[Python-ideas] Disable all peephole optimizations

[Steven D'Aprano]

On Thu, May 22, 2014 at 03:41:31PM +0000, Steve Dower wrote:

> Why not move the existing optimisation into -O mode and put future 
> optimisations in there too? It may just start having enough value that 
> people switch to using it.

I just had the same idea, you beat me to it.

There's a steady but small stream of people asking "why do we have -O, 
it does so little we might as well get rid of it". If I remember 
correctly (and apologies if I do not), Guido has even suggested getting 
rid of simple constant folding. So let's make -O more attractive, while 
simplifying the default behaviour:


* By default, no optimizations operate at all.

* With -O, you get assert disabling, the tricky string concatenation 
  optimization, constant folding, and whatever else the peepholer does.

* The double -OO switch should be deprecated, for eventual removal
  in the very distant future. (4.0? 5.0?)

* Instead, a separate switch for removing docstrings can be added,
  to support implementations in low-memory devices or other 
  constrained situations.


This will make Python's compilation model a little more familiar to 
people coming from other languages.  It will make -O more attractive, 
instead of being viewed by some as a waste of effort, and ensure that by 
default there are no tricks played with byte-code.

A big advantage: we already have separate .pyo and .pyc files, so no 
risk of confusion.

Downside of this suggestion:

- To the extent that constant folding and other optimizations actually 
  lead to a speed-up, turning them off by default will be a performance 
  regression.

- Experienced programmers ought to know not to rely on the string 
  concatenation optimization, as it is non-portable and prone to 
  surprising failures even in CPython. The optimization really only
  exists for naive programmers, but they are unlikely to know about, or 
  bother using, -O to get that optimization.

- Surely I don't expect PyPy to perform no optimizations at all unless
  the -O switch is given? I'd have to be mad to suggest that.



-- 
Steven