f16 addition difference between x86-64 and ARM64

[stevecheckoway]

I tried this code:

#![feature(f16)]

use std::f16;

fn main() {
    let pos_inf = f16::from_bits(0b0_11111_0000000000);
    let neg_inf = f16::from_bits(0b1_11111_0000000000);
    let sum = pos_inf + neg_inf;

    assert!(sum.is_nan());
    println!("{pos_inf:?} + {neg_inf:?} = {sum:?}");
}

on an ARM64 machine running macOS and on an x86-64 machine running Linux.

This code is adding +infinity + -infinity and producing a NaN.

I expected to see the same output on both systems.

Instead, the sign of the returned NaN differs between the two systems.

On ARM64, I get,

$ cargo +nightly run --quiet
0x7c00 + 0xfc00 = 0x7e00

On x86-64, I get,

$ cargo +nightly run --quiet
0x7c00 + 0xfc00 = 0xfe00

As you can see, the only difference is in the NaN's sign bit.

As I read IEEE 754-2008, §6.3, the sign bit of a NaN is not meaningful and that for addition (and most operations), the standard "does not specify the sign bit of a NaN result."

So in the sense of following the standard, this isn't a bug. But it might be nice to produce the same values on supported platforms. Of course, if hardware implementations differ, there's nothing to do.

If I change the code to this

    let pos_inf = f32::INFINITY;
    let neg_inf = -f32::INFINITY;
    let sum = pos_inf + neg_inf;
    println!("{:08x} + {:08x} = {:08x}", pos_inf.to_bits(), neg_inf.to_bits(), sum.to_bits());

then I get the same output for both systems:

7f800000 + ff800000 = 7fc00000

Meta

$ cargo +nightly --quiet rustc -- --version --verbose
rustc 1.88.0-nightly (2da29dbe8 2025-04-14)
binary: rustc
commit-hash: 2da29dbe8fe23df1c7c4ab1d8740ca3c32b15526
commit-date: 2025-04-14
host: aarch64-apple-darwin
release: 1.88.0-nightly
LLVM version: 20.1.2

$ cargo +nightly --quiet rustc -- --version --verbose
rustc 1.88.0-nightly (2da29dbe8 2025-04-14)
binary: rustc
commit-hash: 2da29dbe8fe23df1c7c4ab1d8740ca3c32b15526
commit-date: 2025-04-14
host: x86_64-unknown-linux-gnu
release: 1.88.0-nightly
LLVM version: 20.1.2

[hanna-kruppe]

As I read IEEE 754-2008, §6.3, the sign bit of a NaN is not meaningful and that for addition (and most operations), the standard "does not specify the sign bit of a NaN result."

More immediately relevant for Rust, https://rust-lang.github.io/rfcs/3514-float-semantics.html decided that the sign bit of any NaN result is non-deterministic.

[stevecheckoway]

Fair enough! I'll close this issue.