Perform "peephole" optimization directly on control-flow graph.

[markshannon]

BPO	41323
Nosy	@tim-one, @rhettinger, @vstinner, @methane, @markshannon
PRs	bpo-41323: Perform 'peephole' optimizations directly on the CFG. #21517 bpo-41323: compiler: Reuse tuple in constant tuple folding #25419

^{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 = 'https://github.com/markshannon'
closed_at = <Date 2020-07-30.09:04:04.543>
created_at = <Date 2020-07-17.10:27:02.814>
labels = ['performance']
title = 'Perform "peephole" optimization directly on control-flow graph.'
updated_at = <Date 2021-04-16.05:01:32.524>
user = 'https://github.com/markshannon'

bugs.python.org fields:

activity = <Date 2021-04-16.05:01:32.524>
actor = 'methane'
assignee = 'Mark.Shannon'
closed = True
closed_date = <Date 2020-07-30.09:04:04.543>
closer = 'Mark.Shannon'
components = []
creation = <Date 2020-07-17.10:27:02.814>
creator = 'Mark.Shannon'
dependencies = []
files = []
hgrepos = []
issue_num = 41323
keywords = ['patch']
message_count = 4.0
messages = ['373811', '373848', '374606', '391165']
nosy_count = 5.0
nosy_names = ['tim.peters', 'rhettinger', 'vstinner', 'methane', 'Mark.Shannon']
pr_nums = ['21517', '25419']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue41323'
versions = []

[markshannon]

Currently we perform various bytecode improvements as a pass on the code objects after generating the code object.

This requires parsing the bytecode to find instructions, recreating the CFG, and rewriting the line number table.

If we perform the optimizations directly on the CFG we can avoid all that additional work.

This would save hundreds of lines of code and avoid coupling the optimization to the line number table format.

[rhettinger]

+1 This looks like a nice improvement. I've long wanted the optimizations to be moved upstream.

At the time the peephole logic was first written, operating directly on the code object was the only option that kept optimizations separate from the core compiler logic. But disassembling and reassembling bytecode was always awkward — it is nice to no longer have to do so.

[markshannon]

New changeset 6e8128f by Mark Shannon in branch 'master':
bpo-41323: Perform 'peephole' optimizations directly on the CFG. (GH-21517)
6e8128f

[methane]

New changeset 8a232c7 by Inada Naoki in branch 'master':
bpo-41323: compiler: Reuse tuple in constant tuple folding (GH-25419)
8a232c7