[Python-Dev] Possible optimization for LOAD_FAST ?

[Alex Gaynor]

Cesare Di Mauro <cesare.di.mauro <at> gmail.com> writes:

> 
> 
> 2010/12/28 Lukas Lueg <lukas.lueg <at> googlemail.com>
> 
> Consider the following code:
> def foobar(x):
>     for i in range(5):
>         x[i] = i
> The bytecode in python 2.7 is the following:
>   2           0 SETUP_LOOP              30 (to 33)
>               3 LOAD_GLOBAL              0 (range)
>               6 LOAD_CONST               1 (5)
>               9 CALL_FUNCTION            1
>              12 GET_ITER
>         >>   13 FOR_ITER                16 (to 32)
>              16 STORE_FAST               1 (i)
>   3          19 LOAD_FAST                1 (i)
>              22 LOAD_FAST                0 (x)
>              25 LOAD_FAST                1 (i)
>              28 STORE_SUBSCR
>              29 JUMP_ABSOLUTE           13
>         >>   32 POP_BLOCK
>         >>   33 LOAD_CONST               0 (None)
>              36 RETURN_VALUE
> Can't we optimize the LOAD_FAST in lines 19 and 25 to a single load
> and put the reference twice on the stack? There is no way that the
> reference of i might change in between the two lines. Also, the
> load_fast in lne 22 to reference x could be taken out of the loop as x will 
always point to the same object....
> 
> Yes, you can, but you need:
> - a better AST evaluator (to mark symbols/variables with proper attributes);
> - a better optimizer (usually located on compile.c) which has a "global 
vision" (not limited to single instructions and/or single expressions).
> 
> It's not that simple, and the results aren't guaranteed to be good.
> 
> Also, consider that Python, as a dynamic-and-not-statically-compiled language 
need to find a good trade-off between compilation time and execution.
> 
> Just to be clear, a C program is usually compiled once, then executed, so you 
can spend even *hours* to better optimize the final binary code.
> 
> With a dynamic language, usually the code is compiled and the executed as 
needed, in "realtime". So it isn't practical neither desirable having to wait 
too much time before execution begins (the "startup" problem).
> 
> Python stays in a "gray area", because modules are usually compiled once (when 
they are first used), and executed many times, but it isn't the only case.
> 
> You cannot assume that optimization techniques used on other (static) 
languages can be used/ported in Python.
> 
> Cesare
> 


No, it's singularly impossible to prove that any global load will be any given 
value at compile time.  Any optimization based on this premise is wrong.

Alex