Auto merge of #116510 - scottmcm:no-1-simd-v2, r=compiler-errors

Copy 1-element arrays as scalars, not vectors

For `[T; 1]` it's silly to copy as `<1 x T>` when we can just copy as `T`.

Inspired by https://github.com/rust-lang/rust/issues/101210#issuecomment-1732470941, which pointed out that `Option<[u8; 1]>` was codegenning worse than `Option<u8>`.

(I'm not sure *why* LLVM doesn't optimize out `<1 x u8>`, but might as well just not emit it in the first place in this codepath.)

---

I think I bit off too much in #116479; let me try just the scalar case first.

r? `@ghost`

1 files changed, 33 insertions, 0 deletions

diff --git a/tests/codegen/array-optimized.rs b/tests/codegen/array-optimized.rs
new file mode 100644
index 00000000000..27448fdcfad
--- /dev/null
+++ b/tests/codegen/array-optimized.rs
@@ -0,0 +1,33 @@
+// compile-flags: -O
+
+#![crate_type = "lib"]
+
+// CHECK-LABEL: @array_copy_1_element
+#[no_mangle]
+pub fn array_copy_1_element(a: &[u8; 1], p: &mut [u8; 1]) {
+    // CHECK-NOT: alloca
+    // CHECK: %[[TEMP:.+]] = load i8, ptr %a, align 1
+    // CHECK: store i8 %[[TEMP]], ptr %p, align 1
+    // CHECK: ret
+    *p = *a;
+}
+
+// CHECK-LABEL: @array_copy_2_elements
+#[no_mangle]
+pub fn array_copy_2_elements(a: &[u8; 2], p: &mut [u8; 2]) {
+    // CHECK-NOT: alloca
+    // CHECK: %[[TEMP:.+]] = load <2 x i8>, ptr %a, align 1
+    // CHECK: store <2 x i8> %[[TEMP]], ptr %p, align 1
+    // CHECK: ret
+    *p = *a;
+}
+
+// CHECK-LABEL: @array_copy_4_elements
+#[no_mangle]
+pub fn array_copy_4_elements(a: &[u8; 4], p: &mut [u8; 4]) {
+    // CHECK-NOT: alloca
+    // CHECK: %[[TEMP:.+]] = load <4 x i8>, ptr %a, align 1
+    // CHECK: store <4 x i8> %[[TEMP]], ptr %p, align 1
+    // CHECK: ret
+    *p = *a;
+}