Use a single generation for the GC

Include/internal/pycore_gc.h

@@ -95,7 +95,7 @@ static inline void _PyGC_SET_FINALIZED(PyObject *op) {
 
 /* If we change this, we need to change the default value in the
    signature of gc.collect. */
-#define NUM_GENERATIONS 3
+#define NUM_GENERATIONS 1
 /*
    NOTE: about untracking of mutable objects.
 
@@ -165,17 +165,25 @@ struct _gc_runtime_state {
     int enabled;
     int debug;
     /* linked lists of container objects */
-    struct gc_generation generations[NUM_GENERATIONS];
-    PyGC_Head *generation0;
+    PyGC_Head head;
     /* a permanent generation which won't be collected */
-    struct gc_generation permanent_generation;
-    struct gc_generation_stats generation_stats[NUM_GENERATIONS];
+    struct gc_generation_stats stats;
     /* true if we are currently running the collector */
     int collecting;
     /* list of uncollectable objects */
     PyObject *garbage;
     /* a list of callbacks to be invoked when collection is performed */
     PyObject *callbacks;
+    /* the number of live GC objects */
+    Py_ssize_t gc_live;
+    /* the threshold at which to trigger a collection */
+    Py_ssize_t gc_threshold;
+    /* The ratio used to compute gc_threshold:
+            gc_threshold = (1 + gc_scale/100) * gc_live
+       A value of 100 means to collect every time the number of live
+       objects doubles. */
+    int gc_scale;
+
     /* This is the number of objects that survived the last full
        collection. It approximates the number of long lived objects
        tracked by the GC.
@@ -192,6 +200,13 @@ struct _gc_runtime_state {
 
 extern void _PyGC_InitState(struct _gc_runtime_state *);
 
+static inline int
+_PyGC_ShouldCollect(struct _gc_runtime_state *gcstate)
+{
+    Py_ssize_t live = gcstate->gc_live;
+    return !gcstate->collecting && gcstate->enabled && live >= gcstate->gc_threshold;
+}
+
 extern Py_ssize_t _PyGC_CollectNoFail(PyThreadState *tstate);
 
 

Include/internal/pycore_object.h

@@ -154,12 +154,12 @@ static inline void _PyObject_GC_TRACK(
                           filename, lineno, __func__);
 
     PyInterpreterState *interp = _PyInterpreterState_GET();
-    PyGC_Head *generation0 = interp->gc.generation0;
-    PyGC_Head *last = (PyGC_Head*)(generation0->_gc_prev);
+    PyGC_Head *head = &interp->gc.head;
+    PyGC_Head *last = (PyGC_Head*)(head->_gc_prev);
     _PyGCHead_SET_NEXT(last, gc);
     _PyGCHead_SET_PREV(gc, last);
-    _PyGCHead_SET_NEXT(gc, generation0);
-    generation0->_gc_prev = (uintptr_t)gc;
+    _PyGCHead_SET_NEXT(gc, head);
+    head->_gc_prev = (uintptr_t)gc;
 }
 
 /* Tell the GC to stop tracking this object.

Include/internal/pycore_runtime_init.h

@@ -119,12 +119,6 @@ extern "C" {
         }, \
         .gc = { \
             .enabled = 1, \
-            .generations = { \
-                /* .head is set in _PyGC_InitState(). */ \
-                { .threshold = 700, }, \
-                { .threshold = 10, }, \
-                { .threshold = 10, }, \
-            }, \
         }, \
         .static_objects = { \
             .singletons = { \