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.

1 files changed, 9 insertions, 0 deletions

diff --git a/Include/frameobject.h b/Include/frameobject.h
index 1e3a01e168..cce598bee2 100644
--- a/Include/frameobject.h
+++ b/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