Re: [PATCH 4/5] lib/list_sort: Simplify and remove MAX_LIST_LENGTH_BITS

[Andy Shevchenko]

On Tue, Mar 05, 2019 at 03:06:44AM +0000, George Spelvin wrote:
> Rather than a fixed-size array of pending sorted runs, use the ->prev
> links to keep track of things.  This reduces stack usage, eliminates
> some ugly overflow handling, and reduces the code size.
> 
> Also:
> * merge() no longer needs to handle NULL inputs, so simplify.
> * The same applies to merge_and_restore_back_links(), which is renamed
>   to the less ponderous merge_final().  (It's a static helper function,
>   so we don't need a super-descriptive name; comments will do.)
> 
> x86-64 code size 1086 -> 740 bytes (-346)

> +	do {
> +		size_t bit;
>  		struct list_head *cur = list;
> +
> +		/* Extract the head of "list" as a single-element list "cur" */
>  		list = list->next;
>  		cur->next = NULL;
>  
> +		/* Do merges corresponding to set lsbits in count */

> +		for (bit = 1; count & bit; bit <<= 1) {
> +			cur = merge(priv, (cmp_func)cmp, pending, cur);
> +			pending = pending->prev;  /* Untouched by merge() */
>  		}

Wouldn't be it the same to

	bit = ffz(count);
	while (bit--) {
		...
	}
?

Though I dunno which one is generating better code.

> +		/* And place the result at the head of "pending" */
> +		cur->prev = pending;
> +		pending = cur;
> +		count++;
> +	} while (list->next);
> +
> +	/* Now merge together last element with all pending lists */
> +	while (pending->prev) {
> +		list = merge(priv, (cmp_func)cmp, pending, list);
> +		pending = pending->prev;
>  	}

-- 
With Best Regards,
Andy Shevchenko