From cc7d8014d720ac800f868a0b7ec854f4a5036e3f Mon Sep 17 00:00:00 2001
From: Scott McMurray <scottmcm@users.noreply.github.com>
Date: Tue, 9 Nov 2021 21:43:20 -0800
Subject: [PATCH 1/2] Specialize array cloning for Copy types

Because after PR 86041, the optimizer no longer load-merges at the LLVM IR level, which might be part of the perf loss.  (I'll run perf and see if this makes a difference.)

Also I added a codegen test so this hopefully won't regress in future -- it passes on stable and with my change here, but not on the 2021-11-09 nightly.
---
 library/core/src/array/mod.rs   | 25 ++++++++++++++++++++++---
 src/test/codegen/array-clone.rs | 15 +++++++++++++++
 2 files changed, 37 insertions(+), 3 deletions(-)
 create mode 100644 src/test/codegen/array-clone.rs

diff --git a/library/core/src/array/mod.rs b/library/core/src/array/mod.rs
index 089f1f3639062..e8e0cbf84b37d 100644
--- a/library/core/src/array/mod.rs
+++ b/library/core/src/array/mod.rs
@@ -339,9 +339,7 @@ impl<T: Copy, const N: usize> Copy for [T; N] {}
 impl<T: Clone, const N: usize> Clone for [T; N] {
     #[inline]
     fn clone(&self) -> Self {
-        // SAFETY: we know for certain that this iterator will yield exactly `N`
-        // items.
-        unsafe { collect_into_array_unchecked(&mut self.iter().cloned()) }
+        SpecArrayClone::clone(self)
     }
 
     #[inline]
@@ -350,6 +348,27 @@ impl<T: Clone, const N: usize> Clone for [T; N] {
     }
 }
 
+#[cfg(not(bootstrap))]
+trait SpecArrayClone: Clone {
+    fn clone<const N: usize>(array: &[Self; N]) -> [Self; N];
+}
+
+#[cfg(not(bootstrap))]
+impl<T: Clone> SpecArrayClone for T {
+    default fn clone<const N: usize>(array: &[T; N]) -> [T; N] {
+        // SAFETY: we know for certain that this iterator will yield exactly `N`
+        // items.
+        unsafe { collect_into_array_unchecked(&mut array.iter().cloned()) }
+    }
+}
+
+#[cfg(not(bootstrap))]
+impl<T: Copy> SpecArrayClone for T {
+    fn clone<const N: usize>(array: &[T; N]) -> [T; N] {
+        *array
+    }
+}
+
 // The Default impls cannot be done with const generics because `[T; 0]` doesn't
 // require Default to be implemented, and having different impl blocks for
 // different numbers isn't supported yet.
diff --git a/src/test/codegen/array-clone.rs b/src/test/codegen/array-clone.rs
new file mode 100644
index 0000000000000..0d42963bcd2ce
--- /dev/null
+++ b/src/test/codegen/array-clone.rs
@@ -0,0 +1,15 @@
+// compile-flags: -O
+
+#![crate_type = "lib"]
+
+// CHECK-LABEL: @array_clone
+#[no_mangle]
+pub fn array_clone(a: &[u8; 2]) -> [u8; 2] {
+    // CHECK-NOT: getelementptr
+    // CHECK-NOT: load i8
+    // CHECK-NOT: zext
+    // CHECK-NOT: shl
+    // CHECK: load i16
+    // CHECK-NEXT: ret i16
+    a.clone()
+}

From 5b115fcb6839b94a14fb77acdd738281b4bb3fae Mon Sep 17 00:00:00 2001
From: Scott McMurray <scottmcm@users.noreply.github.com>
Date: Wed, 10 Nov 2021 11:57:14 -0800
Subject: [PATCH 2/2] Moar #[inline]

---
 library/core/src/array/mod.rs | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/library/core/src/array/mod.rs b/library/core/src/array/mod.rs
index e8e0cbf84b37d..09bb4519170da 100644
--- a/library/core/src/array/mod.rs
+++ b/library/core/src/array/mod.rs
@@ -355,6 +355,7 @@ trait SpecArrayClone: Clone {
 
 #[cfg(not(bootstrap))]
 impl<T: Clone> SpecArrayClone for T {
+    #[inline]
     default fn clone<const N: usize>(array: &[T; N]) -> [T; N] {
         // SAFETY: we know for certain that this iterator will yield exactly `N`
         // items.
@@ -364,6 +365,7 @@ impl<T: Clone> SpecArrayClone for T {
 
 #[cfg(not(bootstrap))]
 impl<T: Copy> SpecArrayClone for T {
+    #[inline]
     fn clone<const N: usize>(array: &[T; N]) -> [T; N] {
         *array
     }