[llvm-dev] ThinLTO and linkonce_odr + unnamed_addr

Steven Wu via llvm-dev llvm-dev at lists.llvm.org
Fri Feb 9 14:00:45 PST 2018

Interesting. Now I understand the full picture. For MachO, linker is picking based on atoms so it picks the atom with more visibility which doesn’t have the constraint you mentioned. Let me do some more digging. Worst case I will add a different rule for MachO but it might be fine.


> On Feb 9, 2018, at 1:40 PM, Rafael Avila de Espindola <rafael.espindola at gmail.com> wrote:
> Steven Wu <stevenwu at apple.com> writes:
>> Rafael, another question for you. IRLinker currently takes min visibility for the symbol (lib/Linker/LinkModules.cpp:120). Should it take the max visibility? At least that is what ld64 is doing and is somewhat related to this change because I want to make sure the behavior is consistent if we mark more stuff as hidden.
> No, it should take the min visibility to be consistent with the ELF
> spec. The spec says
> If different visibility attributes are specified for distinct
> references to or definitions of a symbol, the most constraining
> visibility attribute must be propagated to the resolving symbol in the
> linked object.
> The reason the ELF spec mandates the minimum is that a .o having a
> hidden symbol foo means that that file can access foo in a way that is
> only valid under the assumption it is local. It not might change the pic
> register before doing a call for example.
> Cheers,
> Rafael

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