[$] Relief for retpoline pain

Indirect function calls — calls to a function whose address is stored in a
pointer variable — have never been blindingly fast, but the Spectre
hardware vulnerabilities have made things far worse. The indirect branch
predictor used to speed up indirect calls in the CPU can no longer be
used, and performance has suffered accordingly. The 'retpoline'
mechanism
was a brilliant hack that proved faster than the hardware-based solutions
that were tried at the beginning. While retpolines took a lot of the pain
out of Spectre mitigation, experience over the last year has made it clear
that they still hurt. It is thus not surprising that developers have been
looking for alternatives to retpolines; several of them have shown up on
the kernel lists recently.