INADA Naoki added the comment:
> My motivation is improve speed,

Ah, if the motivation is performance, I would like to see benchmark
results :-) I understand that an AST optimizer would help to produce
more efficient bytecode, right?

> reduce memory usage,

I noticed an issue with the peephole optimizer: the constant folding
step keeps original constants. Moving constant folding to the AST
stage fixes this issue by design.

> and quicker startup time for real world applications.

You mean faster import time on precompiled .pyc files, right? It's
related to the hypothetical faster bytecode.

> If some optimization in FAT optimizer has significant speedup, I want to try it.

See http://fatoptimizer.readthedocs.io/en/latest/microbenchmarks.html#microbench

FYI it took me something like 2 months to build FAT Python
"infrastructure": fix CPython bugs, design guards, design the AST
optimizer, write unit tests, etc. I didn't spend much time on
efficient optimizations. But for my first rule was to not break the
CPython test suite! Not break the Python semantics, otherwise it would
be impossible to put enable the optimizer by default in CPython, which
is my long term goal.