Rollup merge of #116099 - eduardosm:issue-79865-regression, r=oli-obk

Add regression test for issue #79865

Closes https://github.com/rust-lang/rust/issues/79865

tests/ui/codegen/issue-79865-llvm-miscompile.rs

@@ -0,0 +1,38 @@
+// run-pass
+// only-x86_64
+// compile-flags: -C opt-level=3
+
+// Regression test for issue #79865.
+// The assertion will fail when compiled with Rust 1.56..=1.59
+// due to a LLVM miscompilation.
+
+use std::arch::x86_64::*;
+
+fn main() {
+    if is_x86_feature_detected!("avx") {
+        let res: [f64; 4] = unsafe { std::mem::transmute::<_, _>(first()) };
+        assert_eq!(res, [22.0, 44.0, 66.0, 88.0]);
+    }
+}
+
+#[target_feature(enable = "avx")]
+unsafe fn first() -> __m256d {
+    second()
+}
+
+unsafe fn second() -> __m256d {
+    let v0 = _mm256_setr_pd(1.0, 2.0, 3.0, 4.0);
+    let v1 = _mm256_setr_pd(10.0, 20.0, 30.0, 40.0);
+
+    // needs to be called twice to hit the miscompilation
+    let (add, _) = add_sub(v0, v1);
+    let (add, _) = add_sub(add, add);
+    add
+}
+
+#[inline(never)] // needed to hit the miscompilation
+unsafe fn add_sub(v1: __m256d, v0: __m256d) -> (__m256d, __m256d) {
+    let add = _mm256_add_pd(v0, v1);
+    let sub = _mm256_sub_pd(v0, v1);
+    (add, sub)
+}