bpo-40941: Unify implicit and explicit state in the frame and generator objects into a single value. (GH-20803)

* Merge gen and frame state variables into one.

* Replace stack pointer with depth in PyFrameObject. Makes code easier to read and saves a word of memory.

9 files changed, 155 insertions, 95 deletions

diff --git a/Include/cpython/frameobject.h b/Include/cpython/frameobject.h
index c76fbe0..63240b5 100644
--- a/Include/cpython/frameobject.h
+++ b/Include/cpython/frameobject.h
@@ -4,6 +4,21 @@
 #  error "this header file must not be included directly"
 #endif
 
+/* These values are chosen so that the inline functions below all
+ * compare f_state to zero.
+ */
+enum _framestate {
+    FRAME_CREATED = -2,
+    FRAME_SUSPENDED = -1,
+    FRAME_EXECUTING = 0,
+    FRAME_RETURNED = 1,
+    FRAME_UNWINDING = 2,
+    FRAME_RAISED = 3,
+    FRAME_CLEARED = 4
+};
+
+typedef signed char PyFrameState;
+
 typedef struct {
     int b_type;                 /* what kind of block this is */
     int b_handler;              /* where to jump to find handler */
@@ -18,11 +33,8 @@ struct _frame {
     PyObject *f_globals;        /* global symbol table (PyDictObject) */
     PyObject *f_locals;         /* local symbol table (any mapping) */
     PyObject **f_valuestack;    /* points after the last local */
-    /* Next free slot in f_valuestack.  Frame creation sets to f_valuestack.
-       Frame evaluation usually NULLs it, but a frame that yields sets it
-       to the current stack top. */
-    PyObject **f_stacktop;
     PyObject *f_trace;          /* Trace function */
+    int f_stackdepth;           /* Depth of value stack */
     char f_trace_lines;         /* Emit per-line trace events? */
     char f_trace_opcodes;       /* Emit per-opcode trace events? */
 
@@ -37,11 +49,22 @@ struct _frame {
        bytecode index. */
     int f_lineno;               /* Current line number */
     int f_iblock;               /* index in f_blockstack */
-    char f_executing;           /* whether the frame is still executing */
+    PyFrameState f_state;       /* What state the frame is in */
     PyTryBlock f_blockstack[CO_MAXBLOCKS]; /* for try and loop blocks */
     PyObject *f_localsplus[1];  /* locals+stack, dynamically sized */
 };
 
+static inline int _PyFrame_IsRunnable(struct _frame *f) {
+    return f->f_state < FRAME_EXECUTING;
+}
+
+static inline int _PyFrame_IsExecuting(struct _frame *f) {
+    return f->f_state == FRAME_EXECUTING;
+}
+
+static inline int _PyFrameHasCompleted(struct _frame *f) {
+    return f->f_state > FRAME_EXECUTING;
+}
 
 /* Standard object interface */
 
diff --git a/Include/genobject.h b/Include/genobject.h
index 8ffd156..a76dc92 100644
--- a/Include/genobject.h
+++ b/Include/genobject.h
@@ -16,8 +16,6 @@ extern "C" {
     PyObject_HEAD                                                           \
     /* Note: gi_frame can be NULL if the generator is "finished" */         \
     PyFrameObject *prefix##_frame;                                          \
-    /* True if generator is being executed. */                              \
-    char prefix##_running;                                                  \
     /* The code object backing the generator */                             \
     PyObject *prefix##_code;                                                \
     /* List of weak reference. */                                           \
diff --git a/Lib/test/test_generators.py b/Lib/test/test_generators.py
index 3bf1522..a634ccd 100644
--- a/Lib/test/test_generators.py
+++ b/Lib/test/test_generators.py
@@ -881,7 +881,7 @@ And more, added later.
 >>> i.gi_running = 42
 Traceback (most recent call last):
   ...
-AttributeError: readonly attribute
+AttributeError: attribute 'gi_running' of 'generator' objects is not writable
 >>> def g():
 ...     yield me.gi_running
 >>> me = g()
diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
index aaba663..7a5df46 100644
--- a/Lib/test/test_sys.py
+++ b/Lib/test/test_sys.py
@@ -1236,7 +1236,7 @@ class SizeofTest(unittest.TestCase):
         nfrees = len(x.f_code.co_freevars)
         extras = x.f_code.co_stacksize + x.f_code.co_nlocals +\
                   ncells + nfrees - 1
-        check(x, vsize('5P2c4P3ic' + CO_MAXBLOCKS*'3i' + 'P' + extras*'P'))
+        check(x, vsize('4Pi2c4P3ic' + CO_MAXBLOCKS*'3i' + 'P' + extras*'P'))
         # function
         def func(): pass
         check(func, size('13P'))
@@ -1253,7 +1253,7 @@ class SizeofTest(unittest.TestCase):
             check(bar, size('PP'))
         # generator
         def get_gen(): yield 1
-        check(get_gen(), size('Pb2PPP4P'))
+        check(get_gen(), size('P2PPP4P'))
         # iterator
         check(iter('abc'), size('lP'))
         # callable-iterator
diff --git a/Lib/test/test_yield_from.py b/Lib/test/test_yield_from.py
index 4735ef4..d105d8c 100644
--- a/Lib/test/test_yield_from.py
+++ b/Lib/test/test_yield_from.py
@@ -938,6 +938,9 @@ class TestPEP380Operation(unittest.TestCase):
                 res.append(g1.throw(MyErr))
         except StopIteration:
             pass
+        except:
+            self.assertEqual(res, [0, 1, 2, 3])
+            raise
         # Check with close
         class MyIt(object):
             def __iter__(self):
diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index 8a6fce9..8943ebf 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -1847,7 +1847,7 @@ _is_running(PyInterpreterState *interp)
         return 0;
     }
 
-    int executing = (int)(frame->f_executing);
+    int executing = _PyFrame_IsExecuting(frame);
     Py_DECREF(frame);
 
     return executing;
diff --git a/Objects/frameobject.c b/Objects/frameobject.c
index 7c2bce3..8838b80 100644
--- a/Objects/frameobject.c
+++ b/Objects/frameobject.c
@@ -300,17 +300,20 @@ first_line_not_before(int *lines, int len, int line)
 static void
 frame_stack_pop(PyFrameObject *f)
 {
-    PyObject *v = (*--f->f_stacktop);
+    assert(f->f_stackdepth >= 0);
+    f->f_stackdepth--;
+    PyObject *v = f->f_valuestack[f->f_stackdepth];
     Py_DECREF(v);
 }
 
 static void
 frame_block_unwind(PyFrameObject *f)
 {
+    assert(f->f_stackdepth >= 0);
     assert(f->f_iblock > 0);
     f->f_iblock--;
     PyTryBlock *b = &f->f_blockstack[f->f_iblock];
-    intptr_t delta = (f->f_stacktop - f->f_valuestack) - b->b_level;
+    intptr_t delta = f->f_stackdepth - b->b_level;
     while (delta > 0) {
         frame_stack_pop(f);
         delta--;
@@ -352,33 +355,36 @@ frame_setlineno(PyFrameObject *f, PyObject* p_new_lineno, void *Py_UNUSED(ignore
         return -1;
     }
 
-    /* Upon the 'call' trace event of a new frame, f->f_lasti is -1 and
-     * f->f_trace is NULL, check first on the first condition.
-     * Forbidding jumps from the 'call' event of a new frame is a side effect
-     * of allowing to set f_lineno only from trace functions. */
-    if (f->f_lasti == -1) {
-        PyErr_Format(PyExc_ValueError,
+    /*
+     * This code preserves the historical restrictions on
+     * setting the line number of a frame.
+     * Jumps are forbidden on a 'return' trace event (except after a yield).
+     * Jumps from 'call' trace events are also forbidden.
+     * In addition, jumps are forbidden when not tracing,
+     * as this is a debugging feature.
+     */
+    switch(f->f_state) {
+        case FRAME_CREATED:
+            PyErr_Format(PyExc_ValueError,
                      "can't jump from the 'call' trace event of a new frame");
-        return -1;
-    }
-
-    /* You can only do this from within a trace function, not via
-     * _getframe or similar hackery. */
-    if (!f->f_trace) {
-        PyErr_Format(PyExc_ValueError,
-                     "f_lineno can only be set by a trace function");
-        return -1;
-    }
-
-    /* Forbid jumps upon a 'return' trace event (except after executing a
-     * YIELD_VALUE or YIELD_FROM opcode, f_stacktop is not NULL in that case)
-     * and upon an 'exception' trace event.
-     * Jumps from 'call' trace events have already been forbidden above for new
-     * frames, so this check does not change anything for 'call' events. */
-    if (f->f_stacktop == NULL) {
-        PyErr_SetString(PyExc_ValueError,
+            return -1;
+        case FRAME_RETURNED:
+        case FRAME_UNWINDING:
+        case FRAME_RAISED:
+        case FRAME_CLEARED:
+            PyErr_SetString(PyExc_ValueError,
                 "can only jump from a 'line' trace event");
-        return -1;
+            return -1;
+        case FRAME_EXECUTING:
+        case FRAME_SUSPENDED:
+            /* You can only do this from within a trace function, not via
+            * _getframe or similar hackery. */
+            if (!f->f_trace) {
+                PyErr_Format(PyExc_ValueError,
+                            "f_lineno can only be set by a trace function");
+                return -1;
+            }
+            break;
     }
 
     int new_lineno;
@@ -585,11 +591,10 @@ frame_dealloc(PyFrameObject *f)
     }
 
     /* Free stack */
-    if (f->f_stacktop != NULL) {
-        for (PyObject **p = valuestack; p < f->f_stacktop; p++) {
-            Py_XDECREF(*p);
-        }
+    for (int i = 0; i < f->f_stackdepth; i++) {
+        Py_XDECREF(f->f_valuestack[i]);
     }
+    f->f_stackdepth = 0;
 
     Py_XDECREF(f->f_back);
     Py_DECREF(f->f_builtins);
@@ -647,10 +652,8 @@ frame_traverse(PyFrameObject *f, visitproc visit, void *arg)
     }
 
     /* stack */
-    if (f->f_stacktop != NULL) {
-        for (PyObject **p = f->f_valuestack; p < f->f_stacktop; p++) {
-            Py_VISIT(*p);
-        }
+    for (int i = 0; i < f->f_stackdepth; i++) {
+        Py_VISIT(f->f_valuestack[i]);
     }
     return 0;
 }
@@ -663,9 +666,7 @@ frame_tp_clear(PyFrameObject *f)
      * frame may also point to this frame, believe itself to still be
      * active, and try cleaning up this frame again.
      */
-    PyObject **oldtop = f->f_stacktop;
-    f->f_stacktop = NULL;
-    f->f_executing = 0;
+    f->f_state = FRAME_CLEARED;
 
     Py_CLEAR(f->f_trace);
 
@@ -676,18 +677,17 @@ frame_tp_clear(PyFrameObject *f)
     }
 
     /* stack */
-    if (oldtop != NULL) {
-        for (PyObject **p = f->f_valuestack; p < oldtop; p++) {
-            Py_CLEAR(*p);
-        }
+    for (int i = 0; i < f->f_stackdepth; i++) {
+        Py_CLEAR(f->f_valuestack[i]);
     }
+    f->f_stackdepth = 0;
     return 0;
 }
 
 static PyObject *
 frame_clear(PyFrameObject *f, PyObject *Py_UNUSED(ignored))
 {
-    if (f->f_executing) {
+    if (_PyFrame_IsExecuting(f)) {
         PyErr_SetString(PyExc_RuntimeError,
                         "cannot clear an executing frame");
         return NULL;
@@ -898,7 +898,7 @@ _PyFrame_New_NoTrack(PyThreadState *tstate, PyCodeObject *code,
         return NULL;
     }
 
-    f->f_stacktop = f->f_valuestack;
+    f->f_stackdepth = 0;
     f->f_builtins = builtins;
     Py_XINCREF(back);
     f->f_back = back;
@@ -927,7 +927,7 @@ _PyFrame_New_NoTrack(PyThreadState *tstate, PyCodeObject *code,
     f->f_lasti = -1;
     f->f_lineno = code->co_firstlineno;
     f->f_iblock = 0;
-    f->f_executing = 0;
+    f->f_state = FRAME_CREATED;
     f->f_gen = NULL;
     f->f_trace_opcodes = 0;
     f->f_trace_lines = 1;
diff --git a/Objects/genobject.c b/Objects/genobject.c
index a379fa6..809838a 100644
--- a/Objects/genobject.c
+++ b/Objects/genobject.c
@@ -47,7 +47,7 @@ _PyGen_Finalize(PyObject *self)
     PyObject *res = NULL;
     PyObject *error_type, *error_value, *error_traceback;
 
-    if (gen->gi_frame == NULL || gen->gi_frame->f_stacktop == NULL) {
+    if (gen->gi_frame == NULL ||  _PyFrameHasCompleted(gen->gi_frame)) {
         /* Generator isn't paused, so no need to close */
         return;
     }
@@ -143,7 +143,7 @@ gen_send_ex(PyGenObject *gen, PyObject *arg, int exc, int closing)
     PyFrameObject *f = gen->gi_frame;
     PyObject *result;
 
-    if (gen->gi_running) {
+    if (f != NULL && _PyFrame_IsExecuting(f)) {
         const char *msg = "generator already executing";
         if (PyCoro_CheckExact(gen)) {
             msg = "coroutine already executing";
@@ -154,7 +154,7 @@ gen_send_ex(PyGenObject *gen, PyObject *arg, int exc, int closing)
         PyErr_SetString(PyExc_ValueError, msg);
         return NULL;
     }
-    if (f == NULL || f->f_stacktop == NULL) {
+    if (f == NULL || _PyFrameHasCompleted(f)) {
         if (PyCoro_CheckExact(gen) && !closing) {
             /* `gen` is an exhausted coroutine: raise an error,
                except when called from gen_close(), which should
@@ -176,6 +176,7 @@ gen_send_ex(PyGenObject *gen, PyObject *arg, int exc, int closing)
         return NULL;
     }
 
+    assert(_PyFrame_IsRunnable(f));
     if (f->f_lasti == -1) {
         if (arg && arg != Py_None) {
             const char *msg = "can't send non-None value to a "
@@ -194,7 +195,8 @@ gen_send_ex(PyGenObject *gen, PyObject *arg, int exc, int closing)
         /* Push arg onto the frame's value stack */
         result = arg ? arg : Py_None;
         Py_INCREF(result);
-        *(f->f_stacktop++) = result;
+        gen->gi_frame->f_valuestack[gen->gi_frame->f_stackdepth] = result;
+        gen->gi_frame->f_stackdepth++;
     }
 
     /* Generators always return to their most recent caller, not
@@ -203,7 +205,6 @@ gen_send_ex(PyGenObject *gen, PyObject *arg, int exc, int closing)
     assert(f->f_back == NULL);
     f->f_back = tstate->frame;
 
-    gen->gi_running = 1;
     gen->gi_exc_state.previous_item = tstate->exc_info;
     tstate->exc_info = &gen->gi_exc_state;
 
@@ -215,7 +216,6 @@ gen_send_ex(PyGenObject *gen, PyObject *arg, int exc, int closing)
     result = _PyEval_EvalFrame(tstate, f, exc);
     tstate->exc_info = gen->gi_exc_state.previous_item;
     gen->gi_exc_state.previous_item = NULL;
-    gen->gi_running = 0;
 
     /* Don't keep the reference to f_back any longer than necessary.  It
      * may keep a chain of frames alive or it could create a reference
@@ -225,7 +225,7 @@ gen_send_ex(PyGenObject *gen, PyObject *arg, int exc, int closing)
 
     /* If the generator just returned (as opposed to yielding), signal
      * that the generator is exhausted. */
-    if (result && f->f_stacktop == NULL) {
+    if (result &&  _PyFrameHasCompleted(f)) {
         if (result == Py_None) {
             /* Delay exception instantiation if we can */
             if (PyAsyncGen_CheckExact(gen)) {
@@ -264,7 +264,7 @@ gen_send_ex(PyGenObject *gen, PyObject *arg, int exc, int closing)
         _PyErr_FormatFromCause(PyExc_RuntimeError, "%s", msg);
     }
 
-    if (!result || f->f_stacktop == NULL) {
+    if (!result ||  _PyFrameHasCompleted(f)) {
         /* generator can't be rerun, so release the frame */
         /* first clean reference cycle through stored exception traceback */
         _PyErr_ClearExcState(&gen->gi_exc_state);
@@ -327,7 +327,7 @@ _PyGen_yf(PyGenObject *gen)
     PyObject *yf = NULL;
     PyFrameObject *f = gen->gi_frame;
 
-    if (f && f->f_stacktop) {
+    if (f) {
         PyObject *bytecode = f->f_code->co_code;
         unsigned char *code = (unsigned char *)PyBytes_AS_STRING(bytecode);
 
@@ -341,7 +341,8 @@ _PyGen_yf(PyGenObject *gen)
 
         if (code[f->f_lasti + sizeof(_Py_CODEUNIT)] != YIELD_FROM)
             return NULL;
-        yf = f->f_stacktop[-1];
+        assert(f->f_stackdepth > 0);
+        yf = f->f_valuestack[f->f_stackdepth-1];
         Py_INCREF(yf);
     }
 
@@ -356,9 +357,10 @@ gen_close(PyGenObject *gen, PyObject *args)
     int err = 0;
 
     if (yf) {
-        gen->gi_running = 1;
+        PyFrameState state = gen->gi_frame->f_state;
+        gen->gi_frame->f_state = FRAME_EXECUTING;
         err = gen_close_iter(yf);
-        gen->gi_running = 0;
+        gen->gi_frame->f_state = state;
         Py_DECREF(yf);
     }
     if (err == 0)
@@ -405,9 +407,10 @@ _gen_throw(PyGenObject *gen, int close_on_genexit,
                We have to allow some awaits to work it through, hence the
                `close_on_genexit` parameter here.
             */
-            gen->gi_running = 1;
+            PyFrameState state = gen->gi_frame->f_state;
+            gen->gi_frame->f_state = FRAME_EXECUTING;
             err = gen_close_iter(yf);
-            gen->gi_running = 0;
+            gen->gi_frame->f_state = state;
             Py_DECREF(yf);
             if (err < 0)
                 return gen_send_ex(gen, Py_None, 1, 0);
@@ -418,7 +421,6 @@ _gen_throw(PyGenObject *gen, int close_on_genexit,
             PyThreadState *tstate = _PyThreadState_GET();
             PyFrameObject *f = tstate->frame;
 
-            gen->gi_running = 1;
             /* Since we are fast-tracking things by skipping the eval loop,
                we need to update the current frame so the stack trace
                will be reported correctly to the user. */
@@ -427,10 +429,12 @@ _gen_throw(PyGenObject *gen, int close_on_genexit,
             tstate->frame = gen->gi_frame;
             /* Close the generator that we are currently iterating with
                'yield from' or awaiting on with 'await'. */
+            PyFrameState state = gen->gi_frame->f_state;
+            gen->gi_frame->f_state = FRAME_EXECUTING;
             ret = _gen_throw((PyGenObject *)yf, close_on_genexit,
                              typ, val, tb);
+            gen->gi_frame->f_state = state;
             tstate->frame = f;
-            gen->gi_running = 0;
         } else {
             /* `yf` is an iterator or a coroutine-like object. */
             PyObject *meth;
@@ -442,16 +446,19 @@ _gen_throw(PyGenObject *gen, int close_on_genexit,
                 Py_DECREF(yf);
                 goto throw_here;
             }
-            gen->gi_running = 1;
+            PyFrameState state = gen->gi_frame->f_state;
+            gen->gi_frame->f_state = FRAME_EXECUTING;
             ret = PyObject_CallFunctionObjArgs(meth, typ, val, tb, NULL);
-            gen->gi_running = 0;
+            gen->gi_frame->f_state = state;
             Py_DECREF(meth);
         }
         Py_DECREF(yf);
         if (!ret) {
             PyObject *val;
             /* Pop subiterator from stack */
-            ret = *(--gen->gi_frame->f_stacktop);
+            assert(gen->gi_frame->f_stackdepth > 0);
+            gen->gi_frame->f_stackdepth--;
+            ret = gen->gi_frame->f_valuestack[gen->gi_frame->f_stackdepth];
             assert(ret == yf);
             Py_DECREF(ret);
             /* Termination repetition of YIELD_FROM */
@@ -701,6 +708,16 @@ gen_getyieldfrom(PyGenObject *gen, void *Py_UNUSED(ignored))
     return yf;
 }
 
+
+static PyObject *
+gen_getrunning(PyGenObject *gen, void *Py_UNUSED(ignored))
+{
+    if (gen->gi_frame == NULL) {
+        Py_RETURN_FALSE;
+    }
+    return PyBool_FromLong(_PyFrame_IsExecuting(gen->gi_frame));
+}
+
 static PyGetSetDef gen_getsetlist[] = {
     {"__name__", (getter)gen_get_name, (setter)gen_set_name,
      PyDoc_STR("name of the generator")},
@@ -708,12 +725,12 @@ static PyGetSetDef gen_getsetlist[] = {
      PyDoc_STR("qualified name of the generator")},
     {"gi_yieldfrom", (getter)gen_getyieldfrom, NULL,
      PyDoc_STR("object being iterated by yield from, or None")},
+    {"gi_running", (getter)gen_getrunning, NULL, NULL},
     {NULL} /* Sentinel */
 };
 
 static PyMemberDef gen_memberlist[] = {
     {"gi_frame",     T_OBJECT, offsetof(PyGenObject, gi_frame),    READONLY},
-    {"gi_running",   T_BOOL,   offsetof(PyGenObject, gi_running),  READONLY},
     {"gi_code",      T_OBJECT, offsetof(PyGenObject, gi_code),     READONLY},
     {NULL}      /* Sentinel */
 };
@@ -791,7 +808,6 @@ gen_new_with_qualname(PyTypeObject *type, PyFrameObject *f,
     f->f_gen = (PyObject *) gen;
     Py_INCREF(f->f_code);
     gen->gi_code = (PyObject *)(f->f_code);
-    gen->gi_running = 0;
     gen->gi_weakreflist = NULL;
     gen->gi_exc_state.exc_type = NULL;
     gen->gi_exc_state.exc_value = NULL;
@@ -921,6 +937,15 @@ coro_get_cr_await(PyCoroObject *coro, void *Py_UNUSED(ignored))
     return yf;
 }
 
+static PyObject *
+cr_getrunning(PyCoroObject *coro, void *Py_UNUSED(ignored))
+{
+    if (coro->cr_frame == NULL) {
+        Py_RETURN_FALSE;
+    }
+    return PyBool_FromLong(_PyFrame_IsExecuting(coro->cr_frame));
+}
+
 static PyGetSetDef coro_getsetlist[] = {
     {"__name__", (getter)gen_get_name, (setter)gen_set_name,
      PyDoc_STR("name of the coroutine")},
@@ -928,12 +953,12 @@ static PyGetSetDef coro_getsetlist[] = {
      PyDoc_STR("qualified name of the coroutine")},
     {"cr_await", (getter)coro_get_cr_await, NULL,
      PyDoc_STR("object being awaited on, or None")},
+    {"cr_running", (getter)cr_getrunning, NULL, NULL},
     {NULL} /* Sentinel */
 };
 
 static PyMemberDef coro_memberlist[] = {
     {"cr_frame",     T_OBJECT, offsetof(PyCoroObject, cr_frame),    READONLY},
-    {"cr_running",   T_BOOL,   offsetof(PyCoroObject, cr_running),  READONLY},
     {"cr_code",      T_OBJECT, offsetof(PyCoroObject, cr_code),     READONLY},
     {"cr_origin",    T_OBJECT, offsetof(PyCoroObject, cr_origin),   READONLY},
     {NULL}      /* Sentinel */
@@ -1828,7 +1853,7 @@ async_gen_athrow_send(PyAsyncGenAThrow *o, PyObject *arg)
         return NULL;
     }
 
-    if (f == NULL || f->f_stacktop == NULL) {
+    if (f == NULL || _PyFrameHasCompleted(f)) {
         o->agt_state = AWAITABLE_STATE_CLOSED;
         PyErr_SetNone(PyExc_StopIteration);
         return NULL;
diff --git a/Python/ceval.c b/Python/ceval.c
index 0386929..f747faa 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -1349,10 +1349,15 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, PyFrameObject *f, int throwflag)
         assert(f->f_lasti % sizeof(_Py_CODEUNIT) == 0);
         next_instr += f->f_lasti / sizeof(_Py_CODEUNIT) + 1;
     }
-    stack_pointer = f->f_stacktop;
-    assert(stack_pointer != NULL);
-    f->f_stacktop = NULL;       /* remains NULL unless yield suspends frame */
-    f->f_executing = 1;
+    stack_pointer = f->f_valuestack + f->f_stackdepth;
+    /* Set f->f_stackdepth to -1.
+     * Update when returning or calling trace function.
+       Having f_stackdepth <= 0 ensures that invalid
+       values are not visible to the cycle GC.
+       We choose -1 rather than 0 to assist debugging.
+     */
+    f->f_stackdepth = -1;
+    f->f_state = FRAME_EXECUTING;
 
     if (co->co_opcache_flag < OPCACHE_MIN_RUNS) {
         co->co_opcache_flag++;
@@ -1440,7 +1445,7 @@ main_loop:
             int err;
             /* see maybe_call_line_trace
                for expository comments */
-            f->f_stacktop = stack_pointer;
+            f->f_stackdepth = stack_pointer-f->f_valuestack;
 
             err = maybe_call_line_trace(tstate->c_tracefunc,
                                         tstate->c_traceobj,
@@ -1448,10 +1453,8 @@ main_loop:
                                         &instr_lb, &instr_ub, &instr_prev);
             /* Reload possibly changed frame fields */
             JUMPTO(f->f_lasti);
-            if (f->f_stacktop != NULL) {
-                stack_pointer = f->f_stacktop;
-                f->f_stacktop = NULL;
-            }
+            stack_pointer = f->f_valuestack+f->f_stackdepth;
+            f->f_stackdepth = -1;
             if (err)
                 /* trace function raised an exception */
                 goto error;
@@ -2076,6 +2079,8 @@ main_loop:
             retval = POP();
             assert(f->f_iblock == 0);
             assert(EMPTY());
+            f->f_state = FRAME_RETURNED;
+            f->f_stackdepth = 0;
             goto exiting;
         }
 
@@ -2242,10 +2247,11 @@ main_loop:
                 DISPATCH();
             }
             /* receiver remains on stack, retval is value to be yielded */
-            f->f_stacktop = stack_pointer;
             /* and repeat... */
             assert(f->f_lasti >= (int)sizeof(_Py_CODEUNIT));
             f->f_lasti -= sizeof(_Py_CODEUNIT);
+            f->f_state = FRAME_SUSPENDED;
+            f->f_stackdepth = stack_pointer-f->f_valuestack;
             goto exiting;
         }
 
@@ -2261,8 +2267,8 @@ main_loop:
                 }
                 retval = w;
             }
-
-            f->f_stacktop = stack_pointer;
+            f->f_state = FRAME_SUSPENDED;
+            f->f_stackdepth = stack_pointer-f->f_valuestack;
             goto exiting;
         }
 
@@ -3762,11 +3768,15 @@ error:
         /* Log traceback info. */
         PyTraceBack_Here(f);
 
-        if (tstate->c_tracefunc != NULL)
+        if (tstate->c_tracefunc != NULL) {
+            /* Make sure state is set to FRAME_EXECUTING for tracing */
+            assert(f->f_state == FRAME_EXECUTING);
+            f->f_state = FRAME_UNWINDING;
             call_exc_trace(tstate->c_tracefunc, tstate->c_traceobj,
                            tstate, f);
-
+        }
 exception_unwind:
+        f->f_state = FRAME_UNWINDING;
         /* Unwind stacks if an exception occurred */
         while (f->f_iblock > 0) {
             /* Pop the current block. */
@@ -3825,6 +3835,7 @@ exception_unwind:
                     }
                 }
                 /* Resume normal execution */
+                f->f_state = FRAME_EXECUTING;
                 goto main_loop;
             }
         } /* unwind stack */
@@ -3841,7 +3852,8 @@ exception_unwind:
         PyObject *o = POP();
         Py_XDECREF(o);
     }
-
+    f->f_stackdepth = 0;
+    f->f_state = FRAME_RAISED;
 exiting:
     if (tstate->use_tracing) {
         if (tstate->c_tracefunc) {
@@ -3863,7 +3875,6 @@ exit_eval_frame:
     if (PyDTrace_FUNCTION_RETURN_ENABLED())
         dtrace_function_return(f);
     _Py_LeaveRecursiveCall(tstate);
-    f->f_executing = 0;
     tstate->frame = f->f_back;
 
     return _Py_CheckFunctionResult(tstate, NULL, retval, __func__);