The behavior is only well defined if the shift amount is strictly less than the width of the operand. If `size_t` is 32 bits, then shifting right by 32 bits is undefined.

I know of three different ways in which platforms implement shifts by greater than the word size.

Googled it. I think he means undefined behavior.

Reading an uninitialized variable is UB.

An uninitialized variable is UB, not implementation-defined. Thus, the compiler is free to treat the variable as though it doesn't have a value at all, or change it's value at will. It's not uncommon for the value of an uninitalized variable to change at strange places in the code that you wouldn't expect, because the compiler initially said "Variable x will be kept in register %eax", but then without a value to initialize it too, it just uses whatever happens to be in %eax at the time. For example, if they compiled this:

    return x == (1 && x);

Into this:

    movl $0, %eax
    andl $1, %eax
    cmpl %eax, %eax
    # Result in %eax

That will (obviously) return 1 every time, because we compare %eax and %eax. The reason for this is that the value of 'x' changes half-way through the computation (Because the computation is done in %eax, which is where 'x' is assumed to be). This is valid because 'x' is uninitalized, so it doesn't have a defined value.