << "Hello, World!" << std::endl;
    return 0;
}

$ zig c++ -target aarch64-linux-musl hello.cpp

ld.lld: error: undefined symbol: __aarch64_ldadd8_acq_rel
>>> referenced by cxa_exception.cpp
[...]
ld.lld: error: undefined symbol: __aarch64_swp8_acq_rel
>>> referenced by cxa_handlers.cpp
[...]
ld.lld: error: undefined symbol: __aarch64_ldadd4_acq_rel
>>> referenced by ios.cpp
[...]
ld.lld: error: undefined symbol: __aarch64_ldadd8_relax
>>> referenced by memory.cpp
[...]
ld.lld: error: undefined symbol: __aarch64_cas8_acq_rel
>>> referenced by memory.cpp