[Xen-devel] [PATCH v5 12/17] rbtree: optimize fetching of sibling node

[Praveen Kumar <kpraveen.lkml@xxxxxxxxx>]

From: Michel Lespinasse <walken@xxxxxxxxxx>

When looking to fetch a node's sibling, we went through a sequence of:
- check if node is the parent's left child
- if it is, then fetch the parent's right child

This can be replaced with:
- fetch the parent's right child as an assumed sibling
- check that node is NOT the fetched child

This avoids fetching the parent's left child when node is actually
that child. Saves a bit on code size, though it doesn't seem to make
a large difference in speed.

Signed-off-by: Michel Lespinasse <walken@xxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: David Woodhouse <David.Woodhouse@xxxxxxxxx>
Acked-by: Rik van Riel <riel@xxxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Daniel Santos <daniel.santos@xxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
[Linux commit 59633abf34e2f44b8e772a2c12a92132aa7c2220]

Ported to Xen.

Signed-off-by: Praveen Kumar <kpraveen.lkml@xxxxxxxxx>
---
 xen/common/rbtree.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/xen/common/rbtree.c b/xen/common/rbtree.c
index 215d4f3613..d8f1194e20 100644
--- a/xen/common/rbtree.c
+++ b/xen/common/rbtree.c
@@ -107,8 +107,8 @@ void rb_insert_color(struct rb_node *node, struct rb_root 
*root)
 
                gparent = rb_red_parent(parent);
 
-               if (parent == gparent->rb_left) {
-                       tmp = gparent->rb_right;
+               tmp = gparent->rb_right;
+               if (parent != tmp) {    /* parent == gparent->rb_left */
                        if (tmp && rb_is_red(tmp)) {
                                /*
                                 * Case 1 - color flips
@@ -131,7 +131,8 @@ void rb_insert_color(struct rb_node *node, struct rb_root 
*root)
                                continue;
                        }
 
-                       if (parent->rb_right == node) {
+                       tmp = parent->rb_right;
+                       if (node == tmp) {
                                /*
                                 * Case 2 - left rotate at parent
                                 *
@@ -151,6 +152,7 @@ void rb_insert_color(struct rb_node *node, struct rb_root 
*root)
                                                            RB_BLACK);
                                rb_set_parent_color(parent, node, RB_RED);
                                parent = node;
+                               tmp = node->rb_right;
                        }
 
                        /*
@@ -162,7 +164,7 @@ void rb_insert_color(struct rb_node *node, struct rb_root 
*root)
                         *     /                 \
                         *    n                   U
                         */
-                       gparent->rb_left = tmp = parent->rb_right;
+                       gparent->rb_left = tmp;  /* == parent->rb_right */
                        parent->rb_right = gparent;
                        if (tmp)
                                rb_set_parent_color(tmp, gparent, RB_BLACK);
@@ -180,7 +182,8 @@ void rb_insert_color(struct rb_node *node, struct rb_root 
*root)
                                continue;
                        }
 
-                       if (parent->rb_left == node) {
+                       tmp = parent->rb_left;
+                       if (node == tmp) {
                                /* Case 2 - right rotate at parent */
                                parent->rb_left = tmp = node->rb_right;
                                node->rb_right = parent;
@@ -189,10 +192,11 @@ void rb_insert_color(struct rb_node *node, struct rb_root 
*root)
                                                            RB_BLACK);
                                rb_set_parent_color(parent, node, RB_RED);
                                parent = node;
+                               tmp = node->rb_left;
                        }
 
                        /* Case 3 - left rotate at gparent */
-                       gparent->rb_right = tmp = parent->rb_left;
+                       gparent->rb_right = tmp;  /* == parent->rb_left */
                        parent->rb_left = gparent;
                        if (tmp)
                                rb_set_parent_color(tmp, gparent, RB_BLACK);
@@ -223,8 +227,9 @@ static void __rb_erase_color(struct rb_node *node, struct 
rb_node *parent,
                        break;
                } else if (!parent) {
                        break;
-               } else if (parent->rb_left == node) {
-                       sibling = parent->rb_right;
+               }
+               sibling = parent->rb_right;
+               if (node != sibling) {  /* node == parent->rb_left */
                        if (rb_is_red(sibling)) {
                                /*
                                 * Case 1 - left rotate at parent
-- 
2.12.0


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