Both the compiler and the CPU can reorder code, as long as the results don't change from the perspective of the code that is executing.

For example, a compiler could take the following code:

    global_x = 1;
    global_y = 2;
    global_x = 3;

And change it into:

    global_x = 3;
    global_y = 2;

Compilers on all platforms reorder statements like this. You can prevent this by making your variables `volatile`, but 99% of the time when you write the word "volatile" in your code you're making a mistake.

In this case, the compiler could have re-ordered the instructions, but he checked to ensure that it hadn't.

It's not just the compiler reordering statements. The CPU can reorder loads and stores that appear in the machine code.

This is not about the compiler reordering statements. This happens in hardware. If a core executes two store instructions, another core might see them in any order.