LLVM Error with asm!

[Soveu]

I tried this code:

#![feature(asm)]

#[inline(always)]
fn check_bsf() -> (u64, u8) {
    let flags: u8;
    let result: u64;

    unsafe {
        asm!(
            "bsf rbx, rbx",
            "lahf",
            inout("rbx") 0u64 => result,
            out("ah") flags,
            options(nostack, nomem),
        );
    }

    return (result, flags);
}

fn main() {
    let flags1: u8;
    unsafe {
        asm!("lahf", out("ah") flags1, options(nostack, nomem));
    }

    let (result, flags2) = check_bsf();
    let flagsdiff = flags2 ^ flags1;
    let zero_flag = 1 << 6;

    println!("result={}", result);
    assert_eq!(flagsdiff, zero_flag);
}

I expected to see this happen:
The code compiles, like with opt-level 0 or 1

Instead, this happened:
When compiling with opt-level 2 or 3, just this message appears

LLVM ERROR: Cannot encode high byte register in REX-prefixed instruction

Meta

rustc --version --verbose:

rustc 1.53.0-nightly (07e0e2ec2 2021-03-24)
binary: rustc
commit-hash: 07e0e2ec268c140e607e1ac7f49f145612d0f597
commit-date: 2021-03-24
host: x86_64-unknown-linux-gnu
release: 1.53.0-nightly
LLVM version: 12.0.0

No backtrace

[nagisa]

Minimal-ish IR test case:

define dso_local void @main(i64* %arg) unnamed_addr {
  %a = tail call { i64, i8 } asm "", "={bx},={ah}"()
  %_4.0 = extractvalue { i64, i8 } %a, 0
  %_4.1 = extractvalue { i64, i8 } %a, 1
  store i64 %_4.0, i64* %arg
  %1 = xor i8 %_4.1, 0
  call void @banana()
  %_30.not = icmp eq i8 %1, 64
  br i1 %_30.not, label %bb7, label %bb6

bb6:
  unreachable

bb7:
  unreachable
}

declare dso_local void @banana() unnamed_addr

Pending verification if reproducible in LLVM main. https://bugs.llvm.org/show_bug.cgi?id=49729

[Amanieu]

I'm considering just outright banning the use of ah on x86_64 with a note to use ax instead.