We determine machine capabilities by performing an os/platform *query* first, like getauxv(). If the *query* fails, we move onto a cpu *probe*. The cpu *probe* tries to exeute an instruction and then catches a SIGILL on Linux or the exception EXCEPTION_ILLEGAL_INSTRUCTION on Windows. Some OSes fail to hangle a SIGILL gracefully, like Apple OSes. Apple machines corrupt memory and variables around the probe.
Currently the CRYPTOPP_BOOL_XXX macros set the macro value to 0 or 1. If we remove setting the 0 value (the #else part of the expression), then the self tests speed up by about 0.3 seconds. I can't explain it, but I have observed it repeatedly.
This check-in prepares for the removal in Upstream master
Jeffrey Walton