Hi Joannah,
On 03/04/2020 4:13 pm, joannah nanjekye wrote:
Hey all,
From my CS theory, a control flow graph models a program flow and one
of its main characteristics is it has one entry and exit point. IIRC,
CPython's CFG has multiple exit points, but that shouldn't matter.
CPython’s compilation process involves generation of a control flow graph.
I started playing with moving the CFG optimizations from the peepholer
to their own pass back at the core sprints in London.
I've worked on it a bit since, but not for a while.
My branch is here:
https://github.com/python/cpython/compare/master...markshannon:cfg-optimizer
One thing that is blocking progress is the line-number table format.
There is no way to say that an instruction does not have a line number
attached to it. That makes handling and transforming of compiler
generated code (implicit `return None`s, for example) harder.
See https://bugs.python.org/issue39537
It's not an insurmountable obstacle, but it is something to be aware of.
Contrary to peephole optimizations, optimizations on the control flow
graph are more global allowing us to have complex and global
optimizations like branch and checkpoint eliminations etc.
Checkpoints for tracing or concurrent GC are not something we need to
worry about.
Because Python is so dynamic, there is little we can do in the way of
further optimizations in the bytecode compiler apart from control flow
improvements.
However, Python does have some fairly complex control flow, so there is
room for improvement.
I have seen several implementations of control flow optimizations. The
one I am familiar with is the V8 control flow optimizer.
I'm not familiar with it. Is there a summary online?
I tried to investigate this for one of my directed courses last fall but
I want to know if there are people who have been thinking about this for
CPython and what their thoughts are.
There won't be much of a speed up, maybe 2 or 3%, but adding an explicit
CFG optimisation pass will improve maintainability as we won't need to
regenerate the CFG after bytecode layout.
I'd be happy to offer advice and review any PRs.
Cheers,
Mark.
--
//Best,
Joannah Nanjekye
/"You think you know when you learn, are more sure when you can write,
even more when you can teach, but certain when you can program."
Alan J. Perlis/
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