f16::is_normal broken on s390x

[fneddy]

I tried this code on s390x:

./x test tests/ui/float/classify-runtime-const.rs

the failing case condensed is:

#![feature(f16)]
use std::hint::black_box;
use std::process::ExitCode;

fn main() -> ExitCode {
    ExitCode::from(f16::is_normal (1.0 / black_box(f16::MAX)) as u8)
}

I expected to see this happen:
the program should return a 0 because the calculation result is not_normal

Instead, this happened:
with -O (or -C opt-level=3) I see a 1 instead of a 0

Analysis

this is due to a bug in the llvm backend¹. llvm will issue f16 expand instructions that will loose information.

Steps

f16 is feature gated. I guess for now its ok to emit wrong code for f16::is_normal + -C opt-level=3?

We are working on fixing the bug in the llvm backend. with the next backend update it should be fixed.

Footnotes

1. https://github.com/llvm/llvm-project/issues/187518 ↩

[folkertdev]

Thanks for the heads-up.

with the next backend update it should be fixed.

To be clear that would be a 22.x update, right? I think we should try to get this backported to LLVM 22, waiting for LLVM 23 would be unfortunate.

[fneddy]

fix got backported to 22.x llvm/llvm-project#187943