It seems like using transmute causes the upper and lower 64-bit halves to be loaded reversed. More at https://rust-lang.zulipchat.com/#narrow/stream/122651-general/topic/f128.20system.20libraries.20noncompliant.20platforms/near/438489330

#![feature(f128)]

#[no_mangle]
#[inline(never)]
fn add_entry(a: f128, b: f128) -> f128 {
    a + b
}

fn main() {
    let a = f128::from_bits(0x0);
    let b = f128::from_bits(0x1);
    dbg!(a, b);
    let c = add_entry(a, b);
    dbg!(c);
}

Espected b to print as 0x00000000000000000000000000000001, but with rustc add_test.rs -o add_test.rust --target powerpc64-unknown-linux-gnu -Clinker=powerpc64-linux-gnu-gcc -Ctarget-cpu=pwr9 it instead prints 0x00000000000000010000000000000000.