Revert "GH-126491: GC: Mark objects reachable from roots before doing…

… cycle collection (GH-126502)" (#126983)

Include/cpython/pystats.h

@@ -99,8 +99,6 @@ typedef struct _gc_stats {
     uint64_t collections;
     uint64_t object_visits;
     uint64_t objects_collected;
-    uint64_t objects_transitively_reachable;
-    uint64_t objects_not_transitively_reachable;
 } GCStats;
 
 typedef struct _uop_stats {

Include/internal/pycore_dict.h

@@ -43,6 +43,8 @@ extern int _PyDict_Next(
 
 extern int _PyDict_HasOnlyStringKeys(PyObject *mp);
 
+extern void _PyDict_MaybeUntrack(PyObject *mp);
+
 // Export for '_ctypes' shared extension
 PyAPI_FUNC(Py_ssize_t) _PyDict_SizeOf(PyDictObject *);
 

Include/internal/pycore_frame.h

@@ -75,7 +75,6 @@ typedef struct _PyInterpreterFrame {
     _PyStackRef *stackpointer;
     uint16_t return_offset;  /* Only relevant during a function call */
     char owner;
-    char visited;
     /* Locals and stack */
     _PyStackRef localsplus[1];
 } _PyInterpreterFrame;
@@ -208,7 +207,6 @@ _PyFrame_Initialize(
 #endif
     frame->return_offset = 0;
     frame->owner = FRAME_OWNED_BY_THREAD;
-    frame->visited = 0;
 
     for (int i = null_locals_from; i < code->co_nlocalsplus; i++) {
         frame->localsplus[i] = PyStackRef_NULL;
@@ -391,7 +389,6 @@ _PyFrame_PushTrampolineUnchecked(PyThreadState *tstate, PyCodeObject *code, int
     frame->instr_ptr = _PyCode_CODE(code);
 #endif
     frame->owner = FRAME_OWNED_BY_THREAD;
-    frame->visited = 0;
     frame->return_offset = 0;
 
 #ifdef Py_GIL_DISABLED

Include/internal/pycore_gc.h

@@ -10,11 +10,11 @@ extern "C" {
 
 /* GC information is stored BEFORE the object structure. */
 typedef struct {
-    // Tagged pointer to next object in the list.
+    // Pointer to next object in the list.
     // 0 means the object is not tracked
     uintptr_t _gc_next;
 
-    // Tagged pointer to previous object in the list.
+    // Pointer to previous object in the list.
     // Lowest two bits are used for flags documented later.
     uintptr_t _gc_prev;
 } PyGC_Head;
@@ -302,11 +302,6 @@ struct gc_generation_stats {
     Py_ssize_t uncollectable;
 };
 
-enum _GCPhase {
-    GC_PHASE_MARK = 0,
-    GC_PHASE_COLLECT = 1
-};
-
 struct _gc_runtime_state {
     /* List of objects that still need to be cleaned up, singly linked
      * via their gc headers' gc_prev pointers.  */
@@ -330,12 +325,10 @@ struct _gc_runtime_state {
     /* a list of callbacks to be invoked when collection is performed */
     PyObject *callbacks;
 
-    Py_ssize_t prior_heap_size;
     Py_ssize_t heap_size;
     Py_ssize_t work_to_do;
     /* Which of the old spaces is the visited space */
     int visited_space;
-    int phase;
 
 #ifdef Py_GIL_DISABLED
     /* This is the number of objects that survived the last full

Include/internal/pycore_object.h

@@ -466,8 +466,8 @@ static inline void _PyObject_GC_TRACK(
     PyGC_Head *last = (PyGC_Head*)(generation0->_gc_prev);
     _PyGCHead_SET_NEXT(last, gc);
     _PyGCHead_SET_PREV(gc, last);
-    uintptr_t not_visited = 1 ^ interp->gc.visited_space;
-    gc->_gc_next = ((uintptr_t)generation0) | not_visited;
+    /* Young objects will be moved into the visited space during GC, so set the bit here */
+    gc->_gc_next = ((uintptr_t)generation0) | (uintptr_t)interp->gc.visited_space;
     generation0->_gc_prev = (uintptr_t)gc;
 #endif
 }

Include/internal/pycore_runtime_init.h

@@ -134,7 +134,6 @@ extern PyTypeObject _PyExc_MemoryError;
                 { .threshold = 0, }, \
             }, \
             .work_to_do = -5000, \
-            .phase = GC_PHASE_MARK, \
         }, \
         .qsbr = { \
             .wr_seq = QSBR_INITIAL, \

InternalDocs/garbage_collector.md

@@ -351,7 +351,6 @@ follows these steps in order:
    the reference counts fall to 0, triggering the destruction of all unreachable
    objects.
 
-
 Optimization: incremental collection
 ====================================
 
@@ -485,46 +484,6 @@ specifically in a generation by calling `gc.collect(generation=NUM)`.
 ```
 
 
-Optimization: visiting reachable objects
-========================================
-
-An object cannot be garbage if it can be reached.
-
-To avoid having to identify reference cycles across the whole heap, we can
-reduce the amount of work done considerably by first moving most reachable objects
-to the `visited` space. Empirically, most reachable objects can be reached from a
-small set of global objects and local variables.
-This step does much less work per object, so reduces the time spent
-performing garbage collection by at least half.
-
-> [!NOTE]
-> Objects that are not determined to be reachable by this pass are not necessarily
-> unreachable. We still need to perform the main algorithm to determine which objects
-> are actually unreachable.
-
-We use the same technique of forming a transitive closure as the incremental
-collector does to find reachable objects, seeding the list with some global
-objects and the currently executing frames.
-
-This phase moves objects to the `visited` space, as follows:
-
-1. All objects directly referred to by any builtin class, the `sys` module, the `builtins`
-module and all objects directly referred to from stack frames are added to a working
-set of reachable objects.
-2. Until this working set is empty:
-   1. Pop an object from the set and move it to the `visited` space
-   2. For each object directly reachable from that object:
-      * If it is not already in `visited` space and it is a GC object,
-        add it to the working set
-
-
-Before each increment of collection is performed, the stacks are scanned
-to check for any new stack frames that have been created since the last
-increment. All objects directly referred to from those stack frames are
-added to the working set.
-Then the above algorithm is repeated, starting from step 2.
-
-
 Optimization: reusing fields to save memory
 ===========================================
 
@@ -573,8 +532,8 @@ of `PyGC_Head` discussed in the `Memory layout and object structure`_ section:
   currently in.  Instead, when that's needed, ad hoc tricks (like the
   `NEXT_MASK_UNREACHABLE` flag) are employed.
 
-Optimization: delayed untracking of containers
-==============================================
+Optimization: delay tracking containers
+=======================================
 
 Certain types of containers cannot participate in a reference cycle, and so do
 not need to be tracked by the garbage collector. Untracking these objects
@@ -589,8 +548,8 @@ a container:
 As a general rule, instances of atomic types aren't tracked and instances of
 non-atomic types (containers, user-defined objects...) are.  However, some
 type-specific optimizations can be present in order to suppress the garbage
-collector footprint of simple instances. Historically, both dictionaries and
-tuples were untracked during garbage collection. Now it is only tuples:
+collector footprint of simple instances. Some examples of native types that
+benefit from delayed tracking:
 
 - Tuples containing only immutable objects (integers, strings etc,
   and recursively, tuples of immutable objects) do not need to be tracked. The
@@ -599,8 +558,14 @@ tuples were untracked during garbage collection. Now it is only tuples:
   tuples at creation time. Instead, all tuples except the empty tuple are tracked
   when created. During garbage collection it is determined whether any surviving
   tuples can be untracked. A tuple can be untracked if all of its contents are
-  already not tracked. Tuples are examined for untracking when moved from the
-  young to the old generation.
+  already not tracked. Tuples are examined for untracking in all garbage collection
+  cycles. It may take more than one cycle to untrack a tuple.
+
+- Dictionaries containing only immutable objects also do not need to be tracked.
+  Dictionaries are untracked when created. If a tracked item is inserted into a
+  dictionary (either as a key or value), the dictionary becomes tracked. During a
+  full garbage collection (all generations), the collector will untrack any dictionaries
+  whose contents are not tracked.
 
 The garbage collector module provides the Python function `is_tracked(obj)`, which returns
 the current tracking status of the object. Subsequent garbage collections may change the
@@ -613,9 +578,11 @@ tracking status of the object.
       False
       >>> gc.is_tracked([])
       True
-      >>> gc.is_tracked(("a", 1))
+      >>> gc.is_tracked({})
       False
       >>> gc.is_tracked({"a": 1})
+      False
+      >>> gc.is_tracked({"a": []})
       True
 ```
 

Lib/test/test_dict.py

@@ -880,6 +880,115 @@ class C(object):
             gc.collect()
             self.assertIs(ref(), None, "Cycle was not collected")
 
+    def _not_tracked(self, t):
+        # Nested containers can take several collections to untrack
+        gc.collect()
+        gc.collect()
+        self.assertFalse(gc.is_tracked(t), t)
+
+    def _tracked(self, t):
+        self.assertTrue(gc.is_tracked(t), t)
+        gc.collect()
+        gc.collect()
+        self.assertTrue(gc.is_tracked(t), t)
+
+    def test_string_keys_can_track_values(self):
+        # Test that this doesn't leak.
+        for i in range(10):
+            d = {}
+            for j in range(10):
+                d[str(j)] = j
+            d["foo"] = d
+
+    @support.cpython_only
+    def test_track_literals(self):
+        # Test GC-optimization of dict literals
+        x, y, z, w = 1.5, "a", (1, None), []
+
+        self._not_tracked({})
+        self._not_tracked({x:(), y:x, z:1})
+        self._not_tracked({1: "a", "b": 2})
+        self._not_tracked({1: 2, (None, True, False, ()): int})
+        self._not_tracked({1: object()})
+
+        # Dicts with mutable elements are always tracked, even if those
+        # elements are not tracked right now.
+        self._tracked({1: []})
+        self._tracked({1: ([],)})
+        self._tracked({1: {}})
+        self._tracked({1: set()})
+
+    @support.cpython_only
+    def test_track_dynamic(self):
+        # Test GC-optimization of dynamically-created dicts
+        class MyObject(object):
+            pass
+        x, y, z, w, o = 1.5, "a", (1, object()), [], MyObject()
+
+        d = dict()
+        self._not_tracked(d)
+        d[1] = "a"
+        self._not_tracked(d)
+        d[y] = 2
+        self._not_tracked(d)
+        d[z] = 3
+        self._not_tracked(d)
+        self._not_tracked(d.copy())
+        d[4] = w
+        self._tracked(d)
+        self._tracked(d.copy())
+        d[4] = None
+        self._not_tracked(d)
+        self._not_tracked(d.copy())
+
+        # dd isn't tracked right now, but it may mutate and therefore d
+        # which contains it must be tracked.
+        d = dict()
+        dd = dict()
+        d[1] = dd
+        self._not_tracked(dd)
+        self._tracked(d)
+        dd[1] = d
+        self._tracked(dd)
+
+        d = dict.fromkeys([x, y, z])
+        self._not_tracked(d)
+        dd = dict()
+        dd.update(d)
+        self._not_tracked(dd)
+        d = dict.fromkeys([x, y, z, o])
+        self._tracked(d)
+        dd = dict()
+        dd.update(d)
+        self._tracked(dd)
+
+        d = dict(x=x, y=y, z=z)
+        self._not_tracked(d)
+        d = dict(x=x, y=y, z=z, w=w)
+        self._tracked(d)
+        d = dict()
+        d.update(x=x, y=y, z=z)
+        self._not_tracked(d)
+        d.update(w=w)
+        self._tracked(d)
+
+        d = dict([(x, y), (z, 1)])
+        self._not_tracked(d)
+        d = dict([(x, y), (z, w)])
+        self._tracked(d)
+        d = dict()
+        d.update([(x, y), (z, 1)])
+        self._not_tracked(d)
+        d.update([(x, y), (z, w)])
+        self._tracked(d)
+
+    @support.cpython_only
+    def test_track_subtypes(self):
+        # Dict subtypes are always tracked
+        class MyDict(dict):
+            pass
+        self._tracked(MyDict())
+
     def make_shared_key_dict(self, n):
         class C:
             pass