Stefan Behnel, 11.03.2011 15:08:
Vitja Makarov, 11.03.2011 15:04:
2011/3/11 Stefan Behnel:
Personally, I think it would be nice to keep up Python's semantics, but
when
I implemented this I broke quite some code in Sage (you may have noticed
that the sage-build project in Hudson has been red for a while). There are
things in C and especially in C++ that cannot be easily copied into a
temporary variable in order to make sure they are evaluated before the
following arguments. This is not a problem for Python function calls where
all arguments end up being copied (and often converted) anyway. It is a
problem for C function calls, though.
f(g(a), a.x, h(a))
Why could not this be translated into:
tmp1 = g(a)
tmp2 = a.x
tmp3 = h(a)
f(tmp1, tmp2, tmp3)
See above.
To be a little clearer here, it's a problem in C for example with struct
values. Copying them by value into a temp variable can be expensive,
potentially twice as expensive as simply passing them into the function
normally.
Not sure what kind of additional devilry C++ provides here, but I'd expect
that object values can exhibit bizarre behaviour when being assigned. Maybe
others can enlighten me here.
I have no idea how many cases there actually are that we can't handle or
that may lead to a performance degradation when using temps, but the
problem is that they exist at all.
Stefan
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