On 18 July 2016 at 03:09, greg7mdp wrote: > Thanks. However that's exactly what it appears to do, i.e. when I compile > with the option -mpopcnt, the macro __POPCNT__ is defined, but is not > defined when the option is not given. The instruction can be enabled without the -mpopcnt option, e.g. -march=nehalem or -march=skylake etc. should enable it. So testing for an explicit command-line option is the wrong thing to do. Testing for the instruction being enabled (however it got enabled) is the right thing to do, and that's what the macro does.
