[Cython] local variable handling in generators

[Stefan Behnel]

Hi,

I've been looking at the nqueens benchmark for a while, and I think it's 
actually not that a bad benchmark for generators.

http://hg.python.org/benchmarks/file/tip/performance/bm_nqueens.py

A better implementation only for Py2.7/Py3 is here:

https://github.com/cython/cython/blob/master/Demos/benchmarks/nqueens.py

Cython currently runs the first implementation about as fast as Py3.3:

https://sage.math.washington.edu:8091/hudson/job/cython-devel-pybenchmarks-py3k/lastSuccessfulBuild/artifact/chart.html

and the second one more than 3x as fast:

https://sage.math.washington.edu:8091/hudson/view/bench/job/cython-devel-cybenchmarks-py3k/lastSuccessfulBuild/artifact/chart.html

However, I think there's still some space for improvements, and local 
variables are part of that. For generator functions that do non-trivial 
things between yields, I think that local variables will quickly become a 
bottleneck. Currently, they are always closure fields, so any access to 
them will use a pointer indirection to a foreign struct, originally passed 
in as an argument to the function. Given that generators often do Python 
object manipulation through C-API calls, any such call will basically 
require the C compiler to assume that all values in the closure may have 
changed, thus disabling any optimisations for them. The same applies to 
many other object related operations or pointer operations (even DECREF!), 
as the C compiler cannot know that the generator function owns the closure 
during its lifetime exclusively.

I think it would be worth changing the current implementation to use local 
C variables for local Cython variables in the generator, and to copy the 
values back into/from the closure around yields. I'd even let local Python 
references start off as NULL when the generator is created, given that 
Vitek's branch can eliminate None initialisations now.

I started looking into this a bit, but it's not a quick change. The main 
problem I see is the current code duplication between the generator body 
node and the DefNode. It would be better if both could share more code. 
Basically, if local variables become truly local, the only differences will 
be that they come from call arguments in one case and from the closure in 
the other, and that generators have the additional jump-to-yield entry 
step. Everything else could hopefully be identical.

Once again, this goes hand in hand with the still pending DefNode 
refactoring...

Stefan