Auto merge of #40601 - stjepang:sort-unstable, r=alexcrichton

Implement feature sort_unstable

Tracking issue for the feature: #40585

This is essentially integration of [pdqsort](https://github.com/stjepang/pdqsort) into libcore.

There's plenty of unsafe blocks to review. The heart of pdqsort is `fn partition_in_blocks` and is probably the most challenging function to understand. It requires some patience, but let me know if you find it too difficult - comments could always be improved.

#### Changes

* Added `sort_unstable` feature.
* Tweaked insertion sort constants for stable sort. Sorting integers is now up to 5% slower, but sorting big elements is much faster (in particular, `sort_large_big_random` is 35% faster). The old constants were highly optimized for sorting integers, so overall the configuration is more balanced now. A minor regression in case of integers is forgivable as we recently had performance improvements (#39538) that completely make up for it.
* Removed some uninteresting sort benchmarks.
* Added a new sort benchmark for string sorting.

#### Benchmarks

The following table compares stable and unstable sorting:
```
name                                 stable ns/iter        unstable ns/iter     diff ns/iter   diff %
slice::sort_large_ascending          7,240 (11049 MB/s)    7,380 (10840 MB/s)            140    1.93%
slice::sort_large_big_random         1,454,138 (880 MB/s)  910,269 (1406 MB/s)      -543,869  -37.40%
slice::sort_large_descending         13,450 (5947 MB/s)    10,895 (7342 MB/s)         -2,555  -19.00%
slice::sort_large_mostly_ascending   204,041 (392 MB/s)    88,639 (902 MB/s)        -115,402  -56.56%
slice::sort_large_mostly_descending  217,109 (368 MB/s)    99,009 (808 MB/s)        -118,100  -54.40%
slice::sort_large_random             477,257 (167 MB/s)    346,028 (231 MB/s)       -131,229  -27.50%
slice::sort_large_random_expensive   21,670,537 (3 MB/s)   22,710,238 (3 MB/s)     1,039,701    4.80%
slice::sort_large_strings            6,284,499 (38 MB/s)   6,410,896 (37 MB/s)       126,397    2.01%
slice::sort_medium_random            3,515 (227 MB/s)      3,327 (240 MB/s)             -188   -5.35%
slice::sort_small_ascending          42 (1904 MB/s)        41 (1951 MB/s)                 -1   -2.38%
slice::sort_small_big_random         503 (2544 MB/s)       514 (2490 MB/s)                11    2.19%
slice::sort_small_descending         72 (1111 MB/s)        69 (1159 MB/s)                 -3   -4.17%
slice::sort_small_random             369 (216 MB/s)        367 (217 MB/s)                 -2   -0.54%
```

Interesting cases:
* Expensive comparison function and string sorting - it's a really close race, but timsort performs a slightly smaller number of comparisons. This is a natural difference of bottom-up merging versus top-down partitioning.
* `large_descending` - unstable sort is faster, but both sorts should have equivalent performance. Both just check whether the slice is descending and if so, they reverse it. I blame LLVM for the discrepancy.

r? @alexcrichton

Author: bors

src/libcollections/benches/lib.rs

@@ -11,6 +11,7 @@
 #![deny(warnings)]
 
 #![feature(rand)]
+#![feature(sort_unstable)]
 #![feature(test)]
 
 extern crate test;

src/libcollections/benches/slice.rs

@@ -169,6 +169,7 @@ fn random_inserts(b: &mut Bencher) {
         }
     })
 }
+
 #[bench]
 fn random_removes(b: &mut Bencher) {
     let mut rng = thread_rng();
@@ -216,65 +217,76 @@ fn gen_mostly_descending(len: usize) -> Vec<u64> {
     v
 }
 
-fn gen_big_random(len: usize) -> Vec<[u64; 16]> {
+fn gen_strings(len: usize) -> Vec<String> {
     let mut rng = thread_rng();
-    rng.gen_iter().map(|x| [x; 16]).take(len).collect()
-}
-
-fn gen_big_ascending(len: usize) -> Vec<[u64; 16]> {
-    (0..len as u64).map(|x| [x; 16]).take(len).collect()
+    let mut v = vec![];
+    for _ in 0..len {
+        let n = rng.gen::<usize>() % 20 + 1;
+        v.push(rng.gen_ascii_chars().take(n).collect());
+    }
+    v
 }
 
-fn gen_big_descending(len: usize) -> Vec<[u64; 16]> {
-    (0..len as u64).rev().map(|x| [x; 16]).take(len).collect()
+fn gen_big_random(len: usize) -> Vec<[u64; 16]> {
+    let mut rng = thread_rng();
+    rng.gen_iter().map(|x| [x; 16]).take(len).collect()
 }
 
-macro_rules! sort_bench {
-    ($name:ident, $gen:expr, $len:expr) => {
+macro_rules! sort {
+    ($f:ident, $name:ident, $gen:expr, $len:expr) => {
         #[bench]
         fn $name(b: &mut Bencher) {
-            b.iter(|| $gen($len).sort());
+            b.iter(|| $gen($len).$f());
             b.bytes = $len * mem::size_of_val(&$gen(1)[0]) as u64;
         }
     }
 }
 
-sort_bench!(sort_small_random, gen_random, 10);
-sort_bench!(sort_small_ascending, gen_ascending, 10);
-sort_bench!(sort_small_descending, gen_descending, 10);
-
-sort_bench!(sort_small_big_random, gen_big_random, 10);
-sort_bench!(sort_small_big_ascending, gen_big_ascending, 10);
-sort_bench!(sort_small_big_descending, gen_big_descending, 10);
-
-sort_bench!(sort_medium_random, gen_random, 100);
-sort_bench!(sort_medium_ascending, gen_ascending, 100);
-sort_bench!(sort_medium_descending, gen_descending, 100);
-
-sort_bench!(sort_large_random, gen_random, 10000);
-sort_bench!(sort_large_ascending, gen_ascending, 10000);
-sort_bench!(sort_large_descending, gen_descending, 10000);
-sort_bench!(sort_large_mostly_ascending, gen_mostly_ascending, 10000);
-sort_bench!(sort_large_mostly_descending, gen_mostly_descending, 10000);
-
-sort_bench!(sort_large_big_random, gen_big_random, 10000);
-sort_bench!(sort_large_big_ascending, gen_big_ascending, 10000);
-sort_bench!(sort_large_big_descending, gen_big_descending, 10000);
+macro_rules! sort_expensive {
+    ($f:ident, $name:ident, $gen:expr, $len:expr) => {
+        #[bench]
+        fn $name(b: &mut Bencher) {
+            b.iter(|| {
+                let mut v = $gen($len);
+                let mut count = 0;
+                v.$f(|a: &u64, b: &u64| {
+                    count += 1;
+                    if count % 1_000_000_000 == 0 {
+                        panic!("should not happen");
+                    }
+                    (*a as f64).cos().partial_cmp(&(*b as f64).cos()).unwrap()
+                });
+                black_box(count);
+            });
+            b.bytes = $len as u64 * mem::size_of::<u64>() as u64;
+        }
+    }
+}
 
-#[bench]
-fn sort_large_random_expensive(b: &mut Bencher) {
-    let len = 10000;
-    b.iter(|| {
-        let mut v = gen_random(len);
-        let mut count = 0;
-        v.sort_by(|a: &u64, b: &u64| {
-            count += 1;
-            if count % 1_000_000_000 == 0 {
-                panic!("should not happen");
-            }
-            (*a as f64).cos().partial_cmp(&(*b as f64).cos()).unwrap()
-        });
-        black_box(count);
-    });
-    b.bytes = len as u64 * mem::size_of::<u64>() as u64;
-}
+sort!(sort, sort_small_ascending, gen_ascending, 10);
+sort!(sort, sort_small_descending, gen_descending, 10);
+sort!(sort, sort_small_random, gen_random, 10);
+sort!(sort, sort_small_big_random, gen_big_random, 10);
+sort!(sort, sort_medium_random, gen_random, 100);
+sort!(sort, sort_large_ascending, gen_ascending, 10000);
+sort!(sort, sort_large_descending, gen_descending, 10000);
+sort!(sort, sort_large_mostly_ascending, gen_mostly_ascending, 10000);
+sort!(sort, sort_large_mostly_descending, gen_mostly_descending, 10000);
+sort!(sort, sort_large_random, gen_random, 10000);
+sort!(sort, sort_large_big_random, gen_big_random, 10000);
+sort!(sort, sort_large_strings, gen_strings, 10000);
+sort_expensive!(sort_by, sort_large_random_expensive, gen_random, 10000);
+
+sort!(sort_unstable, sort_unstable_small_ascending, gen_ascending, 10);
+sort!(sort_unstable, sort_unstable_small_descending, gen_descending, 10);
+sort!(sort_unstable, sort_unstable_small_random, gen_random, 10);
+sort!(sort_unstable, sort_unstable_small_big_random, gen_big_random, 10);
+sort!(sort_unstable, sort_unstable_medium_random, gen_random, 100);
+sort!(sort_unstable, sort_unstable_large_ascending, gen_ascending, 10000);
+sort!(sort_unstable, sort_unstable_large_descending, gen_descending, 10000);
+sort!(sort_unstable, sort_unstable_large_mostly_ascending, gen_mostly_ascending, 10000);
+sort!(sort_unstable, sort_unstable_large_mostly_descending, gen_mostly_descending, 10000);
+sort!(sort_unstable, sort_unstable_large_random, gen_random, 10000);
+sort!(sort_unstable, sort_unstable_large_big_random, gen_big_random, 10000);
+sort!(sort_unstable, sort_unstable_large_strings, gen_strings, 10000);
+sort_expensive!(sort_unstable_by, sort_unstable_large_random_expensive, gen_random, 10000);

src/libcollections/lib.rs

@@ -52,6 +52,7 @@
 #![feature(shared)]
 #![feature(slice_get_slice)]
 #![feature(slice_patterns)]
+#![cfg_attr(not(test), feature(sort_unstable))]
 #![feature(specialization)]
 #![feature(staged_api)]
 #![feature(str_internals)]

src/libcollections/slice.rs

@@ -1092,6 +1092,39 @@ impl<T> [T] {
         merge_sort(self, |a, b| a.lt(b));
     }
 
+    /// Sorts the slice using `compare` to compare elements.
+    ///
+    /// This sort is stable (i.e. does not reorder equal elements) and `O(n log n)` worst-case.
+    ///
+    /// # Current implementation
+    ///
+    /// The current algorithm is an adaptive, iterative merge sort inspired by
+    /// [timsort](https://en.wikipedia.org/wiki/Timsort).
+    /// It is designed to be very fast in cases where the slice is nearly sorted, or consists of
+    /// two or more sorted sequences concatenated one after another.
+    ///
+    /// Also, it allocates temporary storage half the size of `self`, but for short slices a
+    /// non-allocating insertion sort is used instead.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut v = [5, 4, 1, 3, 2];
+    /// v.sort_by(|a, b| a.cmp(b));
+    /// assert!(v == [1, 2, 3, 4, 5]);
+    ///
+    /// // reverse sorting
+    /// v.sort_by(|a, b| b.cmp(a));
+    /// assert!(v == [5, 4, 3, 2, 1]);
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[inline]
+    pub fn sort_by<F>(&mut self, mut compare: F)
+        where F: FnMut(&T, &T) -> Ordering
+    {
+        merge_sort(self, |a, b| compare(a, b) == Less);
+    }
+
     /// Sorts the slice using `f` to extract a key to compare elements by.
     ///
     /// This sort is stable (i.e. does not reorder equal elements) and `O(n log n)` worst-case.
@@ -1122,37 +1155,118 @@ impl<T> [T] {
         merge_sort(self, |a, b| f(a).lt(&f(b)));
     }
 
-    /// Sorts the slice using `compare` to compare elements.
+    /// Sorts the slice, but may not preserve the order of equal elements.
     ///
-    /// This sort is stable (i.e. does not reorder equal elements) and `O(n log n)` worst-case.
+    /// This sort is unstable (i.e. may reorder equal elements), in-place (i.e. does not allocate),
+    /// and `O(n log n)` worst-case.
     ///
     /// # Current implementation
     ///
-    /// The current algorithm is an adaptive, iterative merge sort inspired by
-    /// [timsort](https://en.wikipedia.org/wiki/Timsort).
-    /// It is designed to be very fast in cases where the slice is nearly sorted, or consists of
-    /// two or more sorted sequences concatenated one after another.
+    /// The current algorithm is based on Orson Peters' [pdqsort][pattern-defeating quicksort],
+    /// which is a quicksort variant designed to be very fast on certain kinds of patterns,
+    /// sometimes achieving linear time. It is randomized but deterministic, and falls back to
+    /// heapsort on degenerate inputs.
     ///
-    /// Also, it allocates temporary storage half the size of `self`, but for short slices a
-    /// non-allocating insertion sort is used instead.
+    /// It is generally faster than stable sorting, except in a few special cases, e.g. when the
+    /// slice consists of several concatenated sorted sequences.
     ///
     /// # Examples
     ///
     /// ```
+    /// #![feature(sort_unstable)]
+    ///
+    /// let mut v = [-5, 4, 1, -3, 2];
+    ///
+    /// v.sort_unstable();
+    /// assert!(v == [-5, -3, 1, 2, 4]);
+    /// ```
+    ///
+    /// [pdqsort]: https://github.com/orlp/pdqsort
+    // FIXME #40585: Mention `sort_unstable` in the documentation for `sort`.
+    #[unstable(feature = "sort_unstable", issue = "40585")]
+    #[inline]
+    pub fn sort_unstable(&mut self)
+        where T: Ord
+    {
+        core_slice::SliceExt::sort_unstable(self);
+    }
+
+    /// Sorts the slice using `compare` to compare elements, but may not preserve the order of
+    /// equal elements.
+    ///
+    /// This sort is unstable (i.e. may reorder equal elements), in-place (i.e. does not allocate),
+    /// and `O(n log n)` worst-case.
+    ///
+    /// # Current implementation
+    ///
+    /// The current algorithm is based on Orson Peters' [pdqsort][pattern-defeating quicksort],
+    /// which is a quicksort variant designed to be very fast on certain kinds of patterns,
+    /// sometimes achieving linear time. It is randomized but deterministic, and falls back to
+    /// heapsort on degenerate inputs.
+    ///
+    /// It is generally faster than stable sorting, except in a few special cases, e.g. when the
+    /// slice consists of several concatenated sorted sequences.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(sort_unstable)]
+    ///
     /// let mut v = [5, 4, 1, 3, 2];
-    /// v.sort_by(|a, b| a.cmp(b));
+    /// v.sort_unstable_by(|a, b| a.cmp(b));
     /// assert!(v == [1, 2, 3, 4, 5]);
     ///
     /// // reverse sorting
-    /// v.sort_by(|a, b| b.cmp(a));
+    /// v.sort_unstable_by(|a, b| b.cmp(a));
     /// assert!(v == [5, 4, 3, 2, 1]);
     /// ```
-    #[stable(feature = "rust1", since = "1.0.0")]
+    ///
+    /// [pdqsort]: https://github.com/orlp/pdqsort
+    // FIXME #40585: Mention `sort_unstable_by` in the documentation for `sort_by`.
+    #[unstable(feature = "sort_unstable", issue = "40585")]
     #[inline]
-    pub fn sort_by<F>(&mut self, mut compare: F)
+    pub fn sort_unstable_by<F>(&mut self, compare: F)
         where F: FnMut(&T, &T) -> Ordering
     {
-        merge_sort(self, |a, b| compare(a, b) == Less);
+        core_slice::SliceExt::sort_unstable_by(self, compare);
+    }
+
+    /// Sorts the slice using `f` to extract a key to compare elements by, but may not preserve the
+    /// order of equal elements.
+    ///
+    /// This sort is unstable (i.e. may reorder equal elements), in-place (i.e. does not allocate),
+    /// and `O(n log n)` worst-case.
+    ///
+    /// # Current implementation
+    ///
+    /// The current algorithm is based on Orson Peters' [pdqsort][pattern-defeating quicksort],
+    /// which is a quicksort variant designed to be very fast on certain kinds of patterns,
+    /// sometimes achieving linear time. It is randomized but deterministic, and falls back to
+    /// heapsort on degenerate inputs.
+    ///
+    /// It is generally faster than stable sorting, except in a few special cases, e.g. when the
+    /// slice consists of several concatenated sorted sequences.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(sort_unstable)]
+    ///
+    /// let mut v = [-5i32, 4, 1, -3, 2];
+    ///
+    /// v.sort_unstable_by_key(|k| k.abs());
+    /// assert!(v == [1, 2, -3, 4, -5]);
+    /// ```
+    ///
+    /// [pdqsort]: https://github.com/orlp/pdqsort
+    // FIXME #40585: Mention `sort_unstable_by_key` in the documentation for `sort_by_key`.
+    #[unstable(feature = "sort_unstable", issue = "40585")]
+    #[inline]
+    pub fn sort_unstable_by_key<B, F>(&mut self, f: F)
+        where F: FnMut(&T) -> B,
+              B: Ord
+    {
+        core_slice::SliceExt::sort_unstable_by_key(self, f);
     }
 
     /// Copies the elements from `src` into `self`.
@@ -1553,28 +1667,20 @@ unsafe fn merge<T, F>(v: &mut [T], mid: usize, buf: *mut T, is_less: &mut F)
 fn merge_sort<T, F>(v: &mut [T], mut is_less: F)
     where F: FnMut(&T, &T) -> bool
 {
+    // Slices of up to this length get sorted using insertion sort.
+    const MAX_INSERTION: usize = 20;
+    // Very short runs are extended using insertion sort to span at least this many elements.
+    const MIN_RUN: usize = 10;
+
     // Sorting has no meaningful behavior on zero-sized types.
     if size_of::<T>() == 0 {
         return;
     }
 
-    // FIXME #12092: These numbers are platform-specific and need more extensive testing/tuning.
-    //
-    // If `v` has length up to `max_insertion`, simply switch to insertion sort because it is going
-    // to perform better than merge sort. For bigger types `T`, the threshold is smaller.
-    //
-    // Short runs are extended using insertion sort to span at least `min_run` elements, in order
-    // to improve performance.
-    let (max_insertion, min_run) = if size_of::<T>() <= 2 * mem::size_of::<usize>() {
-        (64, 32)
-    } else {
-        (32, 16)
-    };
-
     let len = v.len();
 
     // Short arrays get sorted in-place via insertion sort to avoid allocations.
-    if len <= max_insertion {
+    if len <= MAX_INSERTION {
         if len >= 2 {
             for i in (0..len-1).rev() {
                 insert_head(&mut v[i..], &mut is_less);
@@ -1618,7 +1724,7 @@ fn merge_sort<T, F>(v: &mut [T], mut is_less: F)
 
         // Insert some more elements into the run if it's too short. Insertion sort is faster than
         // merge sort on short sequences, so this significantly improves performance.
-        while start > 0 && end - start < min_run {
+        while start > 0 && end - start < MIN_RUN {
             start -= 1;
             insert_head(&mut v[start..end], &mut is_less);
         }

src/libcollectionstest/slice.rs

@@ -399,9 +399,10 @@ fn test_sort() {
         }
     }
 
-    // shouldn't panic
-    let mut v: [i32; 0] = [];
-    v.sort();
+    // Should not panic.
+    [0i32; 0].sort();
+    [(); 10].sort();
+    [(); 100].sort();
 
     let mut v = [0xDEADBEEFu64];
     v.sort();
@@ -441,13 +442,6 @@ fn test_sort_stability() {
     }
 }
 
-#[test]
-fn test_sort_zero_sized_type() {
-    // Should not panic.
-    [(); 10].sort();
-    [(); 100].sort();
-}
-
 #[test]
 fn test_concat() {
     let v: [Vec<i32>; 0] = [];

src/libcore/lib.rs

@@ -71,26 +71,27 @@
 #![feature(asm)]
 #![feature(associated_type_defaults)]
 #![feature(cfg_target_feature)]
+#![feature(cfg_target_has_atomic)]
 #![feature(concat_idents)]
 #![feature(const_fn)]
-#![feature(cfg_target_has_atomic)]
 #![feature(custom_attribute)]
 #![feature(fundamental)]
+#![feature(i128_type)]
 #![feature(inclusive_range_syntax)]
 #![feature(intrinsics)]
 #![feature(lang_items)]
+#![feature(never_type)]
 #![feature(no_core)]
 #![feature(on_unimplemented)]
 #![feature(optin_builtin_traits)]
-#![feature(unwind_attributes)]
+#![feature(prelude_import)]
 #![feature(repr_simd, platform_intrinsics)]
 #![feature(rustc_attrs)]
 #![feature(specialization)]
 #![feature(staged_api)]
 #![feature(unboxed_closures)]
-#![feature(never_type)]
-#![feature(i128_type)]
-#![feature(prelude_import)]
+#![feature(untagged_unions)]
+#![feature(unwind_attributes)]
 
 #[prelude_import]
 #[allow(unused)]

src/libcore/slice.rs renamed to src/libcore/slice/mod.rs

@@ -1,4 +1,4 @@
-// Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2012-2017 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -51,6 +51,8 @@ use mem;
 use marker::{Copy, Send, Sync, Sized, self};
 use iter_private::TrustedRandomAccess;
 
+mod sort;
+
 #[repr(C)]
 struct Repr<T> {
     pub data: *const T,
@@ -71,86 +73,119 @@ pub trait SliceExt {
 
     #[stable(feature = "core", since = "1.6.0")]
     fn split_at(&self, mid: usize) -> (&[Self::Item], &[Self::Item]);
+
     #[stable(feature = "core", since = "1.6.0")]
     fn iter(&self) -> Iter<Self::Item>;
+
     #[stable(feature = "core", since = "1.6.0")]
     fn split<P>(&self, pred: P) -> Split<Self::Item, P>
-                    where P: FnMut(&Self::Item) -> bool;
+        where P: FnMut(&Self::Item) -> bool;
+
     #[stable(feature = "core", since = "1.6.0")]
     fn splitn<P>(&self, n: usize, pred: P) -> SplitN<Self::Item, P>
-                     where P: FnMut(&Self::Item) -> bool;
+        where P: FnMut(&Self::Item) -> bool;
+
     #[stable(feature = "core", since = "1.6.0")]
     fn rsplitn<P>(&self,  n: usize, pred: P) -> RSplitN<Self::Item, P>
-                      where P: FnMut(&Self::Item) -> bool;
+        where P: FnMut(&Self::Item) -> bool;
+
     #[stable(feature = "core", since = "1.6.0")]
     fn windows(&self, size: usize) -> Windows<Self::Item>;
+
     #[stable(feature = "core", since = "1.6.0")]
     fn chunks(&self, size: usize) -> Chunks<Self::Item>;
+
     #[stable(feature = "core", since = "1.6.0")]
     fn get<I>(&self, index: I) -> Option<&I::Output>
         where I: SliceIndex<Self::Item>;
+
     #[stable(feature = "core", since = "1.6.0")]
     fn first(&self) -> Option<&Self::Item>;
+
     #[stable(feature = "core", since = "1.6.0")]
     fn split_first(&self) -> Option<(&Self::Item, &[Self::Item])>;
+
     #[stable(feature = "core", since = "1.6.0")]
     fn split_last(&self) -> Option<(&Self::Item, &[Self::Item])>;
+
     #[stable(feature = "core", since = "1.6.0")]
     fn last(&self) -> Option<&Self::Item>;
+
     #[stable(feature = "core", since = "1.6.0")]
     unsafe fn get_unchecked<I>(&self, index: I) -> &I::Output
         where I: SliceIndex<Self::Item>;
+
     #[stable(feature = "core", since = "1.6.0")]
     fn as_ptr(&self) -> *const Self::Item;
+
     #[stable(feature = "core", since = "1.6.0")]
     fn binary_search<Q: ?Sized>(&self, x: &Q) -> Result<usize, usize>
         where Self::Item: Borrow<Q>,
               Q: Ord;
+
     #[stable(feature = "core", since = "1.6.0")]
     fn binary_search_by<'a, F>(&'a self, f: F) -> Result<usize, usize>
         where F: FnMut(&'a Self::Item) -> Ordering;
+
     #[stable(feature = "slice_binary_search_by_key", since = "1.10.0")]
     fn binary_search_by_key<'a, B, F, Q: ?Sized>(&'a self, b: &Q, f: F) -> Result<usize, usize>
         where F: FnMut(&'a Self::Item) -> B,
               B: Borrow<Q>,
               Q: Ord;
+
     #[stable(feature = "core", since = "1.6.0")]
     fn len(&self) -> usize;
+
     #[stable(feature = "core", since = "1.6.0")]
     fn is_empty(&self) -> bool { self.len() == 0 }
+
     #[stable(feature = "core", since = "1.6.0")]
     fn get_mut<I>(&mut self, index: I) -> Option<&mut I::Output>
         where I: SliceIndex<Self::Item>;
+
     #[stable(feature = "core", since = "1.6.0")]
     fn iter_mut(&mut self) -> IterMut<Self::Item>;
+
     #[stable(feature = "core", since = "1.6.0")]
     fn first_mut(&mut self) -> Option<&mut Self::Item>;
+
     #[stable(feature = "core", since = "1.6.0")]
     fn split_first_mut(&mut self) -> Option<(&mut Self::Item, &mut [Self::Item])>;
+
     #[stable(feature = "core", since = "1.6.0")]
     fn split_last_mut(&mut self) -> Option<(&mut Self::Item, &mut [Self::Item])>;
+
     #[stable(feature = "core", since = "1.6.0")]
     fn last_mut(&mut self) -> Option<&mut Self::Item>;
+
     #[stable(feature = "core", since = "1.6.0")]
     fn split_mut<P>(&mut self, pred: P) -> SplitMut<Self::Item, P>
-                        where P: FnMut(&Self::Item) -> bool;
+        where P: FnMut(&Self::Item) -> bool;
+
     #[stable(feature = "core", since = "1.6.0")]
     fn splitn_mut<P>(&mut self, n: usize, pred: P) -> SplitNMut<Self::Item, P>
-                     where P: FnMut(&Self::Item) -> bool;
+        where P: FnMut(&Self::Item) -> bool;
+
     #[stable(feature = "core", since = "1.6.0")]
     fn rsplitn_mut<P>(&mut self,  n: usize, pred: P) -> RSplitNMut<Self::Item, P>
-                      where P: FnMut(&Self::Item) -> bool;
+        where P: FnMut(&Self::Item) -> bool;
+
     #[stable(feature = "core", since = "1.6.0")]
     fn chunks_mut(&mut self, chunk_size: usize) -> ChunksMut<Self::Item>;
+
     #[stable(feature = "core", since = "1.6.0")]
     fn swap(&mut self, a: usize, b: usize);
+
     #[stable(feature = "core", since = "1.6.0")]
     fn split_at_mut(&mut self, mid: usize) -> (&mut [Self::Item], &mut [Self::Item]);
+
     #[stable(feature = "core", since = "1.6.0")]
     fn reverse(&mut self);
+
     #[stable(feature = "core", since = "1.6.0")]
     unsafe fn get_unchecked_mut<I>(&mut self, index: I) -> &mut I::Output
         where I: SliceIndex<Self::Item>;
+
     #[stable(feature = "core", since = "1.6.0")]
     fn as_mut_ptr(&mut self) -> *mut Self::Item;
 
@@ -165,8 +200,22 @@ pub trait SliceExt {
 
     #[stable(feature = "clone_from_slice", since = "1.7.0")]
     fn clone_from_slice(&mut self, src: &[Self::Item]) where Self::Item: Clone;
+
     #[stable(feature = "copy_from_slice", since = "1.9.0")]
     fn copy_from_slice(&mut self, src: &[Self::Item]) where Self::Item: Copy;
+
+    #[unstable(feature = "sort_unstable", issue = "40585")]
+    fn sort_unstable(&mut self)
+        where Self::Item: Ord;
+
+    #[unstable(feature = "sort_unstable", issue = "40585")]
+    fn sort_unstable_by<F>(&mut self, compare: F)
+        where F: FnMut(&Self::Item, &Self::Item) -> Ordering;
+
+    #[unstable(feature = "sort_unstable", issue = "40585")]
+    fn sort_unstable_by_key<B, F>(&mut self, f: F)
+        where F: FnMut(&Self::Item) -> B,
+              B: Ord;
 }
 
 // Use macros to be generic over const/mut
@@ -238,7 +287,9 @@ impl<T> SliceExt for [T] {
     }
 
     #[inline]
-    fn split<P>(&self, pred: P) -> Split<T, P> where P: FnMut(&T) -> bool {
+    fn split<P>(&self, pred: P) -> Split<T, P>
+        where P: FnMut(&T) -> bool
+    {
         Split {
             v: self,
             pred: pred,
@@ -247,8 +298,8 @@ impl<T> SliceExt for [T] {
     }
 
     #[inline]
-    fn splitn<P>(&self, n: usize, pred: P) -> SplitN<T, P> where
-        P: FnMut(&T) -> bool,
+    fn splitn<P>(&self, n: usize, pred: P) -> SplitN<T, P>
+        where P: FnMut(&T) -> bool
     {
         SplitN {
             inner: GenericSplitN {
@@ -260,8 +311,8 @@ impl<T> SliceExt for [T] {
     }
 
     #[inline]
-    fn rsplitn<P>(&self, n: usize, pred: P) -> RSplitN<T, P> where
-        P: FnMut(&T) -> bool,
+    fn rsplitn<P>(&self, n: usize, pred: P) -> RSplitN<T, P>
+        where P: FnMut(&T) -> bool
     {
         RSplitN {
             inner: GenericSplitN {
@@ -422,13 +473,15 @@ impl<T> SliceExt for [T] {
     }
 
     #[inline]
-    fn split_mut<P>(&mut self, pred: P) -> SplitMut<T, P> where P: FnMut(&T) -> bool {
+    fn split_mut<P>(&mut self, pred: P) -> SplitMut<T, P>
+        where P: FnMut(&T) -> bool
+    {
         SplitMut { v: self, pred: pred, finished: false }
     }
 
     #[inline]
-    fn splitn_mut<P>(&mut self, n: usize, pred: P) -> SplitNMut<T, P> where
-        P: FnMut(&T) -> bool
+    fn splitn_mut<P>(&mut self, n: usize, pred: P) -> SplitNMut<T, P>
+        where P: FnMut(&T) -> bool
     {
         SplitNMut {
             inner: GenericSplitN {
@@ -450,7 +503,7 @@ impl<T> SliceExt for [T] {
                 invert: true
             }
         }
-   }
+    }
 
     #[inline]
     fn chunks_mut(&mut self, chunk_size: usize) -> ChunksMut<T> {
@@ -512,7 +565,10 @@ impl<T> SliceExt for [T] {
         m >= n && needle == &self[m-n..]
     }
 
-    fn binary_search<Q: ?Sized>(&self, x: &Q) -> Result<usize, usize> where T: Borrow<Q>, Q: Ord {
+    fn binary_search<Q: ?Sized>(&self, x: &Q) -> Result<usize, usize>
+        where T: Borrow<Q>,
+              Q: Ord
+    {
         self.binary_search_by(|p| p.borrow().cmp(x))
     }
 
@@ -548,6 +604,28 @@ impl<T> SliceExt for [T] {
     {
         self.binary_search_by(|k| f(k).borrow().cmp(b))
     }
+
+    #[inline]
+    fn sort_unstable(&mut self)
+        where Self::Item: Ord
+    {
+        sort::quicksort(self, |a, b| a.lt(b));
+    }
+
+    #[inline]
+    fn sort_unstable_by<F>(&mut self, mut compare: F)
+        where F: FnMut(&Self::Item, &Self::Item) -> Ordering
+    {
+        sort::quicksort(self, |a, b| compare(a, b) == Ordering::Less);
+    }
+
+    #[inline]
+    fn sort_unstable_by_key<B, F>(&mut self, mut f: F)
+        where F: FnMut(&Self::Item) -> B,
+              B: Ord
+    {
+        sort::quicksort(self, |a, b| f(a).lt(&f(b)));
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -2175,6 +2253,15 @@ pub unsafe fn from_raw_parts_mut<'a, T>(p: *mut T, len: usize) -> &'a mut [T] {
     mem::transmute(Repr { data: p, len: len })
 }
 
+// This function is public only because there is no other way to unit test heapsort.
+#[unstable(feature = "sort_internals", reason = "internal to sort module", issue = "0")]
+#[doc(hidden)]
+pub fn heapsort<T, F>(v: &mut [T], mut is_less: F)
+    where F: FnMut(&T, &T) -> bool
+{
+    sort::heapsort(v, &mut is_less);
+}
+
 //
 // Comparison traits
 //

src/libcore/slice/sort.rs

@@ -19,18 +19,20 @@
 #![feature(decode_utf8)]
 #![feature(fixed_size_array)]
 #![feature(flt2dec)]
+#![feature(fmt_internals)]
 #![feature(libc)]
 #![feature(nonzero)]
 #![feature(rand)]
 #![feature(raw)]
 #![feature(sip_hash_13)]
 #![feature(slice_patterns)]
+#![feature(sort_internals)]
+#![feature(sort_unstable)]
 #![feature(step_by)]
 #![feature(test)]
 #![feature(try_from)]
 #![feature(unicode)]
 #![feature(unique)]
-#![feature(fmt_internals)]
 
 extern crate core;
 extern crate test;

src/libcoretest/lib.rs

@@ -8,7 +8,9 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use core::slice::heapsort;
 use core::result::Result::{Ok, Err};
+use rand::{Rng, XorShiftRng};
 
 #[test]
 fn test_binary_search() {
@@ -139,9 +141,6 @@ fn test_chunks_mut_last() {
     assert_eq!(c2.last().unwrap()[0], 4);
 }
 
-
-
-
 #[test]
 fn test_windows_count() {
     let v: &[i32] = &[0, 1, 2, 3, 4, 5];
@@ -224,3 +223,57 @@ fn get_unchecked_mut_range() {
         assert_eq!(v.get_unchecked_mut(1..4), &mut [1, 2, 3][..]);
     }
 }
+
+#[test]
+fn sort_unstable() {
+    let mut v = [0; 600];
+    let mut tmp = [0; 600];
+    let mut rng = XorShiftRng::new_unseeded();
+
+    for len in (2..25).chain(500..510) {
+        let v = &mut v[0..len];
+        let tmp = &mut tmp[0..len];
+
+        for &modulus in &[5, 10, 100, 1000] {
+            for _ in 0..100 {
+                for i in 0..len {
+                    v[i] = rng.gen::<i32>() % modulus;
+                }
+
+                // Sort in default order.
+                tmp.copy_from_slice(v);
+                tmp.sort_unstable();
+                assert!(tmp.windows(2).all(|w| w[0] <= w[1]));
+
+                // Sort in ascending order.
+                tmp.copy_from_slice(v);
+                tmp.sort_unstable_by(|a, b| a.cmp(b));
+                assert!(tmp.windows(2).all(|w| w[0] <= w[1]));
+
+                // Sort in descending order.
+                tmp.copy_from_slice(v);
+                tmp.sort_unstable_by(|a, b| b.cmp(a));
+                assert!(tmp.windows(2).all(|w| w[0] >= w[1]));
+
+                // Test heapsort using `<` operator.
+                tmp.copy_from_slice(v);
+                heapsort(tmp, |a, b| a < b);
+                assert!(tmp.windows(2).all(|w| w[0] <= w[1]));
+
+                // Test heapsort using `>` operator.
+                tmp.copy_from_slice(v);
+                heapsort(tmp, |a, b| a > b);
+                assert!(tmp.windows(2).all(|w| w[0] >= w[1]));
+            }
+        }
+    }
+
+    // Should not panic.
+    [0i32; 0].sort_unstable();
+    [(); 10].sort_unstable();
+    [(); 100].sort_unstable();
+
+    let mut v = [0xDEADBEEFu64];
+    v.sort_unstable();
+    assert!(v == [0xDEADBEEF]);
+}