asm volatile statement reordering

[Jeffrey Walton]

>> GCC guesses wrong on occasion. It will remove code that has value that
>> but does not produce an output because the language does not allow us
>> to express it.
>
> Yes, the compiler is bad at guessing what you mean.  It does not even
> try; it only cares about what you *say*.

I've had both SIGILL probes and memory zeroizers removed by GCC. I was
very clear about what I said should be done.

The language did not give me a way to tell GCC, please don't mess with
this code and please ensure it executes like I've written it.

Jeff