Re: Stack allocation of Lisp objects

[Paul Eggert]

Stefan Monnier wrote:

> Apparently there's an AUTO_STRING somewhere which we stick into a heap
> allocated cons cell.

A plausible scenario is that you got bitten by a GCC bug, where GCC ignores 
___attribute__ ((aligned (8))) on your platform.  At least, that's how I 
reproduced the problem on x86.  I filed a GCC bug report 
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63495>.

If I'm right, the GCC bug cascaded into an Emacs portability bug: Emacs should 
use stack-based strings only if stack-base cons also works, as otherwise 
STACK_CONS could create a heap-based cons cell that points to a stack-based 
string, the symptom you observed.  I fixed that bug in Emacs trunk bzr 118080.

Because of this fix, Emacs no longer uses stack-based allocation on x86 + recent 
GCC, which is a bit of a downer.  It should still work with older GCC though, 
and I expect the GCC folks will fix the bug in newer GCC at some point.