Wolfgang Jenkner wrote:
> So, perhaps, the FreeBSD 9 default base cc (a patched gcc 4.2.1) has
> some stack-related optimization bug, even on amd64?

Thanks for the detective work. It inspired me to search GCC bug reports, where I 
found a couple that appear to be directly relevant. The corresponding bug was 
fixed in 2008 but GCC 4.2.1 predates the fix. I installed the attached patch 
into the emacs-25 branch, and I hope it works around the compiler bug with 
stack-allocated Lisp objects on FreeBSD 9. Please give it a try.