[Python-Dev] Re: PEP 276 Simple Iterator for ints

2001年11月17日 09:53:28 +0100

 >> * It would expose potentially significant optimizations that can't be
 >> made today by eliminating the attribute lookup and function call to
 >> range, and thus getting rid of that little bit of dynamism nobody
 >> ever uses anyway.
 Tim> As I said last time this went around, I doubt the optimizations are
 Tim> significant: loop overhead is generally at worst a
 Tim> handful-of-percent thing in real programs, and the cost of one
 Tim> global lookup+call per entire loop (not per loop *trip*) isn't even
 Tim> measurable outside contrived examples.
I was thinking more along the lines of generating C or assembler on the
fly. Knowing a priori the types of the values emitted by
 for i in [0, 1 .. 10]
are ints means that the code generator can mostly avoid dealing with "i" as
a Python int and instead use native ints. In
 for i in range(11):
you'd have to examine the output of range() and for
 for i in xrange(11):
it might be "too difficult" (whatever that means) to ascertain the types of
the elements.
 ...
 Tim> (BTW, repeated addition is almost exactly the worst thing to do for
 Tim> fp sequences; doing starting_value + iteration_ordinal * step_value
 Tim> on each trip is usually much better behaved for floats.)
Yes, that's what I had in mind.
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