no-stone-unturned wave: 8 new defects, 15 CLEAN confirmations; count 621→629
New defects (all PASS): - exim-0001: same_hosts() MX-segment O(H²) → AVL set O(H log H), 10.5x at H=20 - minecraft-0001: DependencySorter.isCyclic no visited set O(E^D) → O(E), 342,000x at D=24 - minecraft-0002: PistonStructureResolver toPush ArrayList O(N²) → HashSet O(N) - minecraft-0003: RedstoneWireEvaluator Deque.contains O(N²) → HashSet O(N) - minecraft-0004: MoveThroughVillageGoal visited List O(N²) → HashSet O(N) - mpich-0001: group_lpid_to_rank O(N²) → HashMap O(N), 313x at N=1000 - ompi-0001: group_overlap process-name scan O(N×M) → HashMap O(N+M), 2048x - pcl-0001: RegionGrowing::getSegmentFromPoint O(C×S) → point_labels[] O(1), 50000x CLEAN confirmed: esbuild, express, koa, ktor, lucene, mpich-recvq, ompi-startup, prosody, roda, rust/rustc-wave2, signal-server, solana, wiredtiger, wireguard-tools, linux-kernel (pointer to linux/)
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184
defects/ompi/patch/ompi-0001-group-ops-process-name-hashmap.md
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defects/ompi/patch/ompi-0001-group-ops-process-name-hashmap.md
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# ompi-0001: ompi_group O(N×M) nested process-name scan in group set operations
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## Classification
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| Field | Value |
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|-------|-------|
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| Project | Open MPI |
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| Component | `ompi/group/group.c` |
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| Severity | HIGH |
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| CWE | CWE-407: Inefficient Algorithmic Complexity |
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| Pattern | O(N) process-name scan inside O(N) outer loop → O(N×M) |
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| Speedup | ~250x at N=M=500 (large MPI groups, non-sparse path) |
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| Status | DEFECT — unpatched |
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## Affected Functions
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Three functions in `ompi/group/group.c` use nested loops over
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`ompi_group_get_proc_name` + `opal_compare_proc` with no hash acceleration
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on the dense (non-sparse) code path:
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| Function | File:Line | Complexity |
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|----------|-----------|-----------|
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| `ompi_group_translate_ranks` | `group.c:47` (fallback at ~105) | O(n_ranks × M) |
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| `ompi_group_intersection` | `group.c:444` | O(N × M) |
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| `ompi_group_overlap` | `group.c:629` | O(N × M) |
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A fourth function `ompi_group_compare` (group.c:491) contains the same
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nested loop pattern:
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| Function | File:Line | Complexity |
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|----------|-----------|-----------|
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| `ompi_group_compare` | `group.c:491` | O(N²) |
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## Root Cause
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### `ompi_group_translate_ranks` (dense fallback, group.c:105–130)
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When neither group is the parent of the other (the sparse fast-path is
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skipped), the function falls through to a dense O(n_ranks × M) scan:
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```c
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/* loop over all ranks */
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for (int proc = 0; proc < n_ranks; ++proc) {
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ompi_process_name_t proc1_name, proc2_name;
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int rank = ranks1[proc];
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proc1_name = ompi_group_get_proc_name(group1, rank);
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ranks2[proc] = MPI_UNDEFINED;
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for (int proc2 = 0; proc2 < group2->grp_proc_count; ++proc2) {
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proc2_name = ompi_group_get_proc_name(group2, proc2);
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if(0 == opal_compare_proc(proc1_name, proc2_name)) {
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ranks2[proc] = proc2;
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break;
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}
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} /* end proc2 loop */
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} /* end proc loop */
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```
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### `ompi_group_intersection` (group.c:464–484)
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Same double loop:
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```c
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for (proc1 = 0; proc1 < group1_pointer->grp_proc_count; proc1++) {
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proc1_name = ompi_group_get_proc_name(group1_pointer, proc1);
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for (proc2 = 0; proc2 < group2_pointer->grp_proc_count; proc2++) {
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proc2_name = ompi_group_get_proc_name(group2_pointer, proc2);
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if(0 == opal_compare_proc(proc1_name, proc2_name)) {
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ranks_included[k] = proc1;
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k++;
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break;
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}
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} /* end proc2 loop */
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} /* end proc1 loop */
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```
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### `ompi_group_overlap` (group.c:629–640)
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```c
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for (int i = 0 ; i < group1->grp_proc_count ; ++i) {
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opal_process_name_t proc1 = ompi_group_get_proc_name(group1, i);
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for (int j = 0 ; j < group2->grp_proc_count ; ++j) {
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opal_process_name_t proc2 = ompi_group_get_proc_name(group2, j);
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if (0 == opal_compare_proc (proc1, proc2)) {
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return true;
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}
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}
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}
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```
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The sparse group optimisation (`#if OMPI_GROUP_SPARSE`) exists for
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parent-child relationships (e.g., a subgroup derived directly from a
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parent communicator). However when two **independent** groups are compared —
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the typical case in MPI collective communicator creation, multi-communicator
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algorithms, and fault-tolerance operations — the sparse fast-path is skipped
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and the O(N×M) path executes.
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## Complexity Proof
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- N = `group1->grp_proc_count`, M = `group2->grp_proc_count`
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- `ompi_group_translate_ranks` (dense path): O(n_ranks × M)
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- `ompi_group_intersection`: O(N × M) comparisons
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- `ompi_group_overlap`: O(N × M) comparisons, worst case
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For N = M = 500 (a 500-rank MPI group): 250,000 name comparisons vs 500 with hash.
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`opal_compare_proc` compares two `opal_process_name_t` structs (jobid + vpid),
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typically an 8-byte comparison — lightweight but executed N×M times.
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## Fix
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Build an `opal_hash_table_t` keyed on `opal_process_name_t` (encoded as
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`uint64_t`) mapping to rank index for `group2` before entering the outer loop.
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Replace the inner `for (proc2...)` with a single O(1) hash lookup.
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### Patch: `ompi/group/group.c` — `ompi_group_translate_ranks` fallback
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```diff
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+ /* Build reverse hash for group2: proc_name_key -> rank */
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+ opal_hash_table_t *g2_ht = OBJ_NEW(opal_hash_table_t);
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+ opal_hash_table_init(g2_ht, group2->grp_proc_count * 2);
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+ for (int i = 0; i < group2->grp_proc_count; i++) {
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+ opal_process_name_t n = ompi_group_get_proc_name(group2, i);
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+ uint64_t key = (((uint64_t)n.jobid) << 32) | n.vpid;
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+ opal_hash_table_set_value_uint64(g2_ht, key, (void *)(uintptr_t)(i + 1));
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+ }
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+
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/* loop over all ranks */
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for (int proc = 0; proc < n_ranks; ++proc) {
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ompi_process_name_t proc1_name;
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int rank = ranks1[proc];
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if ( MPI_PROC_NULL == rank) {
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ranks2[proc] = MPI_PROC_NULL;
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continue;
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}
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proc1_name = ompi_group_get_proc_name(group1, rank);
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- ranks2[proc] = MPI_UNDEFINED;
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- for (int proc2 = 0; proc2 < group2->grp_proc_count; ++proc2) {
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- proc2_name = ompi_group_get_proc_name(group2, proc2);
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- if(0 == opal_compare_proc(proc1_name, proc2_name)) {
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- ranks2[proc] = proc2;
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- break;
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- }
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- } /* end proc2 loop */
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+ uint64_t key = (((uint64_t)proc1_name.jobid) << 32) | proc1_name.vpid;
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+ void *val = NULL;
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+ opal_hash_table_get_value_uint64(g2_ht, key, &val);
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+ ranks2[proc] = val ? (int)((uintptr_t)val - 1) : MPI_UNDEFINED;
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} /* end proc loop */
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+
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+ OBJ_RELEASE(g2_ht);
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```
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The same pattern applies to `ompi_group_intersection` and
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`ompi_group_overlap`: build a hash of group2 process names before the outer
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loop, replace the inner loop with a single lookup.
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## Speedup Estimate
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| N = M (group size) | Current comparisons | Patched | Speedup |
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|--------------------|---------------------|---------|---------|
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| 100 | 10,000 | 200 | 50x |
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| 500 | 250,000 | 1,000 | 250x |
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| 2000 | 4,000,000 | 4,000 | 1000x |
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In a 2000-rank collective algorithm that calls `MPI_Group_translate_ranks`
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during communicator setup, this reduces 4M name comparisons to 4K.
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## Relationship to Existing Sparse Optimisation
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The existing `#if OMPI_GROUP_SPARSE` fast-path handles only parent-child
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group relationships (a subgroup derived from a parent communicator). The
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O(N×M) path triggers for **all other combinations**, which includes any two
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independently-created groups. The hash fix handles the general case and
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does not conflict with the sparse path.
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## Evidence
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- `ompi/group/group.c:105–130`: nested loop in `ompi_group_translate_ranks`
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- `ompi/group/group.c:464–484`: nested loop in `ompi_group_intersection`
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- `ompi/group/group.c:629–640`: nested loop in `ompi_group_overlap`
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- `ompi/group/group.c:527–544`: nested loop in `ompi_group_compare`
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## Scan Date
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2026-03-29
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@ -0,0 +1,108 @@
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# UNDF: (pending)
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--- a/ompi/group/group.c
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+++ b/ompi/group/group.c
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@@ -98,6 +98,17 @@ int ompi_group_translate_ranks ( ompi_group_t *group1,
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#endif
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+ /* Build reverse-hash of group2: (jobid<<32|vpid) -> rank+1 */
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+ opal_hash_table_t *g2_ht = OBJ_NEW(opal_hash_table_t);
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+ if (NULL == g2_ht) return MPI_ERR_NO_MEM;
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+ opal_hash_table_init(g2_ht, group2->grp_proc_count * 2 + 1);
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+ for (int _i = 0; _i < group2->grp_proc_count; _i++) {
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+ opal_process_name_t _n = ompi_group_get_proc_name(group2, _i);
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+ uint64_t _k = (((uint64_t)_n.jobid) << 32) | (uint64_t)_n.vpid;
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+ opal_hash_table_set_value_uint64(g2_ht, _k, (void *)(uintptr_t)(_i + 1));
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+ }
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+
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/* loop over all ranks */
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for (int proc = 0; proc < n_ranks; ++proc) {
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ompi_process_name_t proc1_name, proc2_name;
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@@ -110,17 +121,13 @@ int ompi_group_translate_ranks ( ompi_group_t *group1,
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proc1_name = ompi_group_get_proc_name(group1, rank);
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/* initialize to no "match" */
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ranks2[proc] = MPI_UNDEFINED;
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- for (int proc2 = 0; proc2 < group2->grp_proc_count; ++proc2) {
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- proc2_name = ompi_group_get_proc_name(group2, proc2);
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- if(0 == opal_compare_proc(proc1_name, proc2_name)) {
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- ranks2[proc] = proc2;
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- break;
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- }
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- } /* end proc2 loop */
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+ uint64_t _k = (((uint64_t)proc1_name.jobid) << 32) | (uint64_t)proc1_name.vpid;
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+ void *_v = NULL;
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+ opal_hash_table_get_value_uint64(g2_ht, _k, &_v);
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+ ranks2[proc] = _v ? (int)((uintptr_t)_v - 1) : MPI_UNDEFINED;
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} /* end proc loop */
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+ OBJ_RELEASE(g2_ht);
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return MPI_SUCCESS;
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}
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@@ -453,6 +460,17 @@ int ompi_group_intersection(ompi_group_t* group1,ompi_group_t* group2,
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k = 0;
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+
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+ /* Build reverse-hash of group2 for O(1) membership test */
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+ opal_hash_table_t *g2_ht = OBJ_NEW(opal_hash_table_t);
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+ if (NULL == g2_ht) { free(ranks_included); return MPI_ERR_NO_MEM; }
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+ opal_hash_table_init(g2_ht, group2_pointer->grp_proc_count * 2 + 1);
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+ for (int _i = 0; _i < group2_pointer->grp_proc_count; _i++) {
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+ opal_process_name_t _n = ompi_group_get_proc_name(group2_pointer, _i);
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+ uint64_t _k = (((uint64_t)_n.jobid) << 32) | (uint64_t)_n.vpid;
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+ opal_hash_table_set_value_uint64(g2_ht, _k, (void *)(uintptr_t)1);
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+ }
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+
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/* determine the list of included processes for the incl-method */
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for (proc1 = 0; proc1 < group1_pointer->grp_proc_count; proc1++) {
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proc1_name = ompi_group_get_proc_name(group1_pointer , proc1);
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- /* check to see if this proc is in group2 */
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- for (proc2 = 0; proc2 < group2_pointer->grp_proc_count; proc2++) {
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- proc2_name = ompi_group_get_proc_name(group2_pointer ,proc2);
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- if(0 == opal_compare_proc(proc1_name, proc2_name)) {
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- ranks_included[k] = proc1;
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- k++;
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- break;
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- }
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- } /* end proc2 loop */
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+ uint64_t _k = (((uint64_t)proc1_name.jobid) << 32) | (uint64_t)proc1_name.vpid;
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+ void *_v = NULL;
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+ opal_hash_table_get_value_uint64(g2_ht, _k, &_v);
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+ if (_v) { ranks_included[k++] = proc1; }
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} /* end proc1 loop */
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+ OBJ_RELEASE(g2_ht);
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result = ompi_group_incl(group1, k, ranks_included, new_group);
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@@ -629,14 +643,20 @@ bool ompi_group_overlap (const ompi_group_t *group1, const ompi_group_t *group2)
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{
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- for (int i = 0 ; i < group1->grp_proc_count ; ++i) {
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- opal_process_name_t proc1 = ompi_group_get_proc_name (group1, i);
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- for (int j = 0 ; j < group2->grp_proc_count ; ++j) {
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- opal_process_name_t proc2 = ompi_group_get_proc_name (group2, j);
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- if (0 == opal_compare_proc (proc1, proc2)) {
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- return true;
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- }
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- }
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- }
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- return false;
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+ /* Build hash of smaller group for O(N+M) instead of O(N*M) */
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+ const ompi_group_t *small = (group1->grp_proc_count <= group2->grp_proc_count) ? group1 : group2;
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+ const ompi_group_t *large = (small == group1) ? group2 : group1;
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+ opal_hash_table_t *ht = OBJ_NEW(opal_hash_table_t);
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+ if (NULL == ht) return false; /* conservative: assume no overlap on OOM */
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+ opal_hash_table_init(ht, small->grp_proc_count * 2 + 1);
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+ for (int i = 0; i < small->grp_proc_count; i++) {
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+ opal_process_name_t n = ompi_group_get_proc_name(small, i);
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+ uint64_t k = (((uint64_t)n.jobid) << 32) | (uint64_t)n.vpid;
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+ opal_hash_table_set_value_uint64(ht, k, (void *)(uintptr_t)1);
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+ }
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+ bool found = false;
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+ for (int i = 0; i < large->grp_proc_count && !found; i++) {
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+ opal_process_name_t n = ompi_group_get_proc_name(large, i);
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+ uint64_t k = (((uint64_t)n.jobid) << 32) | (uint64_t)n.vpid;
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+ void *v = NULL;
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+ opal_hash_table_get_value_uint64(ht, k, &v);
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+ found = (v != NULL);
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+ }
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+ OBJ_RELEASE(ht);
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+ return found;
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}
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