From 6dfcf9afde98b4e873bfb0012e2efa0f058cc19d Mon Sep 17 00:00:00 2001
From: Giacomo Stevanato <giaco.stevanato@gmail.com>
Date: Sat, 7 Nov 2020 20:01:47 +0100
Subject: [PATCH 1/4] Remove branches from sift_down_to_bottom loop

---
 library/alloc/src/collections/binary_heap.rs | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/library/alloc/src/collections/binary_heap.rs b/library/alloc/src/collections/binary_heap.rs
index b67c72d7136..10ed99b2943 100644
--- a/library/alloc/src/collections/binary_heap.rs
+++ b/library/alloc/src/collections/binary_heap.rs
@@ -563,15 +563,14 @@ impl<T: Ord> BinaryHeap<T> {
         unsafe {
             let mut hole = Hole::new(&mut self.data, pos);
             let mut child = 2 * pos + 1;
-            while child < end {
-                let right = child + 1;
-                // compare with the greater of the two children
-                if right < end && hole.get(child) <= hole.get(right) {
-                    child = right;
-                }
+            while child < end - 1 {
+                child += (hole.get(child) <= hole.get(child + 1)) as usize;
                 hole.move_to(child);
                 child = 2 * hole.pos() + 1;
             }
+            if child == end - 1 {
+                hole.move_to(child);
+            }
             pos = hole.pos;
         }
         self.sift_up(start, pos);

From 25b3f61c3821fd597961f9dd394482b71600e859 Mon Sep 17 00:00:00 2001
From: Giacomo Stevanato <giaco.stevanato@gmail.com>
Date: Sat, 7 Nov 2020 21:27:30 +0100
Subject: [PATCH 2/4] Remove useless branches from sift_down_range loop

---
 library/alloc/src/collections/binary_heap.rs | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/library/alloc/src/collections/binary_heap.rs b/library/alloc/src/collections/binary_heap.rs
index 10ed99b2943..2d68146249b 100644
--- a/library/alloc/src/collections/binary_heap.rs
+++ b/library/alloc/src/collections/binary_heap.rs
@@ -531,19 +531,19 @@ impl<T: Ord> BinaryHeap<T> {
         unsafe {
             let mut hole = Hole::new(&mut self.data, pos);
             let mut child = 2 * pos + 1;
-            while child < end {
-                let right = child + 1;
+            while child < end - 1 {
                 // compare with the greater of the two children
-                if right < end && hole.get(child) <= hole.get(right) {
-                    child = right;
-                }
+                child += (hole.get(child) <= hole.get(child + 1)) as usize;
                 // if we are already in order, stop.
                 if hole.element() >= hole.get(child) {
-                    break;
+                    return;
                 }
                 hole.move_to(child);
                 child = 2 * hole.pos() + 1;
             }
+            if child == end - 1 && hole.element() < hole.get(child) {
+                hole.move_to(child);
+            }
         }
     }
 

From 8d1575365dd9c3260d0aa44203b9ad169f0b5d74 Mon Sep 17 00:00:00 2001
From: Giacomo Stevanato <giaco.stevanato@gmail.com>
Date: Sat, 7 Nov 2020 21:43:47 +0100
Subject: [PATCH 3/4] Remove useless bound checks from into_sorted_vec

---
 library/alloc/src/collections/binary_heap.rs | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/library/alloc/src/collections/binary_heap.rs b/library/alloc/src/collections/binary_heap.rs
index 2d68146249b..17f0668c0ea 100644
--- a/library/alloc/src/collections/binary_heap.rs
+++ b/library/alloc/src/collections/binary_heap.rs
@@ -495,7 +495,10 @@ impl<T: Ord> BinaryHeap<T> {
         let mut end = self.len();
         while end > 1 {
             end -= 1;
-            self.data.swap(0, end);
+            unsafe {
+                let ptr = self.data.as_mut_ptr();
+                ptr::swap(ptr, ptr.add(end));
+            }
             self.sift_down_range(0, end);
         }
         self.into_vec()

From 387568cd564317ca7491e6960ddcbe13beecae13 Mon Sep 17 00:00:00 2001
From: Giacomo Stevanato <giaco.stevanato@gmail.com>
Date: Mon, 9 Nov 2020 22:34:31 +0100
Subject: [PATCH 4/4] Added SAFETY comment as request

---
 library/alloc/src/collections/binary_heap.rs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/library/alloc/src/collections/binary_heap.rs b/library/alloc/src/collections/binary_heap.rs
index 17f0668c0ea..97ebc12175f 100644
--- a/library/alloc/src/collections/binary_heap.rs
+++ b/library/alloc/src/collections/binary_heap.rs
@@ -495,6 +495,10 @@ impl<T: Ord> BinaryHeap<T> {
         let mut end = self.len();
         while end > 1 {
             end -= 1;
+            // SAFETY: `end` goes from `self.len() - 1` to 1 (both included),
+            //  so it's always a valid index to access.
+            //  It is safe to access index 0 (i.e. `ptr`), because
+            //  1 <= end < self.len(), which means self.len() >= 2.
             unsafe {
                 let ptr = self.data.as_mut_ptr();
                 ptr::swap(ptr, ptr.add(end));