#include <windows.h>
#include <stdio.h>
#define _USE_MATH_DEFINES
#include <math.h>
#include <conio.h>

const double Log10 = log(10.0);

int main()
{
    int x = 100;
    printf("%.20f\n\n", log((double)x) / Log10);
    printf("%.20f\n\n", log((double)x) / Log10);
    while (!kbhit()) {}
}

/Od /Ob2 /Op /D "WIN32" /D "NDEBUG" /D "_CONSOLE" /D "_ATL_MIN_CRT" /D "_MBCS" /FD /MT /GS /Fo"Release/" /Fd"Release/vc70.pdb" /W3 /nologo /c /Wp64 /Zi /TP

    printf("%.20f\n\n", log((double)x) / Log10);
0040103D DB 45 FC         fild        dword ptr [x] 
00401040 DD 5D EC         fstp        qword ptr [ebp-14h] 
00401043 DD 45 EC         fld         qword ptr [ebp-14h] 
00401046 83 EC 08         sub         esp,8 
00401049 DD 1C 24         fstp        qword ptr [esp] 
0040104C E8 CF 00 00 00   call        log (401120h) 
00401051 83 C4 08         add         esp,8 
00401054 DC 35 00 02 41 00 fdiv        qword ptr [Log10 (410200h)] 
0040105A 83 EC 08         sub         esp,8 
0040105D DD 1C 24         fstp        qword ptr [esp] 
00401060 68 34 C1 40 00   push        offset KERNEL32_NULL_THUNK_DATA+30h (40C134h) 
00401065 E8 44 00 00 00   call        printf (4010AEh) 
0040106A 83 C4 0C         add         esp,0Ch

Y_P>#include <windows.h>
Y_P>#include <stdio.h>
Y_P>#define _USE_MATH_DEFINES
Y_P>#include <math.h>
Y_P>#include <conio.h>

Y_P>const double Log10 = log(10.0);

Y_P>int main()
Y_P>{
Y_P>    int x = 100;
Y_P>    printf("%.20f\n\n", log((double)x) / Log10);
Y_P>    printf("%.20f\n\n", log((double)x) / Log10);
Y_P>    while (!kbhit()) {}
Y_P>}
Y_P>

This option improves the consistency of floating-point tests for equality and inequality by disabling optimizations that could change the precision of floating-point calculations.

By default, the compiler uses the coprocessor's 80-bit registers to hold the intermediate results of floating-point calculations. This increases program speed and decreases program size. However, because the calculation involves floating-point data types that are represented in memory by less than 80 bits, carrying the extra bits of precision (80 bits minus the number of bits in a smaller floating-point type) through a lengthy calculation can produce inconsistent results.

With /Op, the compiler loads data from memory prior to each floating-point operation and, if assignment occurs, writes the results back to memory upon completion. Loading the data prior to each operation guarantees that the data does not retain any significance greater than the capacity of its type.

A program compiled with /Op may be slower and larger than one compiled without /Op.

2.00000000000000000000

2.00000000000000000000

#include <windows.h>
#include <stdio.h>
#define _USE_MATH_DEFINES
#include <math.h>
#include <conio.h>

const double Log10 = log(10.0);

int main()
{
    int x = 100;
    double a = log((double)x) / Log10;
    double b = log((double)x) / Log10;
    printf("%.20f\n\n", a);
    printf("%.20f\n\n", b);
    while (!kbhit()) {}
}

double a = .....;
// здесь
double b = .....;

// и

double a = .....;
printf("%.20f\n", a); // внутри тоже пользуется плавающей арифметикой - для перевода double в десятичный вид
// здесь
double b = .....;