GCC: Mitigation against unsafe data speculation (CVE-2017-5753)

The patches I posted earlier this year for mitigating against
CVE-2017-5753 (Spectre variant 1) attracted some useful feedback, from
which it became obvious that a rethink was needed. This mail, and the
following patches attempt to address that feedback and present a new
approach to mitigating against this form of attack surface.[…]

https://gcc.gnu.org/ml/gcc-patches/2018-07/msg00423.html