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;
>>>> }
>>>
