llvm dev - Feb 2018 - ThinLTO and linkonce_odr + unnamed_addr

Steven Wu <stevenwu at apple.com> writes:
> Add Rafael here. Because he basically ask me the same question in my
commit.
>
> I wonder why clang choose to do unnamed_addr instead of just do visibility
hidden. If linkonce_odr + unnamed_addr is just hidden, is the only difference
linkonce_odr + unnamed_addr not in the symbol table?
A local symbol being in the symbol table is mostly an implementation
detail.

Not marking a linkonce_odr + *global* unnamed_addr hidden is a missing
optimization on non-MachO.

Cheers,
Rafael

I agree with your analysis. I will do the GlobalOpts part as well. The current
fix in thinLTO is still good to have for the input that doesn’t have this
optimization and I will argue to keep it.

Steven
> On Feb 9, 2018, at 11:49 AM, Rafael Avila de Espindola <rafael.espindola
at gmail.com> wrote:
> 
> Steven Wu <stevenwu at apple.com> writes:
> 
>> Add Rafael here. Because he basically ask me the same question in my
commit.
>> 
>> I wonder why clang choose to do unnamed_addr instead of just do
visibility hidden. If linkonce_odr + unnamed_addr is just hidden, is the only
difference linkonce_odr + unnamed_addr not in the symbol table?
> 
> A local symbol being in the symbol table is mostly an implementation
> detail.
> 
> Not marking a linkonce_odr + *global* unnamed_addr hidden is a missing
> optimization on non-MachO.
> 
> Cheers,
> Rafael

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.

Steven
> On Feb 9, 2018, at 12:04 PM, Steven Wu via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> I agree with your analysis. I will do the GlobalOpts part as well. The
current fix in thinLTO is still good to have for the input that doesn’t have
this optimization and I will argue to keep it.
> 
> Steven
> 
>> On Feb 9, 2018, at 11:49 AM, Rafael Avila de Espindola
<rafael.espindola at gmail.com> wrote:
>> 
>> Steven Wu <stevenwu at apple.com> writes:
>> 
>>> Add Rafael here. Because he basically ask me the same question in
my commit.
>>> 
>>> I wonder why clang choose to do unnamed_addr instead of just do
visibility hidden. If linkonce_odr + unnamed_addr is just hidden, is the only
difference linkonce_odr + unnamed_addr not in the symbol table?
>> 
>> A local symbol being in the symbol table is mostly an implementation
>> detail.
>> 
>> Not marking a linkonce_odr + *global* unnamed_addr hidden is a missing
>> optimization on non-MachO.
>> 
>> Cheers,
>> Rafael
> 
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