gh-104635: Expand optimization for dead store elimination

[corona10]

Explain

We might also want to expand this so it can look more than one instruction ahead for another write to the same location, skipping over an allowlist of instructions that we know don't read that same local, don't execute arbitrary code, and can't raise an exception. E.g. LOAD_FAST of a different local, POP_TOP, NOP, probably others.

This PR is motivated by @carljm 's comment from #105040 (comment)
The limitation of #105320 is that the optimization only covers the next redundant opcode, but with this patch, the optimization can be applied to more than one instruction ahead for another write to the same location.

Benchmark

import pyperf

runner = pyperf.Runner()
runner.timeit(name="bench dead_store",
              stmt="""
_, a, b, c, _ = 1, 2, 3, 4, 5
""")

Mean +- std dev: [base] 13.1 ns +- 0.2 ns -> [opt] 12.6 ns +- 0.2 ns: 1.04x faster

TODO (Done)

• Fix test_dis.py
• Add testcase for this optimization.

• Issue: dead store elimination in the compiler #104635

[corona10]

./_bootstrap_python ./Programs/_freeze_module.py runpy ./Lib/runpy.py Python/frozen_modules/runpy.h
Assertion failed: (OPCODE_HAS_ARG(opcode) || HAS_TARGET(opcode) || oparg == 0), function instr_sequence_addop, file compile.c, line 243.
make: *** [Python/frozen_modules/runpy.h] Abort trap: 6
make: *** Waiting for unfinished jobs....

Interesting crash; this crash was not triggered at my local machine. I will take a look.

-> Done

[corona10]

@carljm Please take a look :)

[corona10]

Umm, this patch is missing the following case. (same line, multi variable multi target case)
I will improve for such case, do not merge yet :)

a, a, b, b = 1, 2, 3, 4

[corona10]

@carljm Ready to review, I wanted the linear time complexity for the first time, but it will require more.

[corona10]

>>> def f():
...     a = 42; b = a + 54; a = 54
...     return a, b
...
>>> f()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in f
UnboundLocalError: cannot access local variable 'a' where it is not associated with a value

Found more edge case, I will close the PR. We may need some kinds of use-def analysis

[markshannon]

I'm still unconvinced that this can do anything that a superoptimizer could not do.

_, a, b, c, _ = 1, 2, 3, 4, 5 is obviously equivalent to a, b, c, _ = 2, 3, 4, 5. A superoptimizer would produce an optimal sequence of code for this.

If you are only looking at linear sequences of instructions that are on a single line then, almost by definition, a superoptimizer will do at least as well.

[corona10]

If you are only looking at linear sequences of instructions that are on a single line then, almost by definition, a superoptimizer will do at least as well.

Yeah, I agree if the super-optimizer is added, I expect that most of the cases will be covered, but I just tried it because the super-optimizer infra does not exist at this moment.

More digging will be more expensive than I expect, so I am not going to cover those cases at this moment.

(At least, we may need to add use-def analysis, but it should be compared to adding a super-optimizer process)