PowerPC AltiVec source vector subnormal values are not flushed to zero

Host environment

• Operating system: Ubuntu 20.04
• OS/kernel version: Linux ab4ce1e724be 5.10.47-linuxkit #1 SMP Sat Jul 3 21:51:47 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
• Architecture: x86-64
• QEMU flavor: qemu-ppc
• QEMU version: 7.0.0
• QEMU command line:

  qemu-ppc -cpu e600 /target/powerpc-unknown-linux-gnu/debug/ppc-error

Emulated/Virtualized environment

• Operating system: Ubuntu
• OS/kernel version: Linux ab4ce1e724be 5.10.47-linuxkit #1 SMP Sat Jul 3 21:51:47 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
• Architecture: PowerPC

Description of problem

AltiVec specifies that source and result vectors are flushed to zero (in NJ mode). Only result vectors are flushed to zero.

Steps to reproduce

1. Compile the attached Rust program (e.g. cargo +nightly build --target powerpc-unknown-linux-gnu)
2. Run the program and expect assertions to pass.
3. See assertions fail.

Additional information

See the offending Rust program:

#![feature(stdsimd, powerpc_target_feature)]

use std::arch::powerpc::*;

#[target_feature(enable = "altivec")]
unsafe fn add(x: f32, y: f32) -> f32 {
    let array: [f32; 4] = unsafe { std::mem::transmute(vec_add(vec_splats(x), vec_splats(y))) };
    array[0]
}

pub fn main() {
    let result = unsafe { add(-1.0857398e-38, 0.) };
    assert_eq!(result, 0.);

    // if the input is flushed, the result will be +0
    // if only the output is flushed, the result is -0
    assert!(result.is_sign_positive());
}