BUG: Incorrect Calculations with Both /Og and /G5

MORE INFORMATION

The /G5 compiler switch tells the compiler to place the assembly instructions generated into an order that will be more beneficial to the pipelined architecture of the Pentium processor. The /Og switch enables global optimizations.

The following sample code illustrates the problem. Remove the comment slashes (//) from the #pragma line to optimze the line and illustrate a workaround to the problem.

Sample Code

/* Compile options needed: /G5 /Og
*/ 

#include <stdio.h>

//#pragma optimize("g", off)    //uncomment to fix the problem
void main()
{
   double   polylgd[5] = {0.0, 0.0, 0.0, 1.0, 1.0};
   double   valpol,an,an1,temp,dtemp;
   double   *polyptr;
   int      i;

   polyptr = &polylgd[4];
   an  = *polyptr--;
   an1 = *polyptr--;

   for (i = 4; i >= 2; i--)
   {
      dtemp = (double) (i - 1.0) / (double) i ;
      temp  =  dtemp * an;
      temp  = *polyptr - temp;
      *polyptr--;
      an    = an1 + (dtemp + 1.0) * (-0.5) * an;
      an1   = temp;
   }

   valpol = an1 + (-0.5) * an;
   printf("value should be:  0.14843750\n");
   printf("value this run is %.8lf\n", valpol);
}