[PATCH] D34355: [LLD][ELF] Define _GLOBAL_OFFSET_TABLE_ to base of .got for ARM

[Rafael Avila de Espindola via llvm-commits]

Peter Smith via Phabricator <reviews at reviews.llvm.org> writes:

> Index: ELF/Writer.cpp
> ===================================================================
> --- ELF/Writer.cpp
> +++ ELF/Writer.cpp
> @@ -825,9 +825,16 @@
>    // The situation is even stranger on x86_64 where the assembly doesn't
>    // need the magical symbol, but gas still puts _GLOBAL_OFFSET_TABLE_ as
>    // an undefined symbol in the .o files.
> -  // Given that the symbol is effectively unused, we just create a dummy
> -  // hidden one to avoid the undefined symbol error.
> -  Symtab<ELFT>::X->addIgnored("_GLOBAL_OFFSET_TABLE_");
> +  // For ARM the clang assembler may not have treated _GLOBAL_OFFSET_TABLE_ as
> +  // special so we may encounter a R_ARM_REL32 to _GLOBAL_OFFSET_TABLE_ which
> +  // will only evaluate correctly if _GLOBAL_OFFSET_TABLE_ is at the base of the
> +  // .got section. for other architectures, given that the symbol is
> +  // effectively unused, we just create a dummy hidden one to avoid the
> +  // undefined symbol error.
> +  if (Config->EMachine == EM_ARM && InX::Got)
> +    addOptionalRegular<ELFT>("_GLOBAL_OFFSET_TABLE_", InX::Got, 0);
> +  else
> +    Symtab<ELFT>::X->addIgnored("_GLOBAL_OFFSET_TABLE_");

Looking at the llvm side, I kept wondering why do ABIs even have
relocations like R_386_GOTPC. Why not just have linker defined symbols
_GLOBAL_OFFSET_TABLE_ and _GLOBAL_OFFSET_TABLE_END_?

I also found more cases where MC doesn't convert _GLOBAL_OFFSET_TABLE_,
so think I am not that opposed to something like this, but I would change
it a bit. _GLOBAL_OFFSET_TABLE_ should always point to the start or end
of the got depending on the architecture.

Cheers,
Rafael