On Sun, Oct 10, 2021 at 09:15:30AM +0000, Patrick Reader wrote:
> - a LOAD_FAST cannot possibly have any side-effects outside the
> interpreter stack [1]
> [1]: global variables are probably supposed to have the same
> guarantee, but in practice this is not the case
I don't think that lookups are guaranteed to be side-effect free, since
raising an exception is a side-effect.
def func():
x
can raise NameError if there is no global or builtin "x". So that cannot
be optimized away. Similarly for locals:
def func(x):
if random.random() > 0.5:
del x
x
Will that raise or not? No way of telling without actually running the
code.
Now I guess a sufficiently smart optimizer might be able to optimize
away the lookup of a local variable if it cannot possibly be undefined,
as in your initial example, but that brings us to how much benefit is
gained for how much developer effort and code complexity.
If the benefit is small, or negligible, then the developer effort
had better be even smaller.
In this case, the cost of the lookup is small (it is a *FAST* lookup).
And furthermore, the likelihood that anyone will actually write that
code is even smaller.
(There are times that we might test for the existence of a global, by
looking up the name. There might even be times that we would do the same
for a local. We might even, under some rare circumstances, do so without
guarding it in a try...except block. But why on earth would we test for
the existence of a local that cannot possibly fail to exist, which is
the only case that is safe to optimize away?)
So the benefit is correspondingly tiny, and optimizing it away had
better be effectively free in both developer effort and ongoing code
complexity to make it worthwhile.
--
Steve
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