[PATCH 11/17] rbtree: low level optimizations in __rb_erase_color()

[Praveen Kumar <kpraveen.lkml@gmail.com>]

In __rb_erase_color(), we often already have pointers to the nodes being
rotated and/or know what their colors must be, so we can generate more
efficient code than the generic __rb_rotate_left() and __rb_rotate_right()
functions.

Also when the current node is red or when flipping the sibling's color,
the parent is already known so we can use the more efficient
rb_set_parent_color() function to set the desired color.

commit 6280d2356fd8ad0936a63c10dc1e6accf48d0c61 from linux tree

Signed-off-by: Michel Lespinasse <walken@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: David Woodhouse <David.Woodhouse@intel.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Daniel Santos <daniel.santos@pobox.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
 xen/common/rbtree.c | 197 ++++++++++++++++++++++++++++------------------------
 1 file changed, 107 insertions(+), 90 deletions(-)

diff --git a/xen/common/rbtree.c b/xen/common/rbtree.c
index 1e4cb1ed2c..253861d889 100644
--- a/xen/common/rbtree.c
+++ b/xen/common/rbtree.c
@@ -38,7 +38,8 @@
  *  5), then the longest possible path due to 4 is 2B.
  *
  *  We shall indicate color with case, where black nodes are uppercase and red
- *  nodes will be lowercase.
+ *  nodes will be lowercase. Unknown color nodes shall be drawn as red within
+ *  parentheses and have some accompanying text comment.
  */
 
 #define    RB_RED    0
@@ -47,17 +48,11 @@
 #define rb_color(r)     ((r)->__rb_parent_color & 1)
 #define rb_is_red(r)    (!rb_color(r))
 #define rb_is_black(r)  rb_color(r)
-#define rb_set_red(r)   do { (r)->__rb_parent_color &= ~1; } while (0)
-#define rb_set_black(r) do { (r)->__rb_parent_color |= 1; } while (0)
 
 static inline void rb_set_parent(struct rb_node *rb, struct rb_node *p)
 {
     rb->__rb_parent_color = rb_color(rb) | (unsigned long)p;
 }
-static inline void rb_set_color(struct rb_node *rb, int color)
-{
-    rb->__rb_parent_color = (rb->__rb_parent_color & ~1) | color;
-}
 
 static inline void rb_set_parent_color(struct rb_node *rb,
                                       struct rb_node *p, int color)
@@ -70,52 +65,6 @@ static inline struct rb_node *rb_red_parent(struct rb_node *red)
     return (struct rb_node *)red->__rb_parent_color;
 }
 
-static void __rb_rotate_left(struct rb_node *node, struct rb_root *root)
-{
-    struct rb_node *right = node->rb_right;
-    struct rb_node *parent = rb_parent(node);
-
-    if ((node->rb_right = right->rb_left))
-        rb_set_parent(right->rb_left, node);
-    right->rb_left = node;
-
-    rb_set_parent(right, parent);
-
-    if (parent)
-    {
-        if (node == parent->rb_left)
-            parent->rb_left = right;
-        else
-            parent->rb_right = right;
-    }
-    else
-        root->rb_node = right;
-    rb_set_parent(node, right);
-}
-
-static void __rb_rotate_right(struct rb_node *node, struct rb_root *root)
-{
-    struct rb_node *left = node->rb_left;
-    struct rb_node *parent = rb_parent(node);
-
-    if ((node->rb_left = left->rb_right))
-        rb_set_parent(left->rb_right, node);
-    left->rb_right = node;
-
-    rb_set_parent(left, parent);
-
-    if (parent)
-    {
-        if (node == parent->rb_right)
-            parent->rb_right = left;
-        else
-            parent->rb_left = left;
-    }
-    else
-        root->rb_node = left;
-    rb_set_parent(node, left);
-}
-
 /*
  * Helper function for rotations:
  * - old's parent and color get assigned to new
@@ -260,7 +209,7 @@ EXPORT_SYMBOL(rb_insert_color);
 static void __rb_erase_color(struct rb_node *node, struct rb_node *parent,
                              struct rb_root *root)
 {
-    struct rb_node *other;
+    struct rb_node *sibling, *tmp1, *tmp2;
 
     while (true)
     {
@@ -275,68 +224,136 @@ static void __rb_erase_color(struct rb_node *node, struct rb_node *parent,
          */
         if (node && rb_is_red(node))
         {
-            rb_set_black(node);
+            rb_set_parent_color(node, parent, RB_BLACK);
             break;
         } else if (!parent) {
             break;
         } else if (parent->rb_left == node) {
-            other = parent->rb_right;
-            if (rb_is_red(other))
-            {
-                rb_set_black(other);
-                rb_set_red(parent);
-                __rb_rotate_left(parent, root);
-                other = parent->rb_right;
+            sibling = parent->rb_right;
+            if (rb_is_red(sibling)) {
+                /*
+                 * Case 1 - left rotate at parent
+                 *
+                 *     P               S
+                 *    / \             / \
+                 *   N   s    -->    p   Sr
+                 *      / \         / \
+                 *     Sl  Sr      N   Sl
+                 */
+                parent->rb_right = tmp1 = sibling->rb_left;
+                sibling->rb_left = parent;
+                rb_set_parent_color(tmp1, parent, RB_BLACK);
+                __rb_rotate_set_parents(parent, sibling, root, RB_RED);
+                sibling = tmp1;
             }
-            if (!other->rb_right || rb_is_black(other->rb_right))
+            tmp1 = sibling->rb_right;
+            if (!tmp1 || rb_is_black(tmp1))
             {
-                if (!other->rb_left || rb_is_black(other->rb_left))
+                tmp2 = sibling->rb_left;
+                if (!tmp2 || rb_is_black(tmp2))
                 {
-                    rb_set_red(other);
+                    /*
+                     * Case 2 - sibling color flip
+                     * (p could be either color here)
+                     *
+                     *    (p)           (p)
+                     *    / \           / \
+                     *   N   S    -->  N   s
+                     *      / \           / \
+                     *     Sl  Sr        Sl  Sr
+                     *
+                     * This leaves us violating 5), so
+                     * recurse at p. If p is red, the
+                     * recursion will just flip it to black
+                     * and exit. If coming from Case 1,
+                     * p is known to be red.
+                     */
+                    rb_set_parent_color(sibling, parent, RB_RED);
                     node = parent;
                     parent = rb_parent(node);
                     continue;
 
                 }
-                rb_set_black(other->rb_left);
-                rb_set_red(other);
-                __rb_rotate_right(other, root);
-                other = parent->rb_right;
+                /*
+                 * Case 3 - right rotate at sibling
+                 * (p could be either color here)
+                 *
+                 *   (p)           (p)
+                 *   / \           / \
+                 *  N   S    -->  N   Sl
+                 *     / \             \
+                 *    sl  Sr            s
+                 *                       \
+                 *                        Sr
+                 */
+                sibling->rb_left = tmp1 = tmp2->rb_right;
+                tmp2->rb_right = sibling;
+                parent->rb_right = tmp2;
+                if (tmp1)
+                    rb_set_parent_color(tmp1, sibling, RB_BLACK);
+                tmp1 = sibling;
+                sibling = tmp2;
             }
-            rb_set_color(other, rb_color(parent));
-            rb_set_black(parent);
-            rb_set_black(other->rb_right);
-            __rb_rotate_left(parent, root);
+            /*
+             * Case 4 - left rotate at parent + color flips
+             * (p and sl could be either color here.
+             *  After rotation, p becomes black, s acquires
+             *  p's color, and sl keeps its color)
+             *
+             *      (p)             (s)
+             *      / \             / \
+             *     N   S     -->   P   Sr
+             *        / \         / \
+             *      (sl) sr      N  (sl)
+             */
+            parent->rb_right = tmp2 = sibling->rb_left;
+            sibling->rb_left = parent;
+            rb_set_parent_color(tmp1, sibling, RB_BLACK);
+            if (tmp2)
+                rb_set_parent(tmp2, parent);
+            __rb_rotate_set_parents(parent, sibling, root, RB_BLACK);
             break;
         }
         else
         {
-            other = parent->rb_left;
-            if (rb_is_red(other))
+            sibling = parent->rb_left;
+            if (rb_is_red(sibling))
             {
-                rb_set_black(other);
-                rb_set_red(parent);
-                __rb_rotate_right(parent, root);
-                other = parent->rb_left;
+                /* Case 1 - right rotate at parent */
+                parent->rb_left = tmp1 = sibling->rb_right;
+                sibling->rb_right = parent;
+                rb_set_parent_color(tmp1, parent, RB_BLACK);
+                __rb_rotate_set_parents(parent, sibling, root, RB_RED);
+                sibling = tmp1;
             }
-            if (!other->rb_left || rb_is_black(other->rb_left))
+            tmp1 = sibling->rb_left;
+            if (!tmp1 || rb_is_black(tmp1))
             {
-                if (!other->rb_right || rb_is_black(other->rb_right))
+                tmp2 = sibling->rb_right;
+                if (!tmp2 || rb_is_black(tmp2))
                 {
-                    rb_set_red(other);
+                    /* Case 2 - sibling color flip */
+                    rb_set_parent_color(sibling, parent, RB_RED);
                     node = parent;
                     parent = rb_parent(node);
                     continue;
                 }
-                rb_set_black(other->rb_right);
-                rb_set_red(other);
-                __rb_rotate_left(other, root);
-                other = parent->rb_left;
+                /* Case 3 - right rotate at sibling */
+                sibling->rb_right = tmp1 = tmp2->rb_left;
+                tmp2->rb_left = sibling;
+                parent->rb_left = tmp2;
+                if (tmp1)
+                    rb_set_parent_color(tmp1, sibling, RB_BLACK);
+                tmp1 = sibling;
+                sibling = tmp2;
             }
-            rb_set_color(other, rb_color(parent));
-            rb_set_black(parent);
-            rb_set_black(other->rb_left);
-            __rb_rotate_right(parent, root);
+            /* Case 4 - left rotate at parent + color flips */
+            parent->rb_left = tmp2 = sibling->rb_right;
+            sibling->rb_right = parent;
+            rb_set_parent_color(tmp1, sibling, RB_BLACK);
+            if (tmp2)
+                rb_set_parent(tmp2, parent);
+            __rb_rotate_set_parents(parent, sibling, root, RB_BLACK);
             break;
         }
     }
-- 
2.12.0


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