Stefan Behnel, 15.07.2010 19:34:
> In most cases,
> nodes and their results have to be reused, and there isn't currently a good
> way to do that, especially not before type inference and analysis, which
> may end up wanting to assign different types to the same node result in the
> different contexts.
Let's exercise this a bit. The min/max transformation is a perfect example
here. It expands this code
x = min(a,b,c)
into this code
x = a
if x > b: x = b
if x > c: x = c
Now, you have to evaluate each input parameter exactly once, and each
evaluation can potentially throw an exception. How would you do this in a
source-level transformation? The only way I see is to introduce temp nodes
already at this point, i.e. you'd make it
x, _b, _c = a, b, c
if x > _b: x = _b
if x > _c: x = _c
Next, to make the comparisons efficient if x has a Python type, _b and _c
must have the same type as x. Otherwise, both the comparison and the
assignment would lead to redundant coercions. How would you assure that?
Note that type inference will not work here, as it only impacts the type of
x. All other type analysis steps are local.
I can see us making it less local by injecting back-references to those
nodes that depend on a node's type. That might allow us to minimise the
number of neccessary coercions for a value. However, to do this right, we
need flow analysis to even see which code paths actually lead to a coercion.
Stefan
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