[PATCH v4 12/17] rbtree: optimize fetching of sibling node

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

From: Michel Lespinasse <walken@google.com>

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@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: David Woodhouse <David.Woodhouse@intel.com>
Acked-by: 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>
[Linux commit 59633abf34e2f44b8e772a2c12a92132aa7c2220]

Ported to Xen.

Signed-off-by: Praveen Kumar <kpraveen.lkml@gmail.com>
---
 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 e24f4c4d39..56c815b4a5 100644
--- a/xen/common/rbtree.c
+++ b/xen/common/rbtree.c
@@ -106,8 +106,8 @@ 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
@@ -130,7 +130,8 @@ 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
 				 *
@@ -150,6 +151,7 @@ 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;
 			}
 
 			/*
@@ -161,7 +163,7 @@ 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);
@@ -179,7 +181,8 @@ 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;
@@ -188,10 +191,11 @@ 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);
@@ -222,8 +226,9 @@ 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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