[Bug lto/99898] Possible LTO object incompatibility on gcc-10 branch

["hubicka at ucw dot cz" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99898

--- Comment #10 from Jan Hubicka <hubicka at ucw dot cz> ---
> Many of the *.opt changes are target specific, so you'd need to test it also
> across all targets, and furthermore it depends on what exactly is being
> saved/restored, many options might be at the same spot.
> So perhaps we want to compute some hash of the options stuff (e.g. compute it
> by the awk scripts that emit options*.[ch]) and use that to determine LTO
> compatibility in addition to the version?

That would work.  One does not really do that in lto header, simply
stream the hash before streaming out the optimization_node decl.
Bit sad would be that w/o version info you have no indication if you
mixed new compiler with old objects or vice versa, but that is minor
anoyance I guess.  It would be good that compiler would just
sorryclaiming that it can not read object files created by different
version..

I believe we already safe a diff from default values rather than
streaming out all values. An option would be tom strea the option names
rather than indexes so adding/removing completely unrelated option does
not disturb the file format.

Honza
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