slow complex<double>'s with g++

[Brian Budge]

Which command line arguments are you using?  Can you make use of sse2?

  Brian

On 3/3/06, Greg Buchholz <greg@sleepingsquirrel.org> wrote:
> /*
>     While writing a C++ version of the Mandelbrot benchmark over at the
> "The Great Computer Language Shootout"...
>
>   http://shootout.alioth.debian.org/gp4/benchmark.php?test=mandelbrot&lang=all
>
> ...I've come across the issue that complex<double>'s seem painfully slow
> unless compiled with -ffast-math. Of course doing that results in
> incorrect answers because of rounding issues.  The speed difference for
> the program below is between 5x-8x depending on the version of g++.  It
> is also about 5 times slower than the corresponding gcc version at...
>
> http://shootout.alioth.debian.org/gp4/benchmark.php?test=mandelbrot&lang=gcc&id=2
>
> ...I'd be interesting in learning the reason for the speed difference.
> Does it have something to do with temporaries not being optimized away,
> or somesuch?  A limitation of the x87 instruction set?  Is it inherent
> in the way the C++ Standard requires complex<double>'s to be calculated?
> My bad coding style?
>
> Curious,
>
> Greg Buchholz
> */
>
> // Takes an integer argument "n" on the command line and generates a
> // PBM bitmap of the Mandelbrot set on stdout.
> // see also: ( http://sleepingsquirrel.org/cpp/mandelbrot.cpp.html )
>
> #include<iostream>
> #include<complex>
>
> int main (int argc, char **argv)
> {
>   char  bit_num = 0, byte_acc = 0;
>   const int iter = 50;
>   const double limit_sqr = 2.0 * 2.0;
>
>   std::ios_base::sync_with_stdio(false);
>   int n = atoi(argv[1]);
>
>   std::cout << "P4\n" << n << " " << n << std::endl;
>
>   for(int y=0; y<n; ++y)
>     for(int x=0; x<n; ++x)
>     {
>        std::complex<double> Z(0.0,0.0);
>        std::complex<double> C(2*(double)x/n - 1.5, 2*(double)y/n - 1.0);
>
>        for (int i=0; i<iter and norm(Z) <= limit_sqr; ++i)  Z = Z*Z + C;
>
>        byte_acc = (byte_acc << 1) | ((norm(Z) > limit_sqr) ? 0x00:0x01);
>
>        if(++bit_num == 8){ std::cout << byte_acc; bit_num = byte_acc = 0; }
>        else if(x == n-1) { byte_acc  <<= (8-n%8);
>                            std::cout << byte_acc;
>                            bit_num = byte_acc = 0; }
>     }
> }
>