Frames should store next_instr instead of lasti

[brandtbucher]

BPO	47177
Nosy	@markshannon, @brandtbucher
PRs	bpo-47177: Replace f_lasti with prev_instr #32208

^{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/brandtbucher'
closed_at = <Date 2022-04-07.19:32:09.034>
created_at = <Date 2022-03-31.01:12:26.728>
labels = ['interpreter-core', '3.11', 'performance']
title = 'Frames should store next_instr instead of lasti'
updated_at = <Date 2022-04-07.19:32:09.033>
user = 'https://github.com/brandtbucher'

bugs.python.org fields:

activity = <Date 2022-04-07.19:32:09.033>
actor = 'brandtbucher'
assignee = 'brandtbucher'
closed = True
closed_date = <Date 2022-04-07.19:32:09.034>
closer = 'brandtbucher'
components = ['Interpreter Core']
creation = <Date 2022-03-31.01:12:26.728>
creator = 'brandtbucher'
dependencies = []
files = []
hgrepos = []
issue_num = 47177
keywords = ['patch']
message_count = 2.0
messages = ['416409', '416938']
nosy_count = 2.0
nosy_names = ['Mark.Shannon', 'brandtbucher']
pr_nums = ['32208']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue47177'
versions = ['Python 3.11']

[brandtbucher]

Rather than maintaining the offset of the "last instruction" (f_lasti), interpreter frames should instead just maintain a pointer to the true next instruction. This has several benefits, most notably reducing the register pressure associated with loading first_instr on every instruction and call in the main interpreter loop:

When entering a frame:

• Before: next_instr = first_instr + frame->f_lasti + 1;
• After: next_instr = frame->next_instr;

When starting a new instruction:

• Before: frame->next_instr = next_instr++ - first_instr;
• After: frame->next_instr = ++next_instr;

Benchmarks suggest that this overhead is surprisingly significant (something like 2%).

[brandtbucher]

New changeset ef6a482 by Brandt Bucher in branch 'main':
bpo-47177: Replace f_lasti with prev_instr (GH-32208)
ef6a482