#include <stdio.h>
#include <string.h>
#include <time.h>
#include <stdlib.h>
#include <math.h>
#include <float.h>

void divide_time(float min_before_reset, float reset_to_value)
{
   float num = reset_to_value;

   clock_t start, end;
   long i;
   long denormals_done = 0;
   long iterations_to_do = 10000000;

   start=clock();

   for (i = 0; i < iterations_to_do; i++)
   {
      if (num <= min_before_reset)
      {  num = reset_to_value;
      }

      num = num / 1.1f;

   // note: if you delete this check, the compiler will
   //  delete all references to 'num'!
      if (num < FLT_MIN)
      {  denormals_done++;
      }

   }

   end=clock();
   printf("denormals processed: %li\n", denormals_done);
   printf("Elapsed time: %f\n",(float)(end-start)/CLOCKS_PER_SEC);
}


int main(int argc, char*argv[])
{


   // normal case
   divide_time(FLT_MIN * 2, 10.0f);

   // forced denormals case
   divide_time(0, FLT_MIN);
}

I measure ~80x in the denormal case.

To make GCC do the same thing, you can also add this code... (thanks to this)

    #include <xmmintrin.h>

    int oldMXCSR = _mm_getcsr(); //read the old MXCSR setting
    int newMXCSR = oldMXCSR | 0x8040; // set DAZ and FZ bits
    _mm_setcsr( newMXCSR ); //write the new MXCSR setting to the MXCSR

    ... // do your work with denormals off here

    //restore old MXCSR settings to turn denormals back on if they were on
    _mm_setcsr( oldMXCSR );

...and compile like this: gcc -O3 -msse3 -mfpmath=sse denormal.c.

If you just use the aformentioned gcc flags but not the code, you get better but still 20x worse performance.

(Netburst, no vectorizer)
# icc -O3 denormal_matt.c -o denorm
denormals processed: 0
Elapsed time: 0.160000
denormals processed: 10000000
Elapsed time: 12.340000

(Netburst, using vectorizer)
# icc -xW -O3 denormal_matt.c -o denorm
denormals processed: 0
Elapsed time: 0.070000
denormals processed: 10000000
Elapsed time: 0.080000

(Merom, gcc, no FTZ)

denormals processed: 0
Elapsed time: 0.100000
denormals processed: 10000000
Elapsed time: 1.700000

-- MattWalsh - 19 May 2004