Why is recursion so slow?

Bruno Desthuilliers bruno.42.desthuilliers at websiteburo.invalid
Mon Jun 30 13:10:54 CEST 2008


Dan Upton a écrit :
> On Sun, Jun 29, 2008 at 1:27 AM, Terry Reedy <tjreedy at udel.edu> wrote:
>>
>> slix wrote:
>>> Recursion is awesome for writing some functions, like searching trees
>>> etc but wow how can it be THAT much slower for computing fibonacci-
>>> numbers?
>> The comparison below has nothing to do with recursion versus iteration.  (It
>> is a common myth.) You (as have others) are comparing an exponential,
>> O(1.6**n), algorithm with a linear, O(n), algorithm.
>>
> 
> FWIW, though, it's entirely possible for a recursive algorithm with
> the same asymptotic runtime to be wall-clock slower, just because of
> all the extra work involved in setting up and tearing down stack
> frames and executing call/return instructions.  (If the function is
> tail-recursive you can get around this, though I don't know exactly
> how CPython is implemented and whether it optimizes that case.)

By decision of the BDFL, based on the argument that it makes debugging 
harder, CPython doesn't optimize tail-recursive calls.



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