Fix race condition, memory management, debug output, and hashtable lookup in sorting.cpp (#5078)

[alibeklfc]

Summary:

Four fixes in faiss/utils/sorting.cpp:

1. OpenMP directive fix in fvec_argsort_parallel

The initialization loop used #pragma omp parallel without the for directive. This caused every thread to execute the entire loop independently rather than distributing iterations. With nt threads, each permA[i] was written by all nt threads concurrently — a data race under the C++ memory model (multiple unsynchronized writes to the same non-atomic location), and O(n * nt) wasted work instead of O(n). Fixed by changing to #pragma omp parallel for.

In practice, all threads write the same value (permA[i] = i), so the output was always correct despite the UB. The fix eliminates the undefined behavior and the redundant work.

2. RAII memory management in fvec_argsort_parallel

Replaced new size_t[n] / delete[] perm2 with std::vector<size_t>. The old code had no realistic exception path between allocation and deallocation (all intermediate code is either C functions or non-throwing OpenMP regions), but the manual new/delete pattern is fragile against future edits that might introduce a throwing path. The std::vector provides RAII lifetime management with no behavioral change.

3. Removed debug printf in fvec_argsort_parallel

A leftover printf("merge %d %d, %d threads\n", ...) in the parallel merge loop wrote to stdout during normal operation. Removed.

4. Missing early termination in hashtable_int64_to_int64_lookup

The linear probing loop did not check for empty slots (tab[slot * 2] == -1). In an open-addressing hash table with no deletion support, an empty slot is definitive proof that the key was not inserted — the insert function would have placed it there or earlier. Without this check, lookups for absent keys probed every slot in the bucket before the wrap-around termination at slot == hk_i. The fix adds the standard empty-slot check, matching the structure of the insert function (hashtable_int64_to_int64_add). This is a performance optimization — the old code always returned the correct result (-1 after a full bucket scan), just slower.

Differential Revision: D100317917

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@alibeklfc has exported this pull request. If you are a Meta employee, you can view the originating Diff in D100317917.

[meta-codesync]

This pull request has been merged in aa3ce37.