Patch 1145039.

set_exc_info(), reset_exc_info():  By exploiting the
likely (who knows?) invariant that when an exception's
`type` is NULL, its `value` and `traceback` are also NULL,
save some cycles in heavily-executed code.

This is a "a kronar saved is a kronar earned" patch:  the
speedup isn't reliably measurable, but it obviously does
reduce the operation count in the normal (no exception
raised) path through PyEval_EvalFrameEx().

The tim-exc_sanity branch tries to push this harder, but
is still blowing up (at least in part due to pre-existing
subtle bugs that appear to have no other visible
consequences!).

Not a bugfix candidate.

Include/frameobject.h

@@ -26,7 +26,16 @@ typedef struct _frame {
        to the current stack top. */
     PyObject **f_stacktop;
     PyObject *f_trace;		/* Trace function */
+
+    /* If an exception is raised in this frame, the next three are used to
+     * record the exception info (if any) originally in the thread state.  See
+     * comments before set_exc_info() -- it's not obvious.
+     * Invariant:  if _type is NULL, then so are _value and _traceback.
+     * Desired invariant:  all three are NULL, or all three are non-NULL.  That
+     * one isn't currently true, but "should be".
+     */
     PyObject *f_exc_type, *f_exc_value, *f_exc_traceback;
+
     PyThreadState *f_tstate;
     int f_lasti;		/* Last instruction if called */
     /* As of 2.3 f_lineno is only valid when tracing is active (i.e. when

Python/ceval.c

@@ -2581,7 +2581,12 @@ fast_yield:
 		}
 	}
 
+	if (tstate->frame->f_exc_type != NULL)
 		reset_exc_info(tstate);
+	else {
+		assert(tstate->frame->f_exc_value == NULL);
+		assert(tstate->frame->f_exc_traceback == NULL);
+	}
 
 	/* pop frame */
     exit_eval_frame:
@@ -2846,6 +2851,7 @@ PyEval_EvalCodeEx(PyCodeObject *co, PyObject *globals, PyObject *locals,
 - Once an exception is caught by an except clause, it is transferred
   from tstate->curexc_ZZZ to tstate->exc_ZZZ, from which sys.exc_info()
   can pick it up.  This is the primary task of set_exc_info().
+  XXX That can't be right:  set_exc_info() doesn't look at tstate->curexc_ZZZ.
 
 - Now let me explain the complicated dance with frame->f_exc_ZZZ.
 
@@ -2900,33 +2906,33 @@ Py_LOCAL(void)
 set_exc_info(PyThreadState *tstate,
 	     PyObject *type, PyObject *value, PyObject *tb)
 {
-	PyFrameObject *frame;
+	PyFrameObject *frame = tstate->frame;
 	PyObject *tmp_type, *tmp_value, *tmp_tb;
 
-	frame = tstate->frame;
+	assert(type != NULL);
+	assert(frame != NULL);
 	if (frame->f_exc_type == NULL) {
-		/* This frame didn't catch an exception before */
-		/* Save previous exception of this thread in this frame */
+		assert(frame->f_exc_value == NULL);
+		assert(frame->f_exc_traceback == NULL);
+		/* This frame didn't catch an exception before. */
+		/* Save previous exception of this thread in this frame. */
 		if (tstate->exc_type == NULL) {
+			/* XXX Why is this set to Py_None? */
 			Py_INCREF(Py_None);
 			tstate->exc_type = Py_None;
 		}
-		tmp_value = frame->f_exc_value;
-		tmp_tb = frame->f_exc_traceback;
-		Py_XINCREF(tstate->exc_type);
+		Py_INCREF(tstate->exc_type);
 		Py_XINCREF(tstate->exc_value);
 		Py_XINCREF(tstate->exc_traceback);
 		frame->f_exc_type = tstate->exc_type;
 		frame->f_exc_value = tstate->exc_value;
 		frame->f_exc_traceback = tstate->exc_traceback;
-		Py_XDECREF(tmp_value);
-		Py_XDECREF(tmp_tb);
 	}
-	/* Set new exception for this thread */
+	/* Set new exception for this thread. */
 	tmp_type = tstate->exc_type;
 	tmp_value = tstate->exc_value;
 	tmp_tb = tstate->exc_traceback;
-	Py_XINCREF(type);
+	Py_INCREF(type);
 	Py_XINCREF(value);
 	Py_XINCREF(tb);
 	tstate->exc_type = type;
@@ -2946,9 +2952,16 @@ reset_exc_info(PyThreadState *tstate)
 {
 	PyFrameObject *frame;
 	PyObject *tmp_type, *tmp_value, *tmp_tb;
+
+	/* It's a precondition that the thread state's frame caught an
+	 * exception -- verify in a debug build.
+	 */
+	assert(tstate != NULL);
 	frame = tstate->frame;
-	if (frame->f_exc_type != NULL) {
-		/* This frame caught an exception */
+	assert(frame != NULL);
+	assert(frame->f_exc_type != NULL);
+
+	/* Copy the frame's exception info back to the thread state. */
 	tmp_type = tstate->exc_type;
 	tmp_value = tstate->exc_value;
 	tmp_tb = tstate->exc_traceback;
@@ -2961,18 +2974,20 @@ reset_exc_info(PyThreadState *tstate)
 	Py_XDECREF(tmp_type);
 	Py_XDECREF(tmp_value);
 	Py_XDECREF(tmp_tb);
+
 	/* For b/w compatibility */
 	PySys_SetObject("exc_type", frame->f_exc_type);
 	PySys_SetObject("exc_value", frame->f_exc_value);
 	PySys_SetObject("exc_traceback", frame->f_exc_traceback);
-	}
+
+	/* Clear the frame's exception info. */
 	tmp_type = frame->f_exc_type;
 	tmp_value = frame->f_exc_value;
 	tmp_tb = frame->f_exc_traceback;
 	frame->f_exc_type = NULL;
 	frame->f_exc_value = NULL;
 	frame->f_exc_traceback = NULL;
-	Py_XDECREF(tmp_type);
+	Py_DECREF(tmp_type);
 	Py_XDECREF(tmp_value);
 	Py_XDECREF(tmp_tb);
 }