PyFrameObject: rename f_stackbottom to f_stacktop, since it points to

the next free valuestack slot, not to the base (in America, stacks push
and pop at the top -- they mutate at the bottom in Australia <winK>).
eval_frame():  assert that f_stacktop isn't NULL upon entry.
frame_delloc():  avoid ordered pointer comparisons involving f_stacktop
when f_stacktop is NULL.

Include/frameobject.h

@@ -21,8 +21,10 @@ typedef struct _frame {
     PyObject *f_globals;	/* global symbol table (PyDictObject) */
     PyObject *f_locals;		/* local symbol table (PyDictObject) */
     PyObject **f_valuestack;	/* points after the last local */
-    PyObject **f_stackbottom;   /* points to the last item on the stack if
-                                  frame has yielded. */
+    /* 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 */
     PyObject *f_exc_type, *f_exc_value, *f_exc_traceback;
     PyThreadState *f_tstate;

Objects/frameobject.c

@@ -78,9 +78,11 @@ frame_dealloc(PyFrameObject *f)
 	}
 
 	/* Free stack */
-	for (p = f->f_valuestack; p < f->f_stackbottom; p++) {
-		Py_XDECREF(*p);
+	if (f->f_stacktop != NULL) {
+		for (p = f->f_valuestack; p < f->f_stacktop; p++)
+			Py_XDECREF(*p);
 	}
+	
 	Py_XDECREF(f->f_back);
 	Py_XDECREF(f->f_code);
 	Py_XDECREF(f->f_builtins);
@@ -226,7 +228,7 @@ PyFrame_New(PyThreadState *tstate, PyCodeObject *code, PyObject *globals,
 		f->f_localsplus[extras] = NULL;
 
 	f->f_valuestack = f->f_localsplus + (f->f_nlocals + ncells + nfrees);
-	f->f_stackbottom = f->f_valuestack;
+	f->f_stacktop = f->f_valuestack;
 
 	return f;
 }

Python/ceval.c

@@ -147,9 +147,8 @@ gen_iternext(genobject *gen)
 				"generator already executing");
 		return NULL;
 	}
-	if (f->f_stackbottom == NULL) {
+	if (f->f_stacktop == NULL)
 		return NULL;
-	}
 
 	/* Generators always return to their most recent caller, not
 	 * necessarily their creator. */
@@ -584,8 +583,9 @@ eval_frame(PyFrameObject *f)
 	freevars = f->f_localsplus + f->f_nlocals;
 	_PyCode_GETCODEPTR(co, &first_instr);
 	next_instr = first_instr + f->f_lasti;
-	stack_pointer = f->f_stackbottom;
-	f->f_stackbottom = NULL;
+	stack_pointer = f->f_stacktop;
+	assert(stack_pointer != NULL);
+	f->f_stacktop = NULL;
 
 #ifdef LLTRACE
 	lltrace = PyDict_GetItemString(f->f_globals,"__lltrace__") != NULL;
@@ -1371,7 +1371,7 @@ eval_frame(PyFrameObject *f)
 
 		case YIELD_VALUE:
 			retval = POP();
-			f->f_stackbottom = stack_pointer;
+			f->f_stacktop = stack_pointer;
 			f->f_lasti = INSTR_OFFSET();
 			why = WHY_YIELD;
 			break;