这段代码打破了严格的别名规则？

Question

I read http://blog.qt.digia.com/blog/2011/06/10/type-punning-and-strict-aliasing/ , and found this piece of code.

QDataStream &QDataStream::operator>>(qint16 &i)
{
...
        register uchar *p = (uchar *)(&i);
        char b[2];
        if (dev->read(b, 2) == 2) {
            *p++ = b[1];
            *p = b[0];
...

The author claims MSVC optimizes away the assignments which I find quite strange.

Does MSVC really exploit strict aliasing rule?

And isn't uchar* specially allowed to be used to do type punning?

Answer 1

It was either a compiler bug or a bug in the code that called this method. The "strict aliasing rule" allows the object i refers to be accessed using character types, regardless of what the type of that object actually is. The code that calls this method doesn't even actually have to pass an reference to an object that's compatible with qint16 for this function to have defined behaviour.