Incorrect LLVM generated with mir-opt-level=2: cannot invoke intrinsic

[tmiasko]

struct A;

impl Drop for A {
    fn drop(&mut self) {
        println!("A");
    }
}

fn main() {
    let a = A;
    std::sync::atomic::spin_loop_hint();
    std::mem::forget(a);
}

$ rustc -Copt-level=0 -Zmir-opt-level=2 a.rs
Cannot invoke an intrinsic other than donothing, patchpoint, statepoint, coro_resume or coro_destroy
  invoke void @llvm.x86.sse2.pause()
          to label %15 unwind label %16
Cannot invoke an intrinsic other than donothing, patchpoint, statepoint, coro_resume or coro_destroy
  invoke void @llvm.x86.sse2.pause()
          to label %15 unwind label %16
in function _ZN1a4main17h5a7c080779719c70E
LLVM ERROR: Broken function found, compilation aborted!

[jonas-schievink]

I guess intrinsics need to be called and not invoked? Maybe the MIR inliner is inlining the intrinsic's wrapper, but keeps the unwind target intact when it shouldn't. Or this should be handled in codegen, not sure.

[tmiasko]

The implementation ends up calling pause intrinsic declared as follows:

#![feature(link_llvm_intrinsics)]

extern "C" {
    #[link_name = "llvm.x86.sse2.pause"]
    fn pause();
}

The rustc codegen is completely unaware that it is dealing with an intrinsic,
it just changes the symbol name. Looks like the whole feature was introduced by
accident, and link_llvm_intrinsics feature gate was introduced after the fact
to make it unstable.