[Python-ideas] Optimizing global names via dict references

[M.-A. Lemburg]

On 19.12.2015 16:06, Victor Stinner wrote:
> Le samedi 19 décembre 2015, Franklin? Lee <leewangzhong+python at gmail.com
> <javascript:_e(%7B%7D,'cvml','leewangzhong%2Bpython at gmail.com');>> a écrit :
>>
>> == Problem statement ==
>>
>> I propose a CPython interpreter optimization for global name lookups.
>>
> 
> Fans of micro optimisation are probably already using various hacks like
> func(len=len): ... to avoid the lookup at runtime. There is an option
> rewriting bytecode to replace load_global with load_const (I don't recall
> the name of the PyPI project, it's a decorator).
> 
> Serhiy also proposed to implement a new syntax to make the lookup when the
> function is defined.
> 
> It would be interesting to mesure the cost of these lookups(ex: number of
> nanoseconds per lookup) and have an idea on how much load_global lookups
> are used in the wild (ratio on the overall number of instructions).
> 
> Since the goal is a speedup, a working proof of concept is required to show
> that it works and it's faster (on macro benchmarks?). Do you feel able to
> implement it?
> 
> As I already wrote, I'm not convinced that it's worth it. Your code looks
> more complex, will use more memory, etc. I don't think that load_global is
> common in hot code (the 10% taking 90% of the runtime). I expect effetcs on
> object lifetime which could be annoying.

The effects are minimal and only show up in overall performance
if the functions in question are used a lot.

I gave a talk about such optimizations last year at PyCon UK
you might want to have a look at:

http://www.egenix.com/library/presentations/PyCon-UK-2014-When-performance-matters/

Slide 38 has the details about such lookup optimization tricks:

https://downloads.egenix.com/python/PyCon-UK-2014-When-performance-matters-Talk.pdf

> I implemented an optimization in FAT Python that replaces builtin lookups
> with load_const. I have to enhance the code to make it safe, but it works
> and it doesn't require deep changes in dict type.
> http://faster-cpython.readthedocs.org/en/latest/fat_python.html#copy-builtin-functions-to-constants
> 
> In short, it only adds a single integer per dict, incremented at each
> modification (create, modify, delete).
> 
> Victor
> 
> 
> 
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