optimization bug by anthonyjoquinto

The programming language is C++ and the bug only happens when in Release, in Debug everything is fine.

When trying to do a sum of the elements in a char array in a loop or one after the other, the result is not the expected one. This problem happens when the sum is made in a char with a succession of "+" and the target char is 0 at the beginning.

ex.:
#include <iostream>

int main()
{
    unsigned char byContent[16] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0xA, 0xB, 0xC, 0xD, 0xE, 0xF };
    unsigned char *data = new unsigned char[2048];

    memcpy(data, byContent, 16);

    unsigned int *actualData = (unsigned int*) data;
    unsigned int uiCheckSum = 0;
    unsigned int cnt = 0;

    //the 4 is 16/4, the 16 is the size given to memcpy
    for (cnt=0; cnt<4; cnt++, actualData++)
    {
        uiCheckSum += *actualData;
    }

    unsigned char checkSum = 0;
    unsigned char *totalCheckSum = (unsigned char*)&uiCheckSum;
    /*for (cnt=0; cnt<4; cnt++)
    {
        //printf("losing time");
        checkSum = (unsigned char)(checkSum + totalCheckSum[cnt]);
    }*/

    //printf("totalCheckSum[0] = %i\n", totalCheckSum[0]);
    //printf("totalCheckSum[1] = %i\n", totalCheckSum[1]);
    //printf("totalCheckSum[2] = %i\n", totalCheckSum[2]);
    //printf("totalCheckSum[3] = %i\n", totalCheckSum[3]);

    checkSum = (unsigned char)(checkSum + totalCheckSum[0]);
    checkSum = (unsigned char)(checkSum + totalCheckSum[1]);
    printf("checkSum = %i and should be equal to 52 (24 + 28)\n", checkSum);
    checkSum = (unsigned char)(checkSum + totalCheckSum[2]);
    checkSum = (unsigned char)(checkSum + totalCheckSum[3]);

    printf("checkSum = %i and should be equal to 120", checkSum);

    while(1) {}
}

The first summation seems to modify the value of the next char in the array. If checkSum is not 0 at the beginning, everything is fine and if the printf() are not commented, it works too because there is no optimization.