This patch restricts the behaviour of referencing via .Lfoo$local
local aliases, introduced in https://reviews.llvm.org/D73230, to
STV_DEFAULT globals only.
Hidden symbols via --fvisiblity=hidden (https://gcc.gnu.org/wiki/Visibility)
is an important scenario.
Benefits:
- Improves the size of object files by using fewer STT_SECTION symbols.
- The code reads a bit better (it was not obvious to me without going
back to the code reviews why the canBenefitFromLocalAlias function
currently doesn't consider visibility).
- There is also a side benefit in restoring the effectiveness of the
--wrap linker option and making the behavior of --wrap consistent
between LTO and normal builds for references within a translation-unit.
Note: this --wrap behavior (which is specific to LLD) should not be
considered reliable. See comments on https://reviews.llvm.org/D73230
for more.
Differential Revision: https://reviews.llvm.org/D85782
-fno-PIC and -fPIE code generally cannot be linked in -shared mode and there is no benefit accessing via local aliases.
Actually, a .Lfoo$local reference will be converted to a STT_SECTION (if no section relaxation) reference which will cause the section symbol (sizeof(Elf64_Sym)=24) to be generated.
A global symbol that is defined in a comdat should not generate an alias since
call sites that would've referred to that symbol will refer to their own
independent local aliases rather than the surviving global comdat one. This
could result in something that looks like:
```
ld.lld: error: relocation refers to a discarded section: .text._ZN3fbl8internal18NullFunctionTargetIvJjjPjEED1Ev.stub
>>> defined in user-x64-clang/obj/system/ulib/minfs/libminfs.a(minfs._sources.file.cc.o)
>>> section group signature: _ZN3fbl8internal18NullFunctionTargetIvJjjPjEED1Ev.stub
>>> prevailing definition is in user-x64-clang/obj/system/ulib/minfs/libminfs.a(minfs._sources.vnode.cc.o)
>>> referenced by function.h:169 (../../zircon/system/ulib/fbl/include/fbl/function.h:169)
>>> minfs._sources.file.cc.o:(minfs::File::AllocateAndCommitData(std::__2::unique_ptr<minfs::Transaction, std::__2::default_delete<minfs::Transaction> >)) in archive user-x64-clang/obj/system/ulib/minfs/libminfs.a
```
We ran into this when experimenting with a new C++ ABI for fuchsia
(refer to D72959) which takes relative offsets between comdat'd functions
which is why the normal C++ user wouldn't run into this.
Differential Revision: https://reviews.llvm.org/D77429
