$ for i in `seq 3`; do gcc-6 -x c-header /dev/null -o x.h.gch; sha256sum x.h.gch; done
98d8093503565836ba6f35b7adf90330d63d9d1c76dfb8e3ad1aeb2d933d1a45 x.h.gch
17e5de099860d94aaa468c5ad103b3f0dd5e663f6cdbd01b4f12cf210023e71c x.h.gch
3cc2f1c0a517b5fedbbd49bb3a34084d9aa1428f33f3c30278a8c61f9ed9ba88 x.h.gch