rbtree: optimize case selection logic in __rb_erase_color()

In __rb_erase_color(), we have to select one of 3 cases depending on the
color on the 'other' node children.  If both children are black, we flip a
few node colors and iterate.  Otherwise, we do either one or two tree
rotations, depending on the color of the 'other' child opposite to 'node',
and then we are done.

The corresponding logic had duplicate checks for the color of the 'other'
child opposite to 'node'.  It was checking it first to determine if both
children are black, and then to determine how many tree rotations are
required.  Rearrange the logic to avoid that extra check.

Signed-off-by: Michel Lespinasse <walken@google.com>
Acked-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[Linux commit e125d1471a4f8f1bf7ea9a83deb8d23cb40bd712]

Ported to Xen.

Signed-off-by: Praveen Kumar <kpraveen.lkml@gmail.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

diff --git a/xen/common/rbtree.c b/xen/common/rbtree.c
index 99a1305d3e52e239b8827d5f6b94a7e50547ad20..61a2f5e0b94e0d94316170dcc9d753e9910f21bd 100644 (file)
--- a/xen/common/rbtree.c
+++ b/xen/common/rbtree.c
@@ -283,28 +283,24 @@ static void __rb_erase_color(struct rb_node *node, struct rb_node *parent,
                                __rb_rotate_left(parent, root);
                                other = parent->rb_right;
                        }
-                       if ((!other->rb_left || rb_is_black(other->rb_left)) &&
-                           (!other->rb_right || rb_is_black(other->rb_right)))
-                       {
-                               rb_set_red(other);
-                               node = parent;
-                               parent = rb_parent(node);
-                       }
-                       else
-                       {
-                               if (!other->rb_right || rb_is_black(other->rb_right))
-                               {
-                                       rb_set_black(other->rb_left);
+                       if (!other->rb_right || rb_is_black(other->rb_right)) {
+                               if (!other->rb_left ||
+                                   rb_is_black(other->rb_left)) {
                                        rb_set_red(other);
-                                       __rb_rotate_right(other, root);
-                                       other = parent->rb_right;
+                                       node = parent;
+                                       parent = rb_parent(node);
+                                       continue;
                                }
-                               rb_set_color(other, rb_color(parent));
-                               rb_set_black(parent);
-                               rb_set_black(other->rb_right);
-                               __rb_rotate_left(parent, root);
-                               break;
+                               rb_set_black(other->rb_left);
+                               rb_set_red(other);
+                               __rb_rotate_right(other, root);
+                               other = parent->rb_right;
                        }
+                       rb_set_color(other, rb_color(parent));
+                       rb_set_black(parent);
+                       rb_set_black(other->rb_right);
+                       __rb_rotate_left(parent, root);
+                       break;
                } else {
                        other = parent->rb_left;
                        if (rb_is_red(other))
@@ -314,28 +310,24 @@ static void __rb_erase_color(struct rb_node *node, struct rb_node *parent,
                                __rb_rotate_right(parent, root);
                                other = parent->rb_left;
                        }
-                       if ((!other->rb_left || rb_is_black(other->rb_left)) &&
-                           (!other->rb_right || rb_is_black(other->rb_right)))
-                       {
-                               rb_set_red(other);
-                               node = parent;
-                               parent = rb_parent(node);
-                       }
-                       else
-                       {
-                               if (!other->rb_left || rb_is_black(other->rb_left))
-                               {
-                                       rb_set_black(other->rb_right);
+                       if (!other->rb_left || rb_is_black(other->rb_left)) {
+                               if (!other->rb_right ||
+                                   rb_is_black(other->rb_right)) {
                                        rb_set_red(other);
-                                       __rb_rotate_left(other, root);
-                                       other = parent->rb_left;
+                                       node = parent;
+                                       parent = rb_parent(node);
+                                       continue;
                                }
-                               rb_set_color(other, rb_color(parent));
-                               rb_set_black(parent);
-                               rb_set_black(other->rb_left);
-                               __rb_rotate_right(parent, root);
-                               break;
+                               rb_set_black(other->rb_right);
+                               rb_set_red(other);
+                               __rb_rotate_left(other, root);
+                               other = parent->rb_left;
                        }
+                       rb_set_color(other, rb_color(parent));
+                       rb_set_black(parent);
+                       rb_set_black(other->rb_left);
+                       __rb_rotate_right(parent, root);
+                       break;
                }
        }
 }