> 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. This is at least the third or fourth time. The one prior to this resulted in CVE-2016-3995. > On a side note, if you want to detect the availability of certain extensions > at runtime have you considered using the hwcaps mechanism? > > https://community.arm.com/groups/android-community/blog/2014/10/10/runtime-detection-of-cpu-features-on-an-armv8-a-cpu Yes, I tired that first. Its not as portable as I hoped. It mostly worked on Linux, but not iOS and a couple other platforms. Its also missing constants on Linux for Aarch32 execution environments on Aarch64. Jeff
