GH-139389: Do not track immutable tuples in PyTuple_Pack

Include/internal/pycore_interp_structs.h

@@ -181,6 +181,7 @@ struct gc_collection_stats {
     Py_ssize_t collected;
     /* total number of uncollectable objects (put into gc.garbage) */
     Py_ssize_t uncollectable;
+    Py_ssize_t untracked_tuples;
 };
 
 /* Running stats per generation */
@@ -191,6 +192,10 @@ struct gc_generation_stats {
     Py_ssize_t collected;
     /* total number of uncollectable objects (put into gc.garbage) */
     Py_ssize_t uncollectable;
+    Py_ssize_t untracked_tuples;
+    Py_ssize_t total_untracked_tuples;
+    Py_ssize_t total_tuples;
+    Py_ssize_t tuples_by_size[33];
 };
 
 enum _GCPhase {

Include/internal/pycore_tuple.h

@@ -11,7 +11,7 @@ extern "C" {
 #include "pycore_object.h"        // _PyObject_GC_IS_TRACKED
 #include "pycore_structs.h"       // _PyStackRef
 
-extern void _PyTuple_MaybeUntrack(PyObject *);
+extern int _PyTuple_MaybeUntrack(PyObject *);
 extern void _PyTuple_DebugMallocStats(FILE *out);
 
 /* runtime lifecycle */

Objects/tupleobject.c

@@ -22,6 +22,44 @@ class tuple "PyTupleObject *" "&PyTuple_Type"
 
 static inline int maybe_freelist_push(PyTupleObject *);
 
+static uint8_t
+_log2_int(Py_ssize_t size)
+{
+    if (size == 0) {
+        return 0;
+    }
+
+    const uint64_t b[] = {0x2, 0xC, 0xF0, 0xFF00, 0xFFFF0000, 0xFFFFFFFF00000000};
+    const uint64_t S[] = {1, 2, 4, 8, 16, 32};
+
+    int64_t v = size;
+    uint8_t r = 0; // result of log2(v) will go here
+    for (int i = 5; i >= 0; i--) // unroll for speed...
+    {
+        if (v & b[i])
+        {
+            v >>= S[i];
+            r |= S[i];
+        }
+    }
+
+#ifdef Py_DEBUG
+    uint8_t x = (uint8_t)log2((double)size);
+    assert(x == r);
+#endif
+
+    return r + 1;
+}
+
+static void
+_count_tuple(Py_ssize_t size)
+{
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    interp->gc.generation_stats[0].total_tuples += 1;
+    uint8_t size_index = _log2_int(size);
+    interp->gc.generation_stats[0].tuples_by_size[size_index] += 1;
+}
+
 
 /* Allocate an uninitialized tuple object. Before making it public, following
    steps must be done:
@@ -46,6 +84,7 @@ tuple_alloc(Py_ssize_t size)
         PyTupleObject *op = _Py_FREELIST_POP(PyTupleObject, tuples[index]);
         if (op != NULL) {
             _PyTuple_RESET_HASH_CACHE(op);
+            _count_tuple(size);
             return op;
         }
     }
@@ -57,6 +96,7 @@ tuple_alloc(Py_ssize_t size)
     PyTupleObject *result = PyObject_GC_NewVar(PyTupleObject, &PyTuple_Type, size);
     if (result != NULL) {
         _PyTuple_RESET_HASH_CACHE(result);
+        _count_tuple(size);
     }
     return result;
 }
@@ -68,6 +108,7 @@ tuple_alloc(Py_ssize_t size)
 static inline PyObject *
 tuple_get_empty(void)
 {
+    _count_tuple(0);
     return (PyObject *)&_Py_SINGLETON(tuple_empty);
 }
 
@@ -134,14 +175,14 @@ PyTuple_SetItem(PyObject *op, Py_ssize_t i, PyObject *newitem)
     return 0;
 }
 
-void
+int
 _PyTuple_MaybeUntrack(PyObject *op)
 {
     PyTupleObject *t;
     Py_ssize_t i, n;
 
     if (!PyTuple_CheckExact(op) || !_PyObject_GC_IS_TRACKED(op))
-        return;
+        return 0;
     t = (PyTupleObject *) op;
     n = Py_SIZE(t);
     for (i = 0; i < n; i++) {
@@ -151,9 +192,10 @@ _PyTuple_MaybeUntrack(PyObject *op)
            them yet. */
         if (!elt ||
             _PyObject_GC_MAY_BE_TRACKED(elt))
-            return;
+            return 0;
     }
     _PyObject_GC_UNTRACK(op);
+    return 1;
 }
 
 PyObject *
@@ -175,12 +217,18 @@ PyTuple_Pack(Py_ssize_t n, ...)
         return NULL;
     }
     items = result->ob_item;
+    bool track = false;
     for (i = 0; i < n; i++) {
         o = va_arg(vargs, PyObject *);
         items[i] = Py_NewRef(o);
+        if (!track && _PyObject_GC_MAY_BE_TRACKED(items[i])) {
+            track = true;
+        }
     }
     va_end(vargs);
-    _PyObject_GC_TRACK(result);
+    if (track) {
+        _PyObject_GC_TRACK(result);
+    }
     return (PyObject *)result;
 }
 
@@ -376,12 +424,18 @@ PyTuple_FromArray(PyObject *const *src, Py_ssize_t n)
     if (tuple == NULL) {
         return NULL;
     }
+    bool track = false;
     PyObject **dst = tuple->ob_item;
     for (Py_ssize_t i = 0; i < n; i++) {
         PyObject *item = src[i];
         dst[i] = Py_NewRef(item);
+        if (!track && _PyObject_GC_MAY_BE_TRACKED(dst[i])) {
+            track = true;
+        }
+    }
+    if (track) {
+        _PyObject_GC_TRACK(tuple);
     }
-    _PyObject_GC_TRACK(tuple);
     return (PyObject *)tuple;
 }
 
@@ -395,11 +449,17 @@ _PyTuple_FromStackRefStealOnSuccess(const _PyStackRef *src, Py_ssize_t n)
     if (tuple == NULL) {
         return NULL;
     }
+    bool track = false;
     PyObject **dst = tuple->ob_item;
     for (Py_ssize_t i = 0; i < n; i++) {
         dst[i] = PyStackRef_AsPyObjectSteal(src[i]);
+        if (!track && _PyObject_GC_MAY_BE_TRACKED(dst[i])) {
+            track = true;
+        }
+    }
+    if (track) {
+        _PyObject_GC_TRACK(tuple);
     }
-    _PyObject_GC_TRACK(tuple);
     return (PyObject *)tuple;
 }
 
@@ -416,12 +476,18 @@ _PyTuple_FromArraySteal(PyObject *const *src, Py_ssize_t n)
         }
         return NULL;
     }
+    bool track = false;
     PyObject **dst = tuple->ob_item;
     for (Py_ssize_t i = 0; i < n; i++) {
         PyObject *item = src[i];
         dst[i] = item;
+        if (!track && _PyObject_GC_MAY_BE_TRACKED(item)) {
+            track = true;
+        }
+    }
+    if (track) {
+        _PyObject_GC_TRACK(tuple);
     }
-    _PyObject_GC_TRACK(tuple);
     return (PyObject *)tuple;
 }
 
@@ -494,7 +560,9 @@ tuple_concat(PyObject *aa, PyObject *bb)
         dest[i] = Py_NewRef(v);
     }
 
-    _PyObject_GC_TRACK(np);
+    if (_PyObject_GC_IS_TRACKED(a) || _PyObject_GC_IS_TRACKED(b)) {
+        _PyObject_GC_TRACK(np);
+    }
     return (PyObject *)np;
 }
 
@@ -543,7 +611,9 @@ tuple_repeat(PyObject *self, Py_ssize_t n)
         _Py_memory_repeat((char *)np->ob_item, sizeof(PyObject *)*output_size,
                           sizeof(PyObject *)*input_size);
     }
-    _PyObject_GC_TRACK(np);
+    if (_PyObject_GC_IS_TRACKED(a)) {
+        _PyObject_GC_TRACK(np);
+    }
     return (PyObject *) np;
 }
 
@@ -821,7 +891,9 @@ tuple_subscript(PyObject *op, PyObject* item)
                 dest[i] = it;
             }
 
-            _PyObject_GC_TRACK(result);
+            if (_PyObject_GC_IS_TRACKED(self)) {
+                _PyObject_GC_TRACK(result);
+            }
             return (PyObject *)result;
         }
     }

Python/gc.c

@@ -753,18 +753,20 @@ move_unreachable(PyGC_Head *young, PyGC_Head *unreachable)
 * and is much faster than a more complex approach that
 * would untrack all relevant tuples.
 */
-static void
+static Py_ssize_t
 untrack_tuples(PyGC_Head *head)
 {
+    Py_ssize_t untracked = 0;
     PyGC_Head *gc = GC_NEXT(head);
     while (gc != head) {
         PyObject *op = FROM_GC(gc);
         PyGC_Head *next = GC_NEXT(gc);
         if (PyTuple_CheckExact(op)) {
-            _PyTuple_MaybeUntrack(op);
+            untracked += _PyTuple_MaybeUntrack(op);
         }
         gc = next;
     }
+    return untracked;
 }
 
 /* Return true if object has a pre-PEP 442 finalization method. */
@@ -1376,7 +1378,7 @@ gc_collect_young(PyThreadState *tstate,
     validate_spaces(gcstate);
     PyGC_Head *young = &gcstate->young.head;
     PyGC_Head *visited = &gcstate->old[gcstate->visited_space].head;
-    untrack_tuples(young);
+    stats->untracked_tuples += untrack_tuples(young);
     GC_STAT_ADD(0, collections, 1);
 
     PyGC_Head survivors;
@@ -1654,7 +1656,7 @@ gc_collect_increment(PyThreadState *tstate, struct gc_collection_stats *stats)
     GC_STAT_ADD(1, collections, 1);
     GCState *gcstate = &tstate->interp->gc;
     gcstate->work_to_do += assess_work_to_do(gcstate);
-    untrack_tuples(&gcstate->young.head);
+    stats->untracked_tuples += untrack_tuples(&gcstate->young.head);
     if (gcstate->phase == GC_PHASE_MARK) {
         Py_ssize_t objects_marked = mark_at_start(tstate);
         GC_STAT_ADD(1, objects_transitively_reachable, objects_marked);
@@ -1716,7 +1718,7 @@ gc_collect_full(PyThreadState *tstate,
     PyGC_Head *young = &gcstate->young.head;
     PyGC_Head *pending = &gcstate->old[gcstate->visited_space^1].head;
     PyGC_Head *visited = &gcstate->old[gcstate->visited_space].head;
-    untrack_tuples(young);
+    stats->untracked_tuples += untrack_tuples(young);
     /* merge all generations into visited */
     gc_list_merge(young, pending);
     gc_list_validate_space(pending, 1-gcstate->visited_space);
@@ -1756,7 +1758,7 @@ gc_collect_region(PyThreadState *tstate,
     gc_list_init(&unreachable);
     deduce_unreachable(from, &unreachable);
     validate_consistent_old_space(from);
-    untrack_tuples(from);
+    stats->untracked_tuples += untrack_tuples(from);
 
   /* Move reachable objects to next generation. */
     validate_consistent_old_space(to);
@@ -2098,12 +2100,30 @@ _PyGC_Collect(PyThreadState *tstate, int generation, _PyGC_Reason reason)
         default:
             Py_UNREACHABLE();
     }
+    gcstate->generation_stats[generation].untracked_tuples += stats.untracked_tuples;
+    gcstate->generation_stats[0].total_untracked_tuples += stats.untracked_tuples;
     if (PyDTrace_GC_DONE_ENABLED()) {
         PyDTrace_GC_DONE(stats.uncollectable + stats.collected);
     }
     if (reason != _Py_GC_REASON_SHUTDOWN) {
         invoke_gc_callback(gcstate, "stop", generation, &stats);
     }
+    else {
+        FILE *out = stderr;
+
+        fprintf(out, "GC[%d] total tuples    : %zd\n", 0, gcstate->generation_stats[0].total_tuples);
+        fprintf(out, "GC[%d] total untracked_tuples    : %zd\n", 0, gcstate->generation_stats[0].total_untracked_tuples);
+        for (int i = 0; i < 33; i++) {
+            fprintf(out, "GC[%d] by size %d  : %zd\n", 0, i, gcstate->generation_stats[0].tuples_by_size[i]);
+        }
+
+        for (int i = 0; i < NUM_GENERATIONS; i++) {
+            fprintf(out, "GC[%d] collections     : %zd\n", i, gcstate->generation_stats[i].collections);
+            fprintf(out, "GC[%d] collected       : %zd\n", i, gcstate->generation_stats[i].collected);
+            fprintf(out, "GC[%d] uncollectable   : %zd\n", i, gcstate->generation_stats[i].uncollectable);
+            fprintf(out, "GC[%d] untracked_tuples: %zd\n", i, gcstate->generation_stats[i].untracked_tuples);
+        }
+    }
     _PyErr_SetRaisedException(tstate, exc);
     GC_STAT_ADD(generation, objects_collected, stats.collected);
 #ifdef Py_STATS