[PATCH 1/2] Optimization of function rb_erase() in lib/rbtree.c

From: Wolfram Strepp
Date: Tue Jan 20 2009 - 16:56:20 EST


i have reviewed the code of the rb-tree implementation
in the kernel, and distilled two small patches for file lib/rbtree.c,
which optimize and cleanup the code of functions rb_erase() and __rb_erase_color().
In summary, there are 5 if()-conditions which can be eliminated.
The patches reduce the code size (normal kernel build on x86)
of this functions by 23 bytes, or 4.5 %.

So although this is not a dramatic change, i think its worth it,
given the many places in the kernel where it is used
(and given the fact that processors dont like if-conditions).

The patches are tested on x86.


The first patch was already posted some years ago, see:

It was finally merged by the original author of the rb-tree
implementation, Andrea Arcangeli, in one of his kernel trees.
Citing from http://lkml.org/lkml/2002/12/25/51:

>Only in 2.4.21pre2aa1: 00_rbtree-cleanups-1
> Merged rbtree cleanups/microoptimizations from Érsek László after
> verifying their math correctness also with the help of Paolo Carlini
> and of some gentle reminder from Rusty, they are obviously right,
> thanks.

But obviousely, it never found is way into the mainline kernel,
so here it is again.

Signed-off-by: Wolfram Strepp <wstrepp@xxxxxx>

--- a/lib/rbtree.c
+++ b/lib/rbtree.c
@@ -163,17 +163,14 @@ static void __rb_erase_color(struct rb_node *node, struct rb_node *parent,
if (!other->rb_right || rb_is_black(other->rb_right))
- struct rb_node *o_left;
- if ((o_left = other->rb_left))
- rb_set_black(o_left);
+ rb_set_black(other->rb_left);
__rb_rotate_right(other, root);
other = parent->rb_right;
rb_set_color(other, rb_color(parent));
- if (other->rb_right)
- rb_set_black(other->rb_right);
+ rb_set_black(other->rb_right);
__rb_rotate_left(parent, root);
node = root->rb_node;
@@ -200,17 +197,14 @@ static void __rb_erase_color(struct rb_node *node, struct rb_node *parent,
if (!other->rb_left || rb_is_black(other->rb_left))
- register struct rb_node *o_right;
- if ((o_right = other->rb_right))
- rb_set_black(o_right);
+ rb_set_black(other->rb_right);
__rb_rotate_left(other, root);
other = parent->rb_left;
rb_set_color(other, rb_color(parent));
- if (other->rb_left)
- rb_set_black(other->rb_left);
+ rb_set_black(other->rb_left);
__rb_rotate_right(parent, root);
node = root->rb_node;

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