Subject: Re: [PATCH 9/9] mergesort: use ranks stack

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Tue, 18 Jan 2022 11:40:43 +0100

On Tue, Jan 18 2022, René Scharfe wrote:

> Am 17.01.22 um 19:22 schrieb René Scharfe:
>> Am 17.01.22 um 18:43 schrieb Ævar Arnfjörð Bjarmason:
>>>
>>> On Fri, Oct 01 2021, René Scharfe wrote:
>>>
>>>
>>>> +/*
>>>> + * Perform an iterative mergesort using an array of sublists.
>>>> + *
>>>> + * n is the number of items.
>>>> + * ranks[i] is undefined if n & 2^i == 0, and assumed empty.
>>>> + * ranks[i] contains a sublist of length 2^i otherwise.
>>>> + *
>>>> + * The number of bits in a void pointer limits the number of objects
>>>> + * that can be created, and thus the number of array elements necessary
>>>> + * to be able to sort any valid list.
>>>> + *
>>>> + * Adding an item to this array is like incrementing a binary number;
>>>> + * positional values for set bits correspond to sublist lengths.
>>>> + */
>>>>  void *llist_mergesort(void *list,
>>>>  		      void *(*get_next_fn)(const void *),
>>>>  		      void (*set_next_fn)(void *, void *),
>>>>  		      int (*compare_fn)(const void *, const void *))
>>>>  {
>>>> -	unsigned long l;
>>>> -
>>>> -	if (!list)
>>>> -		return NULL;
>>>> -	for (l = 1; ; l *= 2) {
>>>> -		void *curr;
>>>> -		struct mergesort_sublist p, q;
>>>> +	void *ranks[bitsizeof(void *)];
>>>> +	size_t n = 0;
>>>> +	int i;
>>>>
>>>> -		p.ptr = list;
>>>> -		q.ptr = get_nth_next(p.ptr, l, get_next_fn);
>>>> -		if (!q.ptr)
>>>> -			break;
>>>> -		p.len = q.len = l;
>>>> +	while (list) {
>>>> +		void *next = get_next_fn(list);
>>>> +		if (next)
>>>> +			set_next_fn(list, NULL);
>>>> +		for (i = 0; n & (1 << i); i++)
>>>> +			list = llist_merge(ranks[i], list, get_next_fn,
>>>> +					   set_next_fn, compare_fn);
>>>> +		n++;
>>>> +		ranks[i] = list;
>>>> +		list = next;
>>>> +	}
>>>
>>> (Commenting on a commit integrated into v2.34.0)
>>>
>>> The aCC compiler on HP/UX notes:
>>>
>>>     "mergesort.c", line 67: warning #2549-D: variable "ranks" is used before its value is set
>>>                         list = llist_merge(ranks[i], list, get_next_fn,
>>>
>>> It's commenting on the ranks[i] within the for-loop-within-while-loop
>>> above.
>>
>> That would be a bug.  I think none of the array elements are read before
>> they are written -- but of course I'm biased.  Can that compiler show a
>> sequence that would lead to reading uninitialized data?  Or anyone else?
>>
>> Initializing the array would memset(3) 128 bytes on 32-bit systems and
>> 512 bytes on 64-bit systems.  Doing that everywhere just to appease a
>> confused compiler on a dying platform would be merciful, but still sad.
>
> Does the warning disappear if you add "ranks[0] = NULL;" before the while
> loop?  And if it does, has adding "if (n & 1) ranks[0] = NULL;" instead
> the same effect?

Both of those make the warning go away.

Anyway, if you think the pre-image in master now is fine let's leave it
as it is. There's no point in just trying to appease aCC here.

I just thought I'd send a quick mail about it because I was looking at
its warning output, most of those warnings point to obviously harmless
issues, but I thought this one *might* point to an actual logic error
(but didn't look carefully enough myself), so I thought I'd send a quick
note about it.

If you think not it's probably best just to leave the code as-is.

>>
>>>
>>>>
>>>> -		if (compare_fn(p.ptr, q.ptr) > 0)
>>>> -			list = curr = pop_item(&q, get_next_fn);
>>>> +	for (i = 0; n; i++, n >>= 1) {
>>>> +		if (!(n & 1))
>>>> +			continue;
>>>> +		if (list)
>>>> +			list = llist_merge(ranks[i], list, get_next_fn,
>>>> +					   set_next_fn, compare_fn);
>>>>  		else
>>>> -			list = curr = pop_item(&p, get_next_fn);
>>>> -
>>>> -		while (p.ptr) {
>>>> -			while (p.len || q.len) {
>>>> -				void *prev = curr;
>>>> -
>>>> -				if (!p.len)
>>>> -					curr = pop_item(&q, get_next_fn);
>>>> -				else if (!q.len)
>>>> -					curr = pop_item(&p, get_next_fn);
>>>> -				else if (compare_fn(p.ptr, q.ptr) > 0)
>>>> -					curr = pop_item(&q, get_next_fn);
>>>> -				else
>>>> -					curr = pop_item(&p, get_next_fn);
>>>> -				set_next_fn(prev, curr);
>>>> -			}
>>>> -			p.ptr = q.ptr;
>>>> -			p.len = l;
>>>> -			q.ptr = get_nth_next(p.ptr, l, get_next_fn);
>>>> -			q.len = q.ptr ? l : 0;
>>>> -
>>>> -		}
>>>> -		set_next_fn(curr, NULL);
>>>> +			list = ranks[i];
>>>>  	}
>>>>  	return list;
>>>>  }
>>>