[Bug boehm-gc/52179] boehm-gc incompatible with aslr on darwin11

["howarth at nitro dot med.uc.edu" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52179

--- Comment #6 from Jack Howarth <howarth at nitro dot med.uc.edu> 2012-02-09 15:05:50 UTC ---
All of these crashes appear in the section in the GC_mark_from subroutine of
mark.c commented as...

       /* Try to prefetch the next pointer to be examined asap.        */
        /* Empirically, this also seems to help slightly without        */
        /* prefetches, at least on linux/X86.  Presumably this loop     */
        /* ends up with less register pressure, and gcc thus ends up    */
        /* generating slightly better code.  Overall gcc code quality   */
        /* for this loop is still not great.                            */

GC_mark_from itself is commented as...

/*
 * Mark objects pointed to by the regions described by
 * mark stack entries between GC_mark_stack and GC_mark_stack_top,
 * inclusive.  Assumes the upper limit of a mark stack entry
 * is never 0.  A mark stack entry never has size 0.
 * We try to traverse on the order of a hblk of memory before we return.
 * Caller is responsible for calling this until the mark stack is empty.
 * Note that this is the most performance critical routine in the
 * collector.  Hence it contains all sorts of ugly hacks to speed
 * things up.  In particular, we avoid procedure calls on the common
 * path, we take advantage of peculiarities of the mark descriptor
 * encoding, we optionally maintain a cache for the block address to
 * header mapping, we prefetch when an object is "grayed", etc.
 */