Timing too good to be true

[Paul Jurczak]

I'm running this test program with multiple compilers on Xubuntu 
and I'm getting impossibly short timings with LDC compared to DMD 
and GDC (timing details in comments below). I would appreciate 
any ideas about what is going on here?

/*
Xubuntu 13.04 64-bit Core i5 3450S 2.8GHz:
GDC 4.8.1:     gdc-4.8 -m64 -march=native -fno-bounds-check 
-frename-registers -frelease -O3
669171001  826ns  e28_0
669171001  824ns  e28_1

DMD64 2.063.2: dmd -O -noboundscheck -inline -release
669171001  1115ns  e28_0
669171001  1955ns  e28_1

LDC 0.11.0: ldmd2 -m64 -O -noboundscheck -inline -release
669171001  25ns  e28_0
669171001  25ns  e28_1
*/

module main;

import std.stdio, std.algorithm, std.range, std.datetime, 
std.typetuple;


int e28_0(int N = 1002) {
	int diagNumber = 1;					
	int sum        = diagNumber;	

	for (int width = 2; width < N; width += 2)	
		for (int j = 0; j < 4; ++j) {			
			diagNumber += width;				
			sum        += diagNumber;			
		}

	return sum;
}


int e28_1(int N = 1002) {
	int diagNumber = 1;					
	int sum        = diagNumber;	

	foreach (width; iota(2, N, 2))
		foreach (_; 0..4) {			
			diagNumber += width;				
			sum        += diagNumber;			
		}

	return sum;
}


void main()
{
   enum      N = 100_000;
   StopWatch sw;

    foreach (iFun, fun; TypeTuple!(e28_0, e28_1)) {
       int [N] results;
       long[N] timings;

       foreach (i; 0..N) {
          sw.reset;
          sw.start;
          results[i] = fun();
          sw.stop;
          timings[i] = sw.peek.nsecs;
       }

       writeln(results.reduce!min, "  ", timings.reduce!min, "ns  
e28_", iFun);
   }
}