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

["René Scharfe" <l.s.r@web.de>]

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.

>
>>
>> -		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;
>>  }
>