Comment 2 for bug 549673

commit e7b2c507c364da9395ad1f9591210dac44f24afd
Author: Nikodemus Siivola <email address hidden>
Date: Mon Aug 1 16:46:26 2011 +0300

    more robust backtraces for syscalls on x86

     * new optimization policy: ALIEN-FUNCALL-SAVES-FP-AND-PC Set to 3 for
       self-build on x86 to get reliable more backtraces there, and 0 for
       other platforms. (1 matches the old SPEED <= DEBUG behaviour.)

     * When using a saved FP, and an interrupt context has a bogus
       FP, assume it is an interrupted syscall frame.