On 31/05/16 21:37, Jeffrey Walton wrote: >> Like Florian said, 'w' is not being used so the compiler may optimise the >> whole sequence away. > > I'm pretty sure this is another instance of a language defect. The > C/C++ language does not allow us to properly express ourselves, and > the result is the removal of critical code. The problem here is that GCC knows that the CRC instructions have no side effects beyond their operands. So GCC doesn't generate the instructions. > This is at least the third or fourth time. The one prior to this > resulted in CVE-2016-3995. Of course GCC deletes dead code. That's one of the most basic optimizations. In your case I'd just use the result as an input operand into an asm. Andrew.
