[PATCH] D45195: Add --check-library-dependency to maintain compatibility with other linkers

[Peter Smith via Phabricator via llvm-commits]

peter.smith added a comment.

Perhaps --check-library-dependency is too strong a name for the cases that this can catch. I think it is really a --find-library-backreferences that can be helpful as part of resolving differences in behaviour between lld and other unix linkers, but there are limitations.

Looking at this from a perspective of someone trying to migrate from GNU to LLD. Consider these cases for which --check-library-dependency will detect a back reference, but the outcomes are different in each case:
Library f1.a containing f1.o that defines f()
Library f2.a containing f2.o that defines f()
Library g.a containing g.o that defines g(), g.o references f()
Object m.o containing main() that references g()

The link lines below all result in the same backward reference warning:
m.o f1.a g.a : result in an undefined symbol error on a GNU linker, succeed with LLD with f1.o from f1.a.
m.o f1.a g.a f1.a : result in GNU Linker and LLD extracting the same library, f1.o from f1.a
m.o f1.a g.a f2.a f1.a : result in different outputs as the GNU Linker will select f2.o from f2.a to define f and LLD will detect f1.a

The next example does not give a backward reference warning but does give different results on GNU and LLD
m.o --start-group f1.a g.a f2.a f1.a --end-group :  result in different outputs as the GNU Linker will select f2.o from f2.a to define f and LLD will detect f1.a

The last example could be problematic for users wanting to start a new project and guarantee that it will work with LLD and GNU assuming --check-library-dependency. I think that we may need a two level index, file level and group level, so that we can check for back-references with the group.


https://reviews.llvm.org/D45195