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

[Wolfram Strepp]

Hello Peter,

> On Tue, 2009-01-20 at 22:58 +0100, Wolfram Strepp wrote:
> > This patch reorganizes some lines, and therefore one if-condition
> > can be omitted. Here an explanation.
> OK, this patch is better done in smaller steps.
> 
> I'll have to admit to not having tested any of the below patches, but
> they represent how I went about understanding your patch.
> 
> ---
> Subject: rbtree: code movement
> From: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> Date: Sat Jan 24 13:14:11 CET 2009
> 
> Move some code around in order to make the next change more obvious.
> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> ---
>  lib/rbtree.c |   19 ++++++++++---------
>  1 file changed, 10 insertions(+), 9 deletions(-)
> 
> Index: linux-2.6/lib/rbtree.c
> ===================================================================
> --- linux-2.6.orig/lib/rbtree.c
> +++ linux-2.6/lib/rbtree.c
> @@ -237,6 +237,16 @@ void rb_erase(struct rb_node *node, stru
>  		node = node->rb_right;
>  		while ((left = node->rb_left) != NULL)
>  			node = left;
> +
> +		if (rb_parent(old))
> +		{
> +			if (rb_parent(old)->rb_left == old)
> +				rb_parent(old)->rb_left = node;
> +			else
> +				rb_parent(old)->rb_right = node;
> +		} else
> +			root->rb_node = node;
> +
>  		child = node->rb_right;
>  		parent = rb_parent(node);
>  		color = rb_color(node);
> @@ -253,15 +263,6 @@ void rb_erase(struct rb_node *node, stru
>  		node->rb_right = old->rb_right;
>  		node->rb_left = old->rb_left;
>  
> -		if (rb_parent(old))
> -		{
> -			if (rb_parent(old)->rb_left == old)
> -				rb_parent(old)->rb_left = node;
> -			else
> -				rb_parent(old)->rb_right = node;
> -		} else
> -			root->rb_node = node;
> -
>  		rb_set_parent(old->rb_left, node);
>  		if (old->rb_right)
>  			rb_set_parent(old->rb_right, node);
> 
> 
> -------------------
> 
> Subject: rbtree: optimize a special case in rb_erase()
> From: Wolfram Strepp <wstrepp@xxxxxx>
> Date: Sat Jan 24 13:17:04 CET 2009
> 
> There are two cases when a node is erased:
> 'normal case': the successor is not the right-hand-child of the node to be
> erased
> 'special case': the successor is the right-hand child of the node to be
> erased
> 
> Here some ascii-art, with following symbols (referring to the code):
> O: node to be deleted
> N: the successor of O
> P: parent of N
> C: child of N
> L: some other node
> 
> normal case:
> 
>                O                         N
>               / \                       / \
>              /   \                     /   \
>             L     \                   L     \
>            / \     P      ---->      / \     P
>                   / \                       / \
>                  /                         /
>                 N                         C
>                  \                       / \
>                   \
>                    C
>                   / \
> 
> 
> special case:
>               O|P                        N
>               / \                       / \
>              /   \                     /   \
>             L     \                   L     \
>            / \     N      ---->      /       C
>                     \                       / \
>                      \
>                       C
>                      / \
> 
> Notice that for the special case we don't have to reconnect C to N.
> 
> Signed-off-by: Wolfram Strepp <wstrepp@xxxxxx>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> ---
>  lib/rbtree.c |    8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> Index: linux-2.6/lib/rbtree.c
> ===================================================================
> --- linux-2.6.orig/lib/rbtree.c
> +++ linux-2.6/lib/rbtree.c
> @@ -251,13 +251,13 @@ void rb_erase(struct rb_node *node, stru
>  		parent = rb_parent(node);
>  		color = rb_color(node);
>  
> -		if (child)
> -			rb_set_parent(child, parent);
>  		if (parent == old) {
> -			parent->rb_right = child;
>  			parent = node;
> -		} else
> +		} else {
> +			if (child)
> +				rb_set_parent(child, parent);
>  			parent->rb_left = child;
> +		}
>  
>  		node->rb_parent_color = old->rb_parent_color;
>  		node->rb_right = old->rb_right;
> 
> 
> --------------
> 
> Subject: rbtree: further optimize the special case
> From: Wolfram Strepp <wstrepp@xxxxxx>
> Date: Sat Jan 24 13:30:39 CET 2009
> 
> 
> normal case:
> 
>                O                         N
>               / \                       / \
>              /   \                     /   \
>             L     \                   L     \
>            / \     P      ---->      / \     P
>                   / \                       / \
>                  /                         /
>                 N                         C
>                  \                       / \
>                   \
>                    C
>                   / \
> 
> 
> special case:
>               O|P                        N
>               / \                       / \
>              /   \                     /   \
>             L     \                   L     \
>            / \     N      ---->      /       C
>                     \                       / \
>                      \
>                       C
>                      / \
> 
> Notice that for the special case we don't have to reconnect N to C.
> 
> Furthermore, notice that the initial checks:
> 
> 	if (!node->rb_left)
> 		child = node->rb_right;
> 	else if (!node->rb_right)
> 		child = node->rb_left;
> 	else
> 	{
> 		...
> 	}
> 
> guarantee that old->rb_right is set in the final else branch, therefore
> we can omit checking that again.
> 
> Signed-off-by: Wolfram Strepp <wstrepp@xxxxxx>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> ---
>  lib/rbtree.c |    8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> Index: linux-2.6/lib/rbtree.c
> ===================================================================
> --- linux-2.6.orig/lib/rbtree.c
> +++ linux-2.6/lib/rbtree.c
> @@ -257,15 +257,15 @@ void rb_erase(struct rb_node *node, stru
>  			if (child)
>  				rb_set_parent(child, parent);
>  			parent->rb_left = child;
> +
> +			node->rb_right = old->rb_right;
> +			rb_set_parent(old->rb_right, node);
>  		}
>  
>  		node->rb_parent_color = old->rb_parent_color;
> -		node->rb_right = old->rb_right;
>  		node->rb_left = old->rb_left;
> -
>  		rb_set_parent(old->rb_left, node);
> -		if (old->rb_right)
> -			rb_set_parent(old->rb_right, node);
> +
>  		goto color;
>  	}
>  
> 
I'm fine with your reorganization.
Im just building a kernel to test it, but quite sure it will work.
Regards,

Wolfram
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