[lld] r259259 - Add comments on a mysterious value in MIPS GOT[1].

[Rafael Espíndola via llvm-commits]

Thanks!
On Jan 29, 2016 5:59 PM, "Rui Ueyama via llvm-commits" <
llvm-commits at lists.llvm.org> wrote:

> Author: ruiu
> Date: Fri Jan 29 16:55:38 2016
> New Revision: 259259
>
> URL: http://llvm.org/viewvc/llvm-project?rev=259259&view=rev
> Log:
> Add comments on a mysterious value in MIPS GOT[1].
>
> Thanks to Simon Atanasyan and Igor Kudrin for describing the code!
>
> Modified:
>     lld/trunk/ELF/Target.cpp
>
> Modified: lld/trunk/ELF/Target.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Target.cpp?rev=259259&r1=259258&r2=259259&view=diff
>
> ==============================================================================
> --- lld/trunk/ELF/Target.cpp (original)
> +++ lld/trunk/ELF/Target.cpp Fri Jan 29 16:55:38 2016
> @@ -1408,9 +1408,24 @@ unsigned MipsTargetInfo<ELFT>::getDynRel
>  template <class ELFT>
>  void MipsTargetInfo<ELFT>::writeGotHeader(uint8_t *Buf) const {
>    typedef typename ELFFile<ELFT>::Elf_Off Elf_Off;
> +  typedef typename ELFFile<ELFT>::uintX_t uintX_t;
> +
> +  // Set the MSB of the second GOT slot. This is not required by any
> +  // MIPS ABI documentation, though.
> +  //
> +  // There is a comment in glibc saying that "The MSB of got[1] of a
> +  // gnu object is set to identify gnu objects," and in GNU gold it
> +  // says "the second entry will be used by some runtime loaders".
> +  // But how this field is being used is unclear.
> +  //
> +  // We are not really willing to mimic other linkers behaviors
> +  // without understanding why they do that, but because all files
> +  // generated by GNU tools have this special GOT value, and because
> +  // we've been doing this for years, it is probably a safe bet to
> +  // keep doing this for now. We really need to revisit this to see
> +  // if we had to do this.
>    auto *P = reinterpret_cast<Elf_Off *>(Buf);
> -  // Module pointer
> -  P[1] = ELFT::Is64Bits ? 0x8000000000000000 : 0x80000000;
> +  P[1] = uintX_t(1) << (ELFT::Is64Bits ? 63 : 31);
>  }
>
>  template <class ELFT>
>
>
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