The bug was reported for AIX before today's changes.
I reproduced the problem on Fedora 17 x86-64 when setting
GC_MARK_STACK by hand, and I presume it occurs with default
configurations on HP-UX and Unixware.
Trunk fix on 2013-01-14 by Dmitry Antipov <dmantipov@yandex.ru>:
Fix compilation with GC_MARK_STACK == GC_USE_GCPROS_AS_BEFORE.
* eval.c (eval_sub): Protect `form' from being GCed before its
car and cdr becomes protected with the backtrace entry.