> Any reason not to cherry-pick this to emacs-26?

No, once we fix it up. Although adding 'volatile' happened to work for Gemini's 
compiler, it won't suffice in general as the C standard does not require 
volatile variables to survive their last access (which is what the patch was 
assuming). Furthermore, Fbyte_code bypasses that patch, so the bug could still 
occur even if 'volatile' cured the bug in the more-common code path.

A simple way to ensure that the constant vector survives GC is to have 
exec_byte_code put the vector into a GC-visible slot. As it happens, there's a 
spare slot that we can appropriate, so this won't cost us stack (or heap) space. 
I installed the first attached patch into master to do that, and backported the 
patch series into emacs-26 via the last two attached patches.

Thanks, Gemini, for the good work in debugging this problem and writing that 
test case. GC bugs can be nasty.