Add a bit vector to optimize watcher dispatch

A bit is set in the bit vector iff there is a watcher set at the
corresponding offset in the watcher array. Only notify watchers
if at least one bit is set.

Include/internal/pycore_interp.h

@@ -174,6 +174,8 @@ struct _is {
 
     PyDict_WatchCallback dict_watchers[DICT_MAX_WATCHERS];
     PyFunction_WatchCallback func_watchers[FUNC_MAX_WATCHERS];
+    // One bit is set for each non-NULL entry in func_watchers
+    uint8_t active_func_watchers;
 
     Py_ssize_t co_extra_user_count;
     freefunc co_extra_freefuncs[MAX_CO_EXTRA_USERS];

Objects/funcobject.c

@@ -9,9 +9,9 @@
 #include "structmember.h"         // PyMemberDef
 
 static void
-handle_func_event(PyFunction_WatchEvent event, PyFunctionObject *func, PyObject *new_value)
+notify_func_watchers(PyInterpreterState *interp, PyFunction_WatchEvent event,
+                     PyFunctionObject *func, PyObject *new_value)
 {
-    PyInterpreterState *interp = _PyInterpreterState_GET();
     for (int i = 0; i < FUNC_MAX_WATCHERS; i++) {
         PyFunction_WatchCallback cb = interp->func_watchers[i];
         if ((cb != NULL) && (cb(event, func, new_value) < 0)) {
@@ -20,6 +20,16 @@ handle_func_event(PyFunction_WatchEvent event, PyFunctionObject *func, PyObject
     }
 }
 
+static inline void
+handle_func_event(PyFunction_WatchEvent event, PyFunctionObject *func,
+                  PyObject *new_value)
+{
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    if (interp->active_func_watchers) {
+        notify_func_watchers(interp, event, func, new_value);
+    }
+}
+
 int
 PyFunction_AddWatcher(PyFunction_WatchCallback callback)
 {
@@ -28,6 +38,7 @@ PyFunction_AddWatcher(PyFunction_WatchCallback callback)
     for (int i = 0; i < FUNC_MAX_WATCHERS; i++) {
         if (interp->func_watchers[i] == NULL) {
             interp->func_watchers[i] = callback;
+            interp->active_func_watchers |= (1 << i);
             return i;
         }
     }
@@ -50,6 +61,7 @@ PyFunction_ClearWatcher(int watcher_id)
         return -1;
     }
     interp->func_watchers[watcher_id] = NULL;
+    interp->active_func_watchers &= ~(1 << watcher_id);
     return 0;
 }
 

Python/pystate.c

@@ -464,6 +464,7 @@ interpreter_clear(PyInterpreterState *interp, PyThreadState *tstate)
     for (int i=0; i < FUNC_MAX_WATCHERS; i++) {
         interp->func_watchers[i] = NULL;
     }
+    interp->active_func_watchers = 0;
 
     // XXX Once we have one allocator per interpreter (i.e.
     // per-interpreter GC) we must ensure that all of the interpreter's