why does dead code costs time?

Bruno Dupuis python.ml.bruno.dupuis at lisael.org
Wed Dec 5 17:40:51 CET 2012


On Wed, Dec 05, 2012 at 04:15:59PM +0000, Neil Cerutti wrote:
> On 2012-12-05, Bruno Dupuis <python.ml.bruno.dupuis at lisael.org> wrote:
> > Hi,
> >
> > I'm interested in compilers optimizations, so I study python
> > compilation process
> >
> > I ran that script:
> >
> >     import timeit
> >
> >     def f(x):
> >         return None
> >
> >     def g(x):
> >         return None
> >         print(x)
> >
> >     number = 10000
> >
> >     print(timeit.timeit('f(1)',setup="from __main__ import f", number=number))
> >     print(timeit.timeit('g(1)',setup="from __main__ import g", number=number))   
> >
> >     print(dis.dis(f))
> >     print(dis.dis(g))
> >
> > It gives this output:
> >
> >     0.003460251959040761
> >     0.004164454061537981
> >      17           0 LOAD_CONST               0 (None) 
> >                   3 RETURN_VALUE         
> >     None
> >      20           0 LOAD_GLOBAL              0 (None) 
> >                   3 RETURN_VALUE         
> >
> >      21           4 LOAD_GLOBAL              1 (print) 
> >                   7 LOAD_FAST                0 (x) 
> >                  10 CALL_FUNCTION            1 (1 positional, 0 keyword pair) 
> >                  13 POP_TOP              
> >     None
> >
> > I do not understand why the dead code `print(x)` takes time (~20% in
> > that case). As we see in the opcode, a call to g(1) returns immediately, so
> > there should be no delay at all. Where am i wrong?
> >
> > mmhh... it comes to me now that the gap must be in function loading time...
> > I'll check ceval.c
> >
> > However, isn't there a room for a slight optim here? (in this case, the
> > dead code is obvious, but it may be hidden by complex loops and
> > conditions)
> 
> Maybe it's the difference between LOAD_CONST and LOAD_GLOBAL. We
> can wonder why g uses the latter.

Good point! I didn't even noticed that. It's weird... Maybe the
difference comes from a peehole optim on f which is not possible on g as
g is to complex.

> 
> -- 
> Neil Cerutti
> -- 
> http://mail.python.org/mailman/listinfo/python-list

-- 
Bruno Dupuis



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