Remove some invariant conditions and assert in ceval

[nnorwitz]

BPO	1145039
Nosy	@tim-one
Files	reset_exc.patch: 1

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2006-05-26.23:17:23.000>
created_at = <Date 2005-02-20.21:31:32.000>
labels = ['interpreter-core']
title = 'Remove some invariant conditions and assert in ceval'
updated_at = <Date 2006-05-26.23:17:23.000>
user = 'https://bugs.python.org/nnorwitz'

bugs.python.org fields:

activity = <Date 2006-05-26.23:17:23.000>
actor = 'tim.peters'
assignee = 'none'
closed = True
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2005-02-20.21:31:32.000>
creator = 'nnorwitz'
dependencies = []
files = ['6514']
hgrepos = []
issue_num = 1145039
keywords = ['patch']
message_count = 3.0
messages = ['47837', '47838', '47839']
nosy_count = 2.0
nosy_names = ['tim.peters', 'nnorwitz']
pr_nums = []
priority = 'normal'
resolution = 'accepted'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue1145039'
versions = ['Python 2.5']

[nnorwitz]

ISTM that if frame->f_exc_type == NULL then exc_value
and exc_traceback will also be NULL. I didn't see that
this is documented, perhaps I missed it or there is
some case when this can occur. If it can occur, we
shoul develop a test for it.

Assuming this condition is invariant, some
simplifications can be made in reset_exc_info which is
called once per eval_frame (on function exit).

Also, I think there is currently an extra Py_INCREF on
Py_None. This occurs when tstate->exc_type == NULL.

This patch seems to have little to no effect on
performance. I did measure a 0.3% speed improvement.

[tim-one]

Logged In: YES
user_id=31435

Note that the patch is out of date.

I agree the invariant you deduced should hold, but in fact
it doesn't now, at least due to insane initialization
problems in exceptions.c:

http://mail.python.org/pipermail/python-dev/2006-May/065248.html

I'd like to ensure & exploit a stronger invariant:

http://mail.python.org/pipermail/python-dev/2006-May/065231.html

but that's stuck for now. I put my work in progress on a
new branch:

svn+ssh://svn.python.org/python/branches/tim-exc_sanity

BTW, I don't agree that the incref on Py_None wasn't needed.
Py_None is getting assigned to two new pointers
(tstate->exc_type and frame->f_exc_type), so should be
incremented twice.

[tim-one]

Logged In: YES
user_id=31435

I got a very tiny bit more out of this and added it to the
trunk. Thanks! Note that I left in the disputed Py_INCREF.

The more ambitious tim-exc_sanity branch is looking like
more trouble than it's worth.