This is an alternative design to D77512.
D45195 added --warn-backrefs to detect
- A. certain input orders which GNU ld either errors ("undefined reference") or has different resolution semantics
- B. (byproduct) some latent multiple definition problems (-ldef1 -lref -ldef2) which I call "linking sandwich problems". def2 may or may not be the same as def1.
When an archive appears more than once (-ldef -lref -ldef), lld and GNU
ld may have the same resolution but --warn-backrefs may warn. This is
not uncommon. For example, currently lld itself has such a problem:
liblldCommon.a liblldCOFF.a ... liblldCommon.a _ZN3lld10DWARFCache13getDILineInfoEmm in liblldCOFF.a refers to liblldCommon.a(DWARF.cpp.o) libLLVMSupport.a also appears twice and has a similar warning
glibc has such problems. It is somewhat destined because of its separate
libc/libpthread/... and arbitrary grouping. The situation is getting
improved over time but I have seen:
-lc __isnanl references -lm -lc _IO_funlockfile references -lpthread
There are also various issues in interaction with other runtime
libraries such as libgcc_eh and libunwind:
-lc __gcc_personality_v0 references -lgcc_eh -lpthread __gcc_personality_v0 references -lgcc_eh -lpthread _Unwind_GetCFA references -lunwind
These problems are actually benign. We want --warn-backrefs to focus on
its main task A and defer task B (which is also useful) to a more
specific future feature (see gold --detect-odr-violations).
Instead of warning immediately, we store the message and only report it
if no subsequent lazy definition exists.
The use of the static variable backrefDiags is similar to undefs in