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

[Praveen Kumar <kpraveen.lkml@xxxxxxxxx>]

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.

commit 59633abf34e2f44b8e772a2c12a92132aa7c2220 from Linux tree

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>
---
 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 253861d889..b65f00ca1f 100644
--- a/xen/common/rbtree.c
+++ b/xen/common/rbtree.c
@@ -108,9 +108,9 @@ 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;
+        if (parent != tmp)    /* parent == gparent->rb_left */
         {
-            tmp = gparent->rb_right;
             if (tmp && rb_is_red(tmp))
             {
                 /*
@@ -134,7 +134,8 @@ 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
@@ -164,7 +165,7 @@ 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);
@@ -183,7 +184,8 @@ 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;
@@ -192,10 +194,11 @@ void rb_insert_color(struct rb_node *node, struct rb_root 
*root)
                     rb_set_parent_color(tmp, parent, 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);
@@ -228,8 +231,10 @@ 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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