Chris Colbert skrev:
> No, the python ** gets translated to a pow statement by cython.
>
> I think the issue is that for some reason, i'm getting stuck in the
> gcc slow pow function....
>
> if i let e2 and e1 be 1 and replace f**2 (which would call pow) with f*f,
>
> my execution time drops to this:
> 10000 loops, best of 3: 108 µs per loop
>
> over 6x improvement just by avoid a few measly pow statements...
> anyone know why i'm stuck in slowpow?
>
pow() is "slow" because it is a general function that can compute any
power, including pow(x, -231436.74638746238746). It is not restricted to
integers only. Therefore,
cdef inline double pow0(double x):
return 1
cdef inline double pow1(double x):
return x
cdef inline double pow2(double x):
return x*x
cdef inline double pow3(double x):
return x*x*x
is (often) much faster than pow(x,0), pow(x,1), pow(x,2), and pow(x,3).
A Fortran compiler would recognize x**3 as x*x*x and do the "right thing".
In C++, one could use template metaprogramming for this:
template<int n>
inline double power<n>(double x)
{
if (n > 0)
return x * power<n-1>(x);
else
return 1.0 / power<-n>(x);
}
template<>
inline double power<1>(double x)
{
return x;
}
template<>
inline double power<0>(double x)
{
return 1;
}
Which BTW is a really disgusting way of coding... I like Fortran better.
S.M.
>
>
> On Tue, Sep 29, 2009 at 6:15 PM, Sturla Molden <[email protected]> wrote:
>
>> Sturla Molden skrev:
>>
>>> Chris Colbert skrev:
>>>
>>>
>>>> and within that loop it is these statements that take the bulk of the time:
>>>>
>>>> F = ((f1**2)**(1/e2) + (f2**2)**(1/e2))**(e2/e1) + (f3**2)**(1/e1)
>>>>
>>>> temperr = (C4 * (F**(e1) - 1))**2
>>>>
>>>> and replacing the powers with serial multiplications don't really help
>>>> any...
>>>>
>>>>
>>>>
>>> Does this help?
>>>
>>> cdef extern from "math.h":
>>> double pow(double, double)
>>>
>>> F = pow(pow((f1*f1),(1/e2)) + pow((f2*f2),(1/e2)),(e2/e1)) \
>>> + pow((f3*f3),(1/e1))
>>>
>>>
>>>
>> cdef double tmp
>>
>> tmp = C4 * (pow(F,e1) - 1)
>>
>> temperr = tmp*tmp
>>
>>
>>
>>
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