[Bug pch/71934] pch cannot be disabled so gcc cannot be position independent

["jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71934

--- Comment #16 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
And if doing it at restore time would be too hard, there is always an option to
precompute a "relocation" table during PCH saving and store it at the very end
of the PCH file, so that normally when successfully loading PCH at the expected
address we don't need to touch that area, but if we map it elsewhere, we can
read it and relocate.  Such relocation table could either contain all the
addresses in the global GTY roots that need += bias (relative to say
gt_ggc_rtab or something similar) and all the addresses in the mapped blob that
need += bias (relative to the start of the mapped blob), ideally sorted during
PCH saving by increasing address.  If that would be too large, another option
would be try to make it more compact, e.g. after that sorting of addresses
represent it as unsigned short delta from the last address, unless that ushort
is 0, which then would be followed by full sizeof(void*) address relative to
gt_ggc_rtab or the map base;
perhaps even ushort 0 followed by address relative to gt_ggc_rtab and ushort 1
followed by address relative to map base.