gh-140358: Bring back elapsed time and unreachable count to gc debug output

Misc/NEWS.d/next/Core_and_Builtins/2025-10-20-11-24-36.gh-issue-140358.UQuKdV.rst

@@ -0,0 +1,3 @@
+Restore elapsed time and unreachable object count in GC debug output. These
+were inadvertently removed during a refactor of ``gc.c``. The debug log now
+again reports elapsed collection time and the number of unreachable objects.

Python/gc.c

@@ -10,6 +10,7 @@
 #include "pycore_interpframe.h"   // _PyFrame_GetLocalsArray()
 #include "pycore_object_alloc.h"  // _PyObject_MallocWithType()
 #include "pycore_pystate.h"       // _PyThreadState_GET()
+#include "pycore_time.h"          // _PyTime_MonotonicRaw()
 #include "pycore_tuple.h"         // _PyTuple_MaybeUntrack()
 #include "pycore_weakref.h"       // _PyWeakref_ClearRef()
 
@@ -2067,6 +2068,8 @@ _PyGC_Collect(PyThreadState *tstate, int generation, _PyGC_Reason reason)
     GCState *gcstate = &tstate->interp->gc;
     assert(tstate->current_frame == NULL || tstate->current_frame->stackpointer != NULL);
 
+    PyTime_t t1;
+
     int expected = 0;
     if (!_Py_atomic_compare_exchange_int(&gcstate->collecting, &expected, 1)) {
         // Don't start a garbage collection if one is already in progress.
@@ -2078,6 +2081,7 @@ _PyGC_Collect(PyThreadState *tstate, int generation, _PyGC_Reason reason)
         invoke_gc_callback(gcstate, "start", generation, &stats);
     }
     if (gcstate->debug & _PyGC_DEBUG_STATS) {
+        (void)PyTime_MonotonicRaw(&t1);
         PySys_WriteStderr("gc: collecting generation %d...\n", generation);
         show_stats_each_generations(gcstate);
     }
@@ -2115,6 +2119,17 @@ _PyGC_Collect(PyThreadState *tstate, int generation, _PyGC_Reason reason)
 #endif
     validate_spaces(gcstate);
     _Py_atomic_store_int(&gcstate->collecting, 0);
+
+    if (gcstate->debug & _PyGC_DEBUG_STATS) {
+        PyTime_t t2;
+        (void)PyTime_MonotonicRaw(&t2);
+        double d = PyTime_AsSecondsDouble(t2 - t1);
+        PySys_WriteStderr(
+            "gc: done, %zd unreachable, %zd uncollectable, %.4fs elapsed\n",
+            stats.collected + stats.uncollectable, stats.uncollectable, d
+        );
+    }
+
     return stats.uncollectable + stats.collected;
 }