add inline attribute and codegen test

Author: edwloef

library/core/src/slice/rotate.rs

@@ -10,6 +10,7 @@ type BufType = [usize; 32];
 /// # Safety
 ///
 /// The specified range must be valid for reading and writing.
+#[inline]
 pub(super) unsafe fn ptr_rotate<T>(left: usize, mid: *mut T, right: usize) {
     if T::IS_ZST {
         return;
@@ -43,6 +44,7 @@ pub(super) unsafe fn ptr_rotate<T>(left: usize, mid: *mut T, right: usize) {
 /// # Safety
 ///
 /// The specified range must be valid for reading and writing.
+#[inline]
 unsafe fn ptr_rotate_memmove<T>(left: usize, mid: *mut T, right: usize) {
     // The `[T; 0]` here is to ensure this is appropriately aligned for T
     let mut rawarray = MaybeUninit::<(BufType, [T; 0])>::uninit();
@@ -114,6 +116,7 @@ unsafe fn ptr_rotate_memmove<T>(left: usize, mid: *mut T, right: usize) {
 /// # Safety
 ///
 /// The specified range must be valid for reading and writing.
+#[inline]
 unsafe fn ptr_rotate_gcd<T>(left: usize, mid: *mut T, right: usize) {
     // Algorithm 2
     // Microbenchmarks indicate that the average performance for random shifts is better all
@@ -218,6 +221,7 @@ unsafe fn ptr_rotate_gcd<T>(left: usize, mid: *mut T, right: usize) {
 /// # Safety
 ///
 /// The specified range must be valid for reading and writing.
+#[inline]
 unsafe fn ptr_rotate_swap<T>(mut left: usize, mut mid: *mut T, mut right: usize) {
     loop {
         if left >= right {

tests/codegen/lib-optimizations/slice_rotate.rs

@@ -0,0 +1,30 @@
+//@ compile-flags: -O
+
+#![crate_type = "lib"]
+
+// Ensure that the simple case of rotating by a constant 1 optimizes to the obvious thing
+
+// CHECK-LABEL: @rotate_left_by_one
+#[no_mangle]
+pub fn rotate_left_by_one(slice: &mut [i32]) {
+    // CHECK-NOT: phi
+    // CHECK-NOT: call
+    // CHECK-NOT: load
+    // CHECK-NOT: store
+    // CHECK-NOT: getelementptr
+    // CHECK: %[[END:.+]] = getelementptr
+    // CHECK-NEXT: %[[DIM:.+]] = getelementptr
+    // CHECK-NEXT: %[[LAST:.+]] = load
+    // CHECK-NEXT: %[[FIRST:.+]] = shl
+    // CHECK-NEXT: call void @llvm.memmove
+    // CHECK-NEXT: store i32 %[[LAST]], ptr %[[DIM:.+]]
+    // CHECK-NOT: phi
+    // CHECK-NOT: call
+    // CHECK-NOT: load
+    // CHECK-NOT: store
+    // CHECK-NOT: getelementptr
+    // CHECK: ret void
+    if !slice.is_empty() {
+        slice.rotate_left(1);
+    }
+}