GCC modifications

Disabled trampoline code
Attributes
__nonnull__ 
__sentinel__ NULL pointer checks for varargs functions
__bounded__ checks on buffers and sizes
-Wstack-larger-than-N

See the gcc-local(1) manual page.
More below in exploit mitigation

Work has been done to ensure that these features are consistent across GCC versions (Some architectures use gcc 2.95.3, others gcc 3.3.5, with 3.3, 3.4, 4.0, 4.1 and 4.2 available as ports)