Failed LLVM8 compilation on Darwin: cmpxchg instructions cannot be unordered

[nmattia]

I tried building rust 1.41 on Darwin with LLVM 8 and it fails with the following:

Installing libLLVM.so to stage 0 (x86_64-apple-darwin)
Copying stage0 rustc from stage0 (x86_64-apple-darwin -> x86_64-apple-darwin / x86_64-apple-darwin)
Assembling stage1 compiler (x86_64-apple-darwin)
Building stage1 std artifacts (x86_64-apple-darwin -> x86_64-apple-darwin)
   Compiling cc v1.0.47
   Compiling core v0.0.0 (/private/tmp/nix-build-rustc-1.41.0.drv-0/rustc-1.41.0-src/src/libcore)
   Compiling libc v0.2.64
   Compiling build_helper v0.1.0 (/private/tmp/nix-build-rustc-1.41.0.drv-0/rustc-1.41.0-src/src/build_helper)
   Compiling autocfg v0.1.6
   Compiling std v0.0.0 (/private/tmp/nix-build-rustc-1.41.0.drv-0/rustc-1.41.0-src/src/libstd)
   Compiling cmake v0.1.38
   Compiling compiler_builtins v0.1.22
   Compiling backtrace-sys v0.1.32
   Compiling unwind v0.0.0 (/private/tmp/nix-build-rustc-1.41.0.drv-0/rustc-1.41.0-src/src/libunwind)
   Compiling rustc-std-workspace-core v1.99.0 (/private/tmp/nix-build-rustc-1.41.0.drv-0/rustc-1.41.0-src/src/tools/rustc-std-workspace-core)
   Compiling hashbrown v0.6.2
   Compiling rustc_tsan v0.0.0 (/private/tmp/nix-build-rustc-1.41.0.drv-0/rustc-1.41.0-src/src/librustc_tsan)
   Compiling rustc_asan v0.0.0 (/private/tmp/nix-build-rustc-1.41.0.drv-0/rustc-1.41.0-src/src/librustc_asan)
cmpxchg instructions cannot be unordered.
  %10 = cmpxchg i128* %9, i128 0, i128 0 unordered monotonic
in function __llvm_memcpy_element_unordered_atomic_16
LLVM ERROR: Broken function found, compilation aborted!
error: could not compile `compiler_builtins`.

Everything works with LLVM9, and everything works on Linux.

[cuviper]

x86_64-apple-darwin supports up to 128-bit atomics thanks to its core2 base, but x86_64-unknown-linux-gnu allows older CPUs and only supports up to 64-bit atomics.

rust/src/librustc_target/spec/x86_64_apple_darwin.rs

Line 6 in 5e1a799

base.max_atomic_width = Some(128); // core2 support cmpxchg16b

rust/src/librustc_target/spec/x86_64_unknown_linux_gnu.rs

Line 6 in 5e1a799

base.max_atomic_width = Some(64);

It may be that this was broken before LLVM 9 though...

[akien-mga]

As I landed here with a google search, I'll document @cuviper's fix for future readers:
https://src.fedoraproject.org/rpms/llvm/pull-request/35

[sanxiyn]

Closing, Rust now requires LLVM >=12.