Loop optimization

[Steven Schveighoffer]

On Thu, 13 May 2010 22:38:40 -0400, kai <kai at nospam.zzz> wrote:

> Hello,
>
> I was evaluating using D for some numerical stuff. However I was  
> surprised to
> find that looping & array indexing was not very speedy compared to
> alternatives (gcc et al). I was using the DMD2 compiler on mac and  
> windows,
> with -O -release. Here is a boiled down test case:
>
> 	void main (string[] args)
> 	{
> 		double [] foo = new double [cast(int)1e6];
> 		for (int i=0;i<1e3;i++)
> 		{
> 			for (int j=0;j<1e6-1;j++)
> 			{
> 				foo[j]=foo[j]+foo[j+1];
> 			}
> 		}
> 	}
>
> Any ideas? Am I somehow not hitting a vital compiler optimization?  
> Thanks for
> your help.

I figured it out.

in D, the default value for doubles is nan, so you are adding countless  
scores of nan's which is costly for some reason (not a big floating point  
guy, so I'm not sure about this).

In C/C++, the default value for doubles is 0.

BTW, without any initialization of the array, what are you expecting the  
code to do?  In the C++ version, I suspect you are simply adding a bunch  
of 0s together.

Equivalent D code which first initializes the array to 0s:

void main (string[] args)
{
     double [] foo = new double [cast(int)1e6];
     foo[] = 0; // probably want to change this to something more meaningful
     for (int i=0;i<cast(int)1e3;i++)
     {
         for (int j=0;j<cast(int)1e6-1;j++)
         {
             foo[j]+=foo[j+1];
         }
     }
}

On my PC, it runs almost exactly at the same speed as the C++ version.

-Steve