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;
>> }
>
