Re: [whopr] Design/implementation alternatives for the driver and WPA

["Rafael Espindola" <espindola@google.com>]

>> In ELF you have to think about symbol overriding.  Let's say you link
>> a.o b.o c.o.  a.o has a reference to symbol S.  b.o has a strong
>> definition.  c.o has a weak definition.  a.o and c.o have LTO
>> information, b.o does not.  ELF requires that a.o call the symbol from
>> b.o, not the symbol from c.o.  I don't see how to make that work with
>> the LLVM interface.
>
> This does work.  There are two parts to it.  First the linker's master
> symbol
> table sees the strong definition of S in b.o and the weak in c.o and
> decides to use the strong one from b.o.  Second (because of that) the linker
> calls  lto_codegen_add_must_preserve_symbol("S"). The LTO engine then
> sees it has a weak global function S and it cannot inline those.  Put
> together
> the LTO engine does generate a copy of S, but the linker throws it away
> and uses the one from b.o.

Interesting. The use of lto_codegen_add_must_preserve_symbol is kind
of the opposite of what I had understood. What do you do in this case:

a.o: IL file that contains a reference to "f"
b.o: IL file that has a weak def of "f"

There is no strong definition. Can you inline f into the use in a.o?

> -Nick
>
>

Cheers,
-- 
Rafael Avila de Espindola

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