[lld] r259781 - [ELF][MIPS] Add handling for __gnu_local_gp symbol

[Rui Ueyama via llvm-commits]

Is there a documentation about __gnu_local_gp? If so, please describe it
briefly and add a link to there. If not, it is still better to write a
brief description and mention that it is an unofficial (undocumented?)
feature that the GNU linker has and what it is used for.

On Thu, Feb 4, 2016 at 4:09 AM, Simon Atanasyan via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

> Author: atanasyan
> Date: Thu Feb  4 06:09:49 2016
> New Revision: 259781
>
> URL: http://llvm.org/viewvc/llvm-project?rev=259781&view=rev
> Log:
> [ELF][MIPS] Add handling for __gnu_local_gp symbol
>
> This symbol is a "fake" symbol like "_gp_disp" and denotes
> the GOT + 0x7FF0 value.
>
> Added:
>     lld/trunk/test/ELF/mips-gp-local.s
> Modified:
>     lld/trunk/ELF/Config.h
>     lld/trunk/ELF/Driver.cpp
>     lld/trunk/ELF/InputSection.cpp
>     lld/trunk/ELF/Writer.cpp
>
> Modified: lld/trunk/ELF/Config.h
> URL:
> http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Config.h?rev=259781&r1=259780&r2=259781&view=diff
>
> ==============================================================================
> --- lld/trunk/ELF/Config.h (original)
> +++ lld/trunk/ELF/Config.h Thu Feb  4 06:09:49 2016
> @@ -37,6 +37,7 @@ enum ELFKind {
>  struct Configuration {
>    SymbolBody *EntrySym = nullptr;
>    SymbolBody *MipsGpDisp = nullptr;
> +  SymbolBody *MipsLocalGp = nullptr;
>    InputFile *FirstElf = nullptr;
>    llvm::StringRef DynamicLinker;
>    llvm::StringRef Entry;
>
> Modified: lld/trunk/ELF/Driver.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Driver.cpp?rev=259781&r1=259780&r2=259781&view=diff
>
> ==============================================================================
> --- lld/trunk/ELF/Driver.cpp (original)
> +++ lld/trunk/ELF/Driver.cpp Thu Feb  4 06:09:49 2016
> @@ -328,6 +328,7 @@ template <class ELFT> void LinkerDriver:
>      // start of function and gp pointer into GOT. Use 'strong' variant of
>      // the addIgnored to prevent '_gp_disp' substitution.
>      Config->MipsGpDisp = Symtab.addIgnored("_gp_disp");
> +    Config->MipsLocalGp = Symtab.addIgnored("__gnu_local_gp");
>
>      // Define _gp for MIPS. st_value of _gp symbol will be updated by
> Writer
>      // so that it points to an absolute address which is relative to GOT.
>
> Modified: lld/trunk/ELF/InputSection.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/InputSection.cpp?rev=259781&r1=259780&r2=259781&view=diff
>
> ==============================================================================
> --- lld/trunk/ELF/InputSection.cpp (original)
> +++ lld/trunk/ELF/InputSection.cpp Thu Feb  4 06:09:49 2016
> @@ -234,6 +234,8 @@ void InputSectionBase<ELFT>::relocate(ui
>          SymVA = getMipsGpAddr<ELFT>() - AddrLoc;
>        else if (Type == R_MIPS_LO16 && Body == Config->MipsGpDisp)
>          SymVA = getMipsGpAddr<ELFT>() - AddrLoc + 4;
> +      else if (Body == Config->MipsLocalGp)
> +        SymVA = getMipsGpAddr<ELFT>();
>      }
>      uintX_t Size = Body->getSize<ELFT>();
>      Target->relocateOne(BufLoc, BufEnd, Type, AddrLoc, SymVA + A, Size +
> A,
>
> Modified: lld/trunk/ELF/Writer.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Writer.cpp?rev=259781&r1=259780&r2=259781&view=diff
>
> ==============================================================================
> --- lld/trunk/ELF/Writer.cpp (original)
> +++ lld/trunk/ELF/Writer.cpp Thu Feb  4 06:09:49 2016
> @@ -388,7 +388,7 @@ void Writer<ELFT>::scanRelocs(
>          // relocation too because that case is possible for executable
> file
>          // linking only.
>          continue;
> -      if (Body == Config->MipsGpDisp)
> +      if (Body == Config->MipsGpDisp || Body == Config->MipsLocalGp)
>          // MIPS _gp_disp designates offset between start of function and
> gp
>          // pointer into GOT therefore any relocations against it do not
> require
>          // dynamic relocation.
>
> Added: lld/trunk/test/ELF/mips-gp-local.s
> URL:
> http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/mips-gp-local.s?rev=259781&view=auto
>
> ==============================================================================
> --- lld/trunk/test/ELF/mips-gp-local.s (added)
> +++ lld/trunk/test/ELF/mips-gp-local.s Thu Feb  4 06:09:49 2016
> @@ -0,0 +1,20 @@
> +# Check handling of relocations against __gnu_local_gp symbol.
> +
> +# RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux %s -o %t.o
> +# RUN: ld.lld -o %t.exe %t.o
> +# RUN: llvm-objdump -d -t %t.exe | FileCheck %s
> +
> +# REQUIRES: mips
> +
> +# CHECK:      Disassembly of section .text:
> +# CHECK-NEXT: __start:
> +# CHECK-NEXT:    20000:  3c 08 00 00  lui   $8, 0
> +# CHECK-NEXT:    20004:  21 08 00 00  addi  $8, $8, 0
> +
> +# CHECK: 00000000  *ABS*  00000000 _gp
> +
> +  .text
> +  .globl  __start
> +__start:
> +  lui    $t0,%hi(__gnu_local_gp)
> +  addi   $t0,$t0,%lo(__gnu_local_gp)
>
>
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