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`.

compiler/rustc_codegen_llvm/src/type_of.rs

@@ -397,7 +397,12 @@ fn scalar_copy_llvm_type<'a>(&self, cx: &CodegenCx<'a, 'tcx>) -> Option<&'a Type
             // extracts all the individual values.
 
             let ety = element.llvm_type(cx);
-            return Some(cx.type_vector(ety, *count));
+            if *count == 1 {
+                // Emitting `<1 x T>` would be silly; just use the scalar.
+                return Some(ety);
+            } else {
+                return Some(cx.type_vector(ety, *count));
+            }
         }
 
         // FIXME: The above only handled integer arrays; surely more things

tests/assembly/x86_64-array-pair-load-store-merge.rs

@@ -0,0 +1,19 @@
+// assembly-output: emit-asm
+// compile-flags: --crate-type=lib -O -C llvm-args=-x86-asm-syntax=intel
+// only-x86_64
+// ignore-sgx
+
+// Depending on various codegen choices, this might end up copying
+// a `<2 x i8>`, an `i16`, or two `i8`s.
+// Regardless of those choices, make sure the instructions use (2-byte) words.
+
+// CHECK-LABEL: array_copy_2_elements:
+#[no_mangle]
+pub fn array_copy_2_elements(a: &[u8; 2], p: &mut [u8; 2]) {
+    // CHECK-NOT: byte
+    // CHECK-NOT: mov
+    // CHECK: mov{{.+}}, word ptr
+    // CHECK-NEXT: mov word ptr
+    // CHECK-NEXT: ret
+    *p = *a;
+}

tests/codegen/array-codegen.rs

@@ -32,3 +32,25 @@
     // CHECK: store <4 x i8> %[[TEMP2]], ptr %p, align 1
     *p = *a;
 }
+
+// CHECK-LABEL: @array_copy_1_element
+#[no_mangle]
+pub fn array_copy_1_element(a: &[u8; 1], p: &mut [u8; 1]) {
+    // CHECK: %[[LOCAL:.+]] = alloca [1 x i8], align 1
+    // CHECK: %[[TEMP1:.+]] = load i8, ptr %a, align 1
+    // CHECK: store i8 %[[TEMP1]], ptr %[[LOCAL]], align 1
+    // CHECK: %[[TEMP2:.+]] = load i8, ptr %[[LOCAL]], align 1
+    // CHECK: store i8 %[[TEMP2]], ptr %p, align 1
+    *p = *a;
+}
+
+// CHECK-LABEL: @array_copy_2_elements
+#[no_mangle]
+pub fn array_copy_2_elements(a: &[u8; 2], p: &mut [u8; 2]) {
+    // CHECK: %[[LOCAL:.+]] = alloca [2 x i8], align 1
+    // CHECK: %[[TEMP1:.+]] = load <2 x i8>, ptr %a, align 1
+    // CHECK: store <2 x i8> %[[TEMP1]], ptr %[[LOCAL]], align 1
+    // CHECK: %[[TEMP2:.+]] = load <2 x i8>, ptr %[[LOCAL]], align 1
+    // CHECK: store <2 x i8> %[[TEMP2]], ptr %p, align 1
+    *p = *a;
+}

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;
+}