为什么这个 if/else 语句似乎被优化掉了？

Question

This is the code in question:

    void DeckTug::StickCallback(unsigned long long evtID, DWORD value)
{
    long int val = value;


    if (evtID == stickXInputID || evtID == stickAxisXInputID)
        stickXpct = (((double)val)) / 325.94;
    else if (evtID == stickYInputID || evtID == stickAxisYInputID) {
        stickYpct = (((double)val)) / 325.94;
        if(isAuto)
        if ((stickYpct < 0.0)) {
            acPullingTug = true;
            tugTBoffset = tugReversed ? towbarAttachAft * (-1.0) : towbarAttachForward;
        }
        else {
            acPullingTug = false;
            tugTBoffset = tugReversed ? towbarAttachAft * (-1.0) : towbarAttachForward;
        }
    }
}

When I compile a debug build, this runs perfectly. When I compile a release build, it does not work. When I attach the visual studio debugger to the release version, I can break on the first if statement and on the closing brace of the function, but I cannot hit a break point anywhere else, and neither stickXpct or stickYpct are ever being assigned anything, although in the debugger I can see that "value" has a valid value, and "evtID" DOES equal one of inputIDs. In conclusion, it looks to me like, in the release version of the code only, both the first "if" statement and the first "else if" statement only evaluate to false, even when one of them should evaluate to true. Does anyone know what is going on here? because I don't.

Thanks so much, Farley

Answer 1

Edit: changed answer in response to comments

Try adding volatility

void DeckTug::StickCallback(unsigned long long evtID, DWORD value)
{
    long int val = value;
    volatile unsigned long long _evtID = evtID;


    if (_evtID == stickXInputID || _evtID == stickAxisXInputID)
        stickXpct = (((double)val)) / 325.94;
    else if (_evtID == stickYInputID || _evtID == stickAxisYInputID) {
        stickYpct = (((double)val)) / 325.94;
        if(isAuto)
            if ((stickYpct < 0.0)) {
                acPullingTug = true;
                tugTBoffset = tugReversed ? towbarAttachAft * (-1.0) : towbarAttachForward;
            }
            else {
                acPullingTug = false;
                tugTBoffset = tugReversed ? towbarAttachAft * (-1.0) : towbarAttachForward;
            }
    }
}

That should prevent the compiler from optimizing those branches until you can track down why it wants to optimize those branches away.