Visual Studio optimizes to different behavior in Release mode

Asked Nov 04 '12 at 14:53

Active Nov 04 '12 at 15:17

Viewed 102 times

I have a C++ application developed with Visual Studio 2010. I have a static method, which is called to decode messages received via serial hardware.

When I use the "Release" configuration, the compiler somehow optimizes the method in a strange way and the method doesn't work as expected. When I use the "Debug" configuration, everything is fine.

Here's the method:

std::vector<BYTE> CMsg::DecodeMsg(std::vector<BYTE> & msg)
{
    std::vector<BYTE> asciiDecoded;
    for (size_t i = 0; i < msg.size() - 1; i++)
    {
        auto val2 = BaseUtils::hex2Ascii(msg.at(i++), msg.at(i));
        asciiDecoded.push_back(val2);
    }

    return asciiDecoded;
}

I think there is something messed up with the i-variable caused by some optimizations (I use /O2) in Release configuration.

asked Nov 04 '12 at 14:53

Simon

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I believe `msg.at(i++), msg.at(i)` is UB. – GSerg Nov 04 '12 at 14:59
The additional increment of the variable inside the for loop in general or just the msg.at(i++) is undefined behavior? – Simon Nov 04 '12 at 15:07
1

No, it's about [using both `i` and `i++` in the same parameter list](http://stackoverflow.com/a/7568163/11683). – GSerg Nov 04 '12 at 15:09
You're right. Using BaseUtils::hex2Ascii(msg.at(i), msg.at(i + 1)); and afterwards just a i++; solved the issue. Thanks! Thanks also for the link – Simon Nov 04 '12 at 15:14
1

@Simon: you should also fix the function to properly handle an empty `msg` argument. – Michael Burr Nov 05 '12 at 01:01
You are right, but this is already checked, before this method can be called – Simon Nov 05 '12 at 15:10
Comment of GSerg solved the issue. – Simon Nov 04 '12 at 15:17

Visual Studio optimizes to different behavior in Release mode

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