linux-kernel - Re: [PATCH v3 2/9] rbtree: add __rb_change

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Date:	Mon, 20 Aug 2012 15:17:10 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Michel Lespinasse <walken@...gle.com>
Cc:	riel@...hat.com, peterz@...radead.org, daniel.santos@...ox.com,
	aarcange@...hat.com, dwmw2@...radead.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org
Subject: Re: [PATCH v3 2/9] rbtree: add __rb_change_child() helper function

On Mon, 20 Aug 2012 15:05:24 -0700
Michel Lespinasse <walken@...gle.com> wrote:

> Add __rb_change_child() as an inline helper function to replace code that
> would otherwise be duplicated 4 times in the source.
> 
> No changes to binary size or speed.
> 
> ...
>
> --- a/lib/rbtree.c
> +++ b/lib/rbtree.c
> @@ -66,6 +66,19 @@ static inline struct rb_node *rb_red_parent(struct rb_node *red)
>  	return (struct rb_node *)red->__rb_parent_color;
>  }
>  
> +static inline void
> +__rb_change_child(struct rb_node *old, struct rb_node *new,
> +		  struct rb_node *parent, struct rb_root *root)
> +{
> +	if (parent) {
> +		if (parent->rb_left == old)
> +			parent->rb_left = new;
> +		else
> +			parent->rb_right = new;
> +	} else
> +		root->rb_node = new;
> +}

I'm inclined to agree with Peter here - "inline" is now a vague,
pathetic and useless thing.  The problem is that the reader just
doesn't *know* whether or not the writer really wanted it to be
inlined.

If we have carefully made a decision to inline a function, we should
(now) use __always_inline.

If we have carefully made a decision to not inline a function, we
should use noinline.

If we don't care, we should omit all such markings.

This leaves no place for "inline"?

Marking it noinline shrinks the text by 60-odd bytes.  Given the number
of args, my gut feel is that this will be slower, despite the cache
benefit.  But that might be wrong.

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