Auto merge of #52553 - Pazzaz:vecdeque-append, r=SimonSapin

Non-naive implementation of `VecDeque.append`

Replaces the old, simple implementation with a more manual (and **unsafe** 😱) one. I've added 1 more test and verified that it covers all 6 code paths in the function.

This new implementation was about 60% faster than the old naive one when I tried benchmarking it.

1 files changed, 159 insertions, 2 deletions

diff --git a/src/liballoc/collections/vec_deque.rs b/src/liballoc/collections/vec_deque.rs
index 7787102ba82..0f759bb8f0b 100644
--- a/src/liballoc/collections/vec_deque.rs
+++ b/src/liballoc/collections/vec_deque.rs
@@ -202,6 +202,23 @@ impl<T> VecDeque<T> {
                                  len);
     }
 
+    /// Returns a pair of slices which contain the contents of the buffer not used by the VecDeque.
+    #[inline]
+    unsafe fn unused_as_mut_slices<'a>(&'a mut self) -> (&'a mut [T], &'a mut [T]) {
+        let head = self.head;
+        let tail = self.tail;
+        let buf = self.buffer_as_mut_slice();
+        if head != tail {
+            // In buf, head..tail contains the VecDeque and tail..head is unused.
+            // So calling `ring_slices` with tail and head swapped returns unused slices.
+            RingSlices::ring_slices(buf, tail, head)
+        } else {
+            // Swapping doesn't help when head == tail.
+            let (before, after) = buf.split_at_mut(head);
+            (after, before)
+        }
+    }
+
     /// Copies a potentially wrapping block of memory len long from src to dest.
     /// (abs(dst - src) + len) must be no larger than cap() (There must be at
     /// most one continuous overlapping region between src and dest).
@@ -1834,8 +1851,148 @@ impl<T> VecDeque<T> {
     #[inline]
     #[stable(feature = "append", since = "1.4.0")]
     pub fn append(&mut self, other: &mut Self) {
-        // naive impl
-        self.extend(other.drain(..));
+        // Copies all values from `src_slice` to the start of `dst_slice`.
+        unsafe fn copy_whole_slice<T>(src_slice: &[T], dst_slice: &mut [T]) {
+            let len = src_slice.len();
+            ptr::copy_nonoverlapping(src_slice.as_ptr(), dst_slice[..len].as_mut_ptr(), len);
+        }
+
+        let src_total = other.len();
+
+        // Guarantees there is space in `self` for `other`.
+        self.reserve(src_total);
+
+        self.head = {
+            let original_head = self.head;
+
+            // The goal is to copy all values from `other` into `self`. To avoid any
+            // mismatch, all valid values in `other` are retrieved...
+            let (src_high, src_low) = other.as_slices();
+            // and unoccupied parts of self are retrieved.
+            let (dst_high, dst_low) = unsafe { self.unused_as_mut_slices() };
+
+            // Then all that is needed is to copy all values from
+            // src (src_high and src_low) to dst (dst_high and dst_low).
+            //
+            // other [o o o . . . . . o o o o]
+            //       [5 6 7]         [1 2 3 4]
+            //       src_low         src_high
+            //
+            // self  [. . . . . . o o o o . .]
+            //       [3 4 5 6 7 .]       [1 2]
+            //       dst_low             dst_high
+            //
+            // Values are not copied one by one but as slices in `copy_whole_slice`.
+            // What slices are used depends on various properties of src and dst.
+            // There are 6 cases in total:
+            //     1. `src` is contiguous and fits in dst_high
+            //     2. `src` is contiguous and does not fit in dst_high
+            //     3. `src` is discontiguous and fits in dst_high
+            //     4. `src` is discontiguous and does not fit in dst_high
+            //        + src_high is smaller than dst_high
+            //     5. `src` is discontiguous and does not fit in dst_high
+            //        + dst_high is smaller than src_high
+            //     6. `src` is discontiguous and does not fit in dst_high
+            //        + dst_high is the same size as src_high
+            let src_contiguous = src_low.is_empty();
+            let dst_high_fits_src = dst_high.len() >= src_total;
+            match (src_contiguous, dst_high_fits_src) {
+                (true, true) => {
+                    // 1.
+                    // other [. . . o o o . . . . . .]
+                    //       []    [1 1 1]
+                    //
+                    // self  [. o o o o o . . . . . .]
+                    //       [.]         [1 1 1 . . .]
+
+                    unsafe {
+                        copy_whole_slice(src_high, dst_high);
+                    }
+                    original_head + src_total
+                }
+                (true, false) => {
+                    // 2.
+                    // other [. . . o o o o o . . . .]
+                    //       []    [1 1 2 2 2]
+                    //
+                    // self  [. . . . . . . o o o . .]
+                    //       [2 2 2 . . . .]     [1 1]
+
+                    let (src_1, src_2) = src_high.split_at(dst_high.len());
+                    unsafe {
+                        copy_whole_slice(src_1, dst_high);
+                        copy_whole_slice(src_2, dst_low);
+                    }
+                    src_total - dst_high.len()
+                }
+                (false, true) => {
+                    // 3.
+                    // other [o o . . . . . . . o o o]
+                    //       [2 2]             [1 1 1]
+                    //
+                    // self  [. o o . . . . . . . . .]
+                    //       [.]   [1 1 1 2 2 . . . .]
+
+                    let (dst_1, dst_2) = dst_high.split_at_mut(src_high.len());
+                    unsafe {
+                        copy_whole_slice(src_high, dst_1);
+                        copy_whole_slice(src_low, dst_2);
+                    }
+                    original_head + src_total
+                }
+                (false, false) => {
+                    if src_high.len() < dst_high.len() {
+                        // 4.
+                        // other [o o o . . . . . . o o o]
+                        //       [2 3 3]           [1 1 1]
+                        //
+                        // self  [. . . . . . o o . . . .]
+                        //       [3 3 . . . .]   [1 1 1 2]
+
+                        let (dst_1, dst_2) = dst_high.split_at_mut(src_high.len());
+                        let (src_2, src_3) = src_low.split_at(dst_2.len());
+                        unsafe {
+                            copy_whole_slice(src_high, dst_1);
+                            copy_whole_slice(src_2, dst_2);
+                            copy_whole_slice(src_3, dst_low);
+                        }
+                        src_3.len()
+                    } else if src_high.len() > dst_high.len() {
+                        // 5.
+                        // other [o o o . . . . . o o o o]
+                        //       [3 3 3]         [1 1 2 2]
+                        //
+                        // self  [. . . . . . o o o o . .]
+                        //       [2 2 3 3 3 .]       [1 1]
+
+                        let (src_1, src_2) = src_high.split_at(dst_high.len());
+                        let (dst_2, dst_3) = dst_low.split_at_mut(src_2.len());
+                        unsafe {
+                            copy_whole_slice(src_1, dst_high);
+                            copy_whole_slice(src_2, dst_2);
+                            copy_whole_slice(src_low, dst_3);
+                        }
+                        dst_2.len() + src_low.len()
+                    } else {
+                        // 6.
+                        // other [o o . . . . . . . o o o]
+                        //       [2 2]             [1 1 1]
+                        //
+                        // self  [. . . . . . . o o . . .]
+                        //       [2 2 . . . . .]   [1 1 1]
+
+                        unsafe {
+                            copy_whole_slice(src_high, dst_high);
+                            copy_whole_slice(src_low, dst_low);
+                        }
+                        src_low.len()
+                    }
+                }
+            }
+        };
+
+        // Some values now exist in both `other` and `self` but are made inaccessible in `other`.
+        other.tail = other.head;
     }
 
     /// Retains only the elements specified by the predicate.