Code shared from the Rust Playground

playground.rs

#[test]
#[allow(clippy::float_cmp)]
fn sse_sanity_tests() {
  // Note(Lokathor): Here is where many sanity checks for our assumptions of
  // `__m128` will go.
  #[cfg(target_arch = "x86")]
  use core::arch::x86::*;
  #[cfg(target_arch = "x86_64")]
  use core::arch::x86_64::*;
  use core::mem::*;
  unsafe {
    // An `__m128` is "basically" an `[f32;4]`, and if you inspect the inside
    // with pointer math stuff you get floats at each offset that are the same
    // as the indexes of the array you transmuted to get the `__m128`.
    let m: __m128 = transmute([5.0_f32, 6.0, 7.0, 8.0]);
    let m_ptr: *const f32 = &m as *const __m128 as *const f32;
    assert_eq!(*m_ptr.offset(0), 5.0);
    assert_eq!(*m_ptr.offset(1), 6.0);
    assert_eq!(*m_ptr.offset(2), 7.0);
    assert_eq!(*m_ptr.offset(3), 8.0);

    // Confusingly, the `set` order places the data into the lanes _backwards_
    // from the array index ordering.
    let m: __m128 = _mm_set_ps(5.0, 6.0, 7.0, 8.0);
    let m_ptr: *const f32 = &m as *const __m128 as *const f32;
    assert_eq!(*m_ptr.offset(0), 8.0);
    assert_eq!(*m_ptr.offset(1), 7.0);
    assert_eq!(*m_ptr.offset(2), 6.0);
    assert_eq!(*m_ptr.offset(3), 5.0);

    // And then the `setr` order places the data into the lanes the same as the
    // array index ordering.
    let m: __m128 = _mm_setr_ps(5.0, 6.0, 7.0, 8.0);
    let m_ptr: *const f32 = &m as *const __m128 as *const f32;
    assert_eq!(*m_ptr.offset(0), 5.0);
    assert_eq!(*m_ptr.offset(1), 6.0);
    assert_eq!(*m_ptr.offset(2), 7.0);
    assert_eq!(*m_ptr.offset(3), 8.0);

    // The "low" lane is offset 0
    let m: __m128 = _mm_set_ss(5.0);
    let m_ptr: *const f32 = &m as *const __m128 as *const f32;
    assert_eq!(*m_ptr.offset(0), 5.0);
    assert_eq!(*m_ptr.offset(1), 0.0);
    assert_eq!(*m_ptr.offset(2), 0.0);
    assert_eq!(*m_ptr.offset(3), 0.0);
    // Some operations affect only the low lane.
    {
      let b: __m128 = _mm_set1_ps(3.0);
      // PACKED (adds all lanes)
      let m_b = _mm_add_ps(m, b);
      let m_b_ptr: *const f32 = &m_b as *const __m128 as *const f32;
      assert_eq!(*m_b_ptr.offset(0), 8.0);
      assert_eq!(*m_b_ptr.offset(1), 3.0);
      assert_eq!(*m_b_ptr.offset(2), 3.0);
      assert_eq!(*m_b_ptr.offset(3), 3.0);
      // SINGLE (adds only offset0, other lanes copy "self", `m`)
      let m_b = _mm_add_ss(m, b);
      let m_b_ptr: *const f32 = &m_b as *const __m128 as *const f32;
      assert_eq!(*m_b_ptr.offset(0), 8.0);
      assert_eq!(*m_b_ptr.offset(1), 0.0);
      assert_eq!(*m_b_ptr.offset(2), 0.0);
      assert_eq!(*m_b_ptr.offset(3), 0.0);
    }
  }
}