test: dont optimize to invalid bitcasts

tests/ui/simd/dont-invalid-bitcast-masks.rs

@@ -0,0 +1,17 @@
+//@ build-pass
+//@ compile-flags: -Copt-level=3
+
+// regression test for https://github.com/rust-lang/rust/issues/110722
+// in --release we were optimizing to invalid bitcasts, due to a combination of MIR inlining and
+// mostly bad repr(simd) lowering which prevented even basic splats from working
+#![crate_type = "rlib"]
+#![feature(portable_simd)]
+use std::simd::*;
+use std::simd::num::*;
+
+pub unsafe fn mask_to_array(mask: u8) -> [i32; 8] {
+    let mut output = [0; 8];
+    let m = masksizex8::from_bitmask(mask as _);
+    output.copy_from_slice(&m.to_int().cast::<i32>().to_array());
+    output
+}

tests/ui/simd/dont-invalid-bitcast-x86_64.rs

@@ -0,0 +1,27 @@
+//@ build-pass
+//@ compile-flags: -Copt-level=3
+//@ only-x86_64
+// ignore-tidy-linelength
+
+// regression test for https://github.com/rust-lang/rust/issues/110707
+// in --release we were optimizing to invalid bitcasts, due to a combination of MIR inlining and
+// mostly bad repr(simd) lowering which prevented even basic splats from working
+
+#![crate_type = "rlib"]
+#![feature(portable_simd)]
+use std::simd::*;
+use std::arch::x86_64::*;
+
+#[target_feature(enable = "sse4.1")]
+pub unsafe fn fast_round_sse(i: f32x8) -> f32x8 {
+    let a = i.to_array();
+    let [low, high]: [[f32; 4]; 2] =
+        unsafe { std::mem::transmute::<[f32; 8], [[f32; 4]; 2]>(a) };
+
+    let low = f32x4::from(_mm_round_ps::<{_MM_FROUND_TO_NEAREST_INT | _MM_FROUND_NO_EXC}>(f32x4::from_array(low).into()));
+    let high = f32x4::from(_mm_round_ps::<{_MM_FROUND_TO_NEAREST_INT | _MM_FROUND_NO_EXC}>(f32x4::from_array(high).into()));
+
+    let a: [f32; 8] =
+        unsafe { std::mem::transmute::<[[f32; 4]; 2], [f32; 8]>([low.to_array(), high.to_array()]) };
+    f32x8::from_array(a)
+}