Finding large difference b/w execution time of c++ and D codes for same problem

[Marco Leise]

I like optimization challenges. This is an excellent test
program to check the effect of different floating point types
on intermediate values. Remember that when you store values in
a float variable, the FPU actually has to round it down to
that precision, store it in a 32-bit memory location, then
load it back in and expand it - you _asked_ for that.

I compiled with LDC2 and these are the results:

D code serial with dimension 32768 ...
  using floats  Total time: 13.399 [sec]
  using doubles Total time:  9.429 [sec]
  using reals   Total time:  8.909 [sec] // <- !!!

You get both, 50% more speed and more precision! It is a
win-win situation. Also take a look at Phobos' std.math that
returns real everywhere.

Modified code:
---8<-------------------------------

module main;

import std.datetime;
import std.metastrings;
import std.stdio;
import std.typetuple;


enum DIM = 32 * 1024;

int juliaValue;

template Julia(TReal)
{
	struct ComplexStruct
	{
		float r;
		float i;
	
		TReal squarePlusMag(const ComplexStruct another)
		{
			TReal r1 = r*r - i*i + another.r;
			TReal i1 = 2.0*i*r + another.i;
			
			r = r1;
			i = i1;
			
			return (r1*r1 + i1*i1);
		}
	}

	int juliaFunction( int x, int y )
	{
		auto c = ComplexStruct(0.8, 0.156);
		auto a = ComplexStruct(x, y);
	
		foreach (i; 0 .. 200)
			if (a.squarePlusMag(c) > 1000)
				return 0;
		return 1;
	}
	
	void kernel()
	{
		foreach (x; 0 .. DIM) {
			foreach (y; 0 .. DIM) {
				juliaValue = juliaFunction( x, y );
			}
		}
	}
}

void main()
{
	writeln("D code serial with dimension " ~ toStringNow!DIM ~ " ...");
	StopWatch sw;
	foreach (Math; TypeTuple!(float, double, real))
	{
		sw.start();
		Julia!(Math).kernel();
		sw.stop();
		writefln("  using %ss Total time: %s [sec]",
		         Math.stringof, (sw.peek().msecs * 0.001));
		sw.reset();
	}
}

------------------------------->8---

-- 
Marco