AST-level Constant folding

[methane]

BPO	29469
Nosy	@brettcannon, @birkenfeld, @rhettinger, @ncoghlan, @vstinner, @benjaminp, @methane, @serhiy-storchaka, @1st1
PRs	bpo-29469: Move constant folding at AST level #2858 bpo-29469: Remove unnecessary peephole optimizer #4863 bpo-29469: Optimize literal lists and sets iterating on the AST level. #4866 bpo-29469: peephole: Remove const_stack #4879
Dependencies	bpo-29463: Add docstring field to AST nodes
Files	ast-constant-folding.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 2017-12-14.18:25:10.101>
created_at = <Date 2017-02-07.04:02:38.365>
labels = ['interpreter-core', 'type-feature', '3.7']
title = 'AST-level Constant folding'
updated_at = <Date 2017-12-18.06:52:57.357>
user = 'https://github.com/methane'

bugs.python.org fields:

activity = <Date 2017-12-18.06:52:57.357>
actor = 'methane'
assignee = 'none'
closed = True
closed_date = <Date 2017-12-14.18:25:10.101>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2017-02-07.04:02:38.365>
creator = 'methane'
dependencies = ['29463']
files = ['46555']
hgrepos = []
issue_num = 29469
keywords = ['patch']
message_count = 14.0
messages = ['287186', '287210', '287214', '287227', '297596', '297599', '308277', '308283', '308296', '308300', '308322', '308327', '308369', '308519']
nosy_count = 9.0
nosy_names = ['brett.cannon', 'georg.brandl', 'rhettinger', 'ncoghlan', 'vstinner', 'benjamin.peterson', 'methane', 'serhiy.storchaka', 'yselivanov']
pr_nums = ['2858', '4863', '4866', '4879']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue29469'
versions = ['Python 3.7']

[methane]

spin off of bpo-11549.
This patch uses code generator to traverse AST.

[vstinner]

I suggest you to look at my AST optimizer, especially the constant folding part:
https://github.com/haypo/fatoptimizer/blob/master/fatoptimizer/const_fold.py

And the unit tests, search for BaseConstantFoldingTests:
https://github.com/haypo/fatoptimizer/blob/master/test_fatoptimizer.py#L1980

IHMO we must have a long test suite on this AST optimizer, because it's common that the AST changes in subtle ways, AST is complex and so we should prevent regressions. You may simply copy my unit tests.

My optimizer implements more optimization: just remove unit tests on cases which you don't want to optimize.

[vstinner]

With this change, the Python compiler doesn't emit ast.Num nor ast.Str, right?

If we merge such change, we should prepare projects using AST. There is something like that in pip, but I failed to remind which one :-/ It should be easy: apply your patch, try to install something using pip and see the traceback ;-)

[methane]

Do you mean https://github.com/pypa/pip/blob/403e398330c8e841e4633aceda859430f5f7b913/pip/_vendor/distlib/markers.py ?

This doesn't affect, maybe.
Constant folding is executed right before producing bytecode.
It doesn't affect to PyCF_ONLY_AST.

BTW, how do you feel asdl_ct.py?

I feel it's too much only for constant folding, but it is too less for other advanced optimizations.
Actually, original patch in bpo-11549 implements other optimizations
ported from peephole in compile.c.
And most tough part of updating this patch is resolve conflict with this commit.
9cea398

I'll try to remove asdl_ct.py and ast_opt.ct in next version.

[serhiy-storchaka]

In general the patch LGTM. I haven't found bugs.

I think that at this stage we can asdl_ct.py and ast_opt.ct and use just their result, ast_opt.c (maybe rename to optimize.c?) The grammar is not often changed, and in that case it would be not hard to modify the code manually. In any case compile.c should be modified manually. After removing no-op functions like astfold_operator() and inlining simple functions like astfold_arg() the code should be clearer.

Instead of converting all kinds of constants to Constant_kind, you could use is_const() and get_const_value() from compile.c.

I suggest to remove the parts of the peepholer that are superseeded by the AST optimizer.

As for preventing expensive calculations, there is a patch for the peepholer that does this. I wait on implementing the AST-level constant folding for adapting that patch for it.

[vstinner]

Naoki: Can you please create a PR for your patch?

[methane]

New changeset 7ea143a by INADA Naoki in branch 'master':
bpo-29469: Move constant folding to AST optimizer (GH-2858)
7ea143a

[rhettinger]

ISTM, the constant-tracking macros for peephole.c can also be removed, essentially reverting back to the simpler cumlc-style code in Py2.5.

[methane]

New changeset eadad1b by INADA Naoki in branch 'master':
bpo-29469: Remove unnecessary peephole optimizer (GH-4863)
eadad1b

[vstinner]

Thank you very much Naoki for taking time to implement this obvious "optimization", rewriting the peephole optimizer at the AST level, rather than modifying bytecode which is more complex to get it right and had annoying side effect (like inefficient stack size, now fixed: bpo-26549).