gh-128807: Add marking phase for free-threaded cyclic GC

Include/internal/pycore_gc.h

@@ -45,12 +45,13 @@ static inline PyObject* _Py_FROM_GC(PyGC_Head *gc) {
  * the per-object lock.
  */
 #ifdef Py_GIL_DISABLED
-#  define _PyGC_BITS_TRACKED        (1)     // Tracked by the GC
-#  define _PyGC_BITS_FINALIZED      (2)     // tp_finalize was called
-#  define _PyGC_BITS_UNREACHABLE    (4)
-#  define _PyGC_BITS_FROZEN         (8)
-#  define _PyGC_BITS_SHARED         (16)
-#  define _PyGC_BITS_DEFERRED       (64)    // Use deferred reference counting
+#  define _PyGC_BITS_TRACKED        (1<<0)     // Tracked by the GC
+#  define _PyGC_BITS_FINALIZED      (1<<1)     // tp_finalize was called
+#  define _PyGC_BITS_UNREACHABLE    (1<<2)
+#  define _PyGC_BITS_FROZEN         (1<<3)
+#  define _PyGC_BITS_SHARED         (1<<4)
+#  define _PyGC_BITS_ALIVE          (1<<5)    // Reachable from a known root.
+#  define _PyGC_BITS_DEFERRED       (1<<6)    // Use deferred reference counting
 #endif
 
 #ifdef Py_GIL_DISABLED
@@ -330,6 +331,9 @@ struct _gc_runtime_state {
        collections, and are awaiting to undergo a full collection for
        the first time. */
     Py_ssize_t long_lived_pending;
+
+    /* True if gc.freeze() has been used. */
+    int freeze_active;
 #endif
 };
 

Misc/NEWS.d/next/Core_and_Builtins/2025-01-13-17-03-49.gh-issue-128807.BGGBxD.rst

@@ -0,0 +1,6 @@
+Add a marking phase to the free-threaded GC.  This is similar to what was
+done in GH-126491.  Since the free-threaded GC does not have generations and
+is not incremental, the marking phase looks for all objects reachable from
+known roots.  The roots are objects known to not be garbage, like the module
+dictionary for :mod:`sys`.  For most programs, this marking phase should
+make the GC a bit faster since typically less work is done per object.