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

["René Scharfe" <l.s.r@web.de>]

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?

>
>>
>>>
>>> -		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;
>>>  }
>>