Eliminate NOPs in the assembler, by emitting zero-width entries in the line number table

[markshannon]

BPO	42719
Nosy	@markshannon

^{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 = None
created_at = <Date 2020-12-22.14:08:13.221>
labels = ['interpreter-core', '3.10', 'performance']
title = 'Eliminate NOPs in the assembler, by emitting zero-width entries in the line number table'
updated_at = <Date 2020-12-22.14:08:13.221>
user = 'https://github.com/markshannon'

bugs.python.org fields:

activity = <Date 2020-12-22.14:08:13.221>
actor = 'Mark.Shannon'
assignee = 'Mark.Shannon'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2020-12-22.14:08:13.221>
creator = 'Mark.Shannon'
dependencies = []
files = []
hgrepos = []
issue_num = 42719
keywords = []
message_count = 1.0
messages = ['383590']
nosy_count = 1.0
nosy_names = ['Mark.Shannon']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue42719'
versions = ['Python 3.10']

[markshannon]

This will require a change to the internal line number table format.

PEP-626 requires all lines are traced, which makes handling of 'continue' and other jump-to-jumps inefficient if spread across multiple lines.

In 3.9 many jump-to-jumps were eliminated, but PEP-626 requires that many are converted to a NOP. Zero-width line number table entries will allow us to eliminate most, if not all, remaining NOPs in the assembler.

[gvanrossum]

Let's make this happen in 3.12. @iritkatriel ?

[markshannon]

I don't think we should. I know I created this issue, but I've changed my mind.

Having multiple line events associated with a single instruction could add a fair bit of complexity, but NOPs are responsible for only a tiny fraction of execution time (well under 0.1%).

[gvanrossum]

OK, closing.