Optimize LOAD_NAME bytecode

[vstinner]

BPO	25557
Nosy	@rhettinger, @vstinner, @benjaminp, @serhiy-storchaka
Files	pydict_loadname.patch pydict_loadname-2.patch

^{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 2015-11-20.08:28:57.355>
created_at = <Date 2015-11-05.10:11:19.444>
labels = ['interpreter-core', 'performance']
title = 'Optimize LOAD_NAME bytecode'
updated_at = <Date 2015-11-20.08:58:24.586>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2015-11-20.08:58:24.586>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2015-11-20.08:28:57.355>
closer = 'vstinner'
components = ['Interpreter Core']
creation = <Date 2015-11-05.10:11:19.444>
creator = 'vstinner'
dependencies = []
files = ['40950', '40954']
hgrepos = []
issue_num = 25557
keywords = ['patch']
message_count = 7.0
messages = ['254097', '254101', '254102', '254112', '254956', '254957', '254959']
nosy_count = 5.0
nosy_names = ['rhettinger', 'vstinner', 'benjamin.peterson', 'python-dev', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'patch review'
status = 'closed'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue25557'
versions = ['Python 3.6']

[vstinner]

The LOAD_GLOBAL bytecode has a fast-path when globals and builtins have exactly the type 'dict'. It calls the _PyDict_LoadGlobal() function.

I propose to implement a similar optimization for LOAD_NAME, see attached patch.

The patch also fixes LOAD_GLOBAL and LOAD_NAME bytecodes when locals, globals or builtins are not exactly the type 'dict'. It clears the KeyError before trying the next PyObject_GetItem().

The patch changes also _PyDict_LoadGlobal() to call PyObject_Hash() if the hash was not computed yet. It might make it a little bit faster.

[serhiy-storchaka]

When LOAD_NAME is generated? Is it worth to optimize this case? Aren't LOAD_FAST and LOAD_GLOBAL used in performance critical code?

It looks to me that there is a bug in fast path of _PyDict_LoadGlobal. If the first lookup fails, it can raise an exception. We have to add

if (PyErr_Occurred())
    return NULL;

before the second lookup.

Just for reference, the fast path for LOAD_GLOBAL was added in 8f8fe990e82c.

[vstinner]

When LOAD_NAME is generated? Is it worth to optimize this case? Aren't LOAD_FAST and LOAD_GLOBAL used in performance critical code?

I guess that it's only used to execute the body of modules.

Is it worth to optimize this case?

Hum, I don't know :-)

It looks to me that there is a bug in fast path of _PyDict_LoadGlobal. If the first lookup fails, it can raise an exception.

Sorry, where exactly? Can you maybe put a comment on the review? I see many checks to handle errors.

[vstinner]

Rebased patch (ceval.c was modified by the changeset of c1414f80ebc9, issue bpo-25556).

[python-dev]

New changeset c35d65c9ded3 by Victor Stinner in branch 'default':
Issue bpo-25557: Refactor _PyDict_LoadGlobal()
https://hg.python.org/cpython/rev/c35d65c9ded3

[vstinner]

Since LOAD_NAME is rare, I don't think that it's worth to optimize it.

I pushed a partial version of pydict_loadname-2.patch to add comments to the code.

[serhiy-storchaka]

Thanks Victor.