Re: [PATCH] merge-ort: only do pointer arithmetic for non-empty lists
- From
Andrzej Hunt <andrzej@ahunt.org>
- Date
- Apr 11, 2021, 09:12 UTC
- Message-ID
- <a5e9bbd6-23bb-2778-2f2a-dc601c842336@ahunt.org>
- In-Reply-To
- <1866b90b-fe07-18df-0d60-e2350d935375@web.de>
On 10/04/2021 13:48, René Scharfe wrote:
Show 32 quoted lines
> Am 10.04.21 um 10:30 schrieb Andrzej Hunt via GitGitGadget:
>> [...]
>> diff --git a/merge-ort.c b/merge-ort.c
>> index 5e118a85ee04..4da4b4688336 100644
>> --- a/merge-ort.c
>> +++ b/merge-ort.c
>> @@ -2504,8 +2504,10 @@ static void write_tree(struct object_id *result_oid,
>> * We won't use relevant_entries again and will let it just pop off the
>> * stack, so there won't be allocation worries or anything.
>> */
>> - relevant_entries.items = versions->items + offset;
>> - relevant_entries.nr = versions->nr - offset;
>> + if (versions->nr) {
>> + relevant_entries.items = versions->items + offset;
>> + relevant_entries.nr = versions->nr - offset;
>> + }
>> QSORT(relevant_entries.items, relevant_entries.nr, tree_entry_order);
>
> Reading the diff I was wondering if QSORT now gets handed uninitialized
> values if version-nr is 0. The answer is no -- relevant_entries is
> initialized at declaration. Otherwise the compiler would have probably
> warned, but sometimes it gets confused.
>
> I wonder why relevant_entries is introduced at all, though. It's not
> referenced later. So how about this instead?
>
> if (versions->nr)
> QSORT(versions->items + offset, nr, tree_entry_order);
>
> The intent to sort the last versions->nr-offset entries of versions,
> but only if it's not empty, is easier to see like this, I think.
> That is much more elegant, I will follow this approach. Thank you for the suggestion!
An alternative might be to keep relevant_entries, and replace all later usages of versions->items[offset+i] with relevant_entries.items[i], but that's more invasive and I don't see any good reason for doing so given that the existing pattern works fine.