http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55431
--- Comment #5 from Alan Modra <amodra at gmail dot com> 2013-02-12 03:04:28 UTC --- Created attachment 29420 --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=29420 use /proc/self/auxv At the time the original code was being developed, linux-2.4.x was in widespread use. /proc/self/auxv was introduced with linux-2.6.0 in Dec 2003. I guess it's reasonable to rely on that nowadays. BTW, it's not entirely an optimization to condition reading of altivec regs on AT_HWCAP. I believe it may be possible to segv if the unwinder tries to dereference a location past the end of struct sigcontext.
