Very slow speed of gcc compiled C-program under Linux - c

Question

I have two OS on my PC with i7-3770 # 3.40 GHz. One OS is latest Linux Kubuntu 18.04, the other OS is Windows 10 Pro running on same HDD.
I have tested a simple funny program written in C language doing some arithmetic calculations from number theory. On Kubuntu compiled with gcc 7.3.0, on Windows compiled with gcc 5.2.0. built by MinGW-W64 project.
The result is amazing, running program was 4-times slower on Linux, than on Windows.
On Windows the elapsed time is just 6 seconds. On Linux is elapsed time 24 seconds! On the same hardware.
I tried on Kubuntu to compile with some CPU specific options like "gcc -corei7" etc., but nothing helped. In the program is used "math.h" library, so the compilation is done with "-lm" on both systems. The source code is the same.
Is there a reason for this slow speed under Linux?
Further more I have compiled the same code also on older 32-bit machine with Core Duo T2250 # 1.73 GHz under Linux Mint 19 with gcc 7.3.0. The elapsed time was 28 seconds! Not much difference than 64-bit machine running on double frequency under Linux.
The sorce code is below, you can compile it and test it.
/* Program for playing with sigma(n) and tau(n) functions */
/* Compilation of code: "gcc name.c -o name -lm" */
#include <stdio.h>
#include <math.h>
#include <time.h>
int main(void)
{
double i, nq, x, zacatek, konec, p;
double odx, soucet, delitel, celkem, ZM;
unsigned long cas1, cas2;
i=(double)0; soucet=(double)0; celkem=(double)0; nq=(double)0;
zacatek=(double)1; konec=(double)1000000; x=zacatek;
ZM=(double)16 / (double)10;
printf("\n Program for playing with sigma(n) and tau(n) functions \n");
printf("---------------------------------------------------------\n");
printf("Calculation is running in range from %.0lf to %.0lf\n\n\n", zacatek, konec);
printf("Finding numbers which have sigma(n)/n = %.3lf\n\n", ZM);
cas1=time(NULL);
while (x <= konec) {
i=1; celkem=0; nq=0;
odx=sqrt(x)+1;
while (i <= odx) {
if (fmod(x, i)==0) {
nq++;
celkem=celkem+x/i+i;
}
i++;
}
nq=2*nq-1;
if ((odx-floor(odx))==0) {celkem=celkem-odx;}
if (fabs(celkem - (ZM*x)) < 0.001) {
printf("%.0lf has sum of all divisors = %.3lf times the number itself (%.0lf, %.0lf)\n", x, ZM, celkem, nq+1);
}
x++;
}
cas2=time(NULL);
printf("\n\nProgram ended.\n\n");
printf("Elapsed time %lu seconds.\n\n", cas2-cas1);
return (0);
}