> Why not enablind the option by default if GCC 5.1 or newer is detected? Is there a risk of breaking third-party extensions?
> Should we pass the MPX compiler options to third-party extensions by way?
On a processor that does not support MPX technology, MPX specific instructions will be replaced by nop's and will introduce a performance loss. Also, third party extensions might need to be patched in order to work with MPX. One example is the math module, which needed the bnd_legacy attribute to disable instrumentation when calling libc functions (which are not instrumented and generated compile errors).

> I'm not sure that it's ok to add such change to Python 2.7. It's border line between new feature and supporting a new architecture.
I believe that Python 2.7 should benefit from this change, since it is still used in real life applications, one example being Openstack Swift.