[PATCH] D62106: [X86] Support -fno-plt __tls_get_addr calls

Sun Jul 7 08:12:08 PDT 2019

MaskRay added a comment.

In D62106#1572600 <https://reviews.llvm.org/D62106#1572600>, @nikic wrote:

> > ld.bfd doesn't allow R_X86_64_GOTPCREL. You can work around the issue with:
> > 
> >   % clang -fno-plt -fpic a.cc -Wa,-mrelax-relocations=yes -fuse-ld=bfd
>
> Thanks, I've verified that setting `RelaxELFRelocations` in `TargetOptions` indeed works around the issue. I'm a bit concerned about what other compatibility issues this will cause though. If I understood correctly, relax relocations is a relatively recent feature and not compatible with older linkers, so may not be a suitable compiler default.

I think `R_X86_64_GOTPCRELX` should be usable in ld.bfd and gold since 2016. In gold, -fno-plt R_X86_64_GOTPCREL was supported in Aug 2016 (https://sourceware.org/bugzilla/show_bug.cgi?id=20216).

Do you know why the Rust lib uses -fno-plt TLS code sequence? I'm thinking if its depended llvm should be configured with `-DENABLE_X86_RELAX_RELOCATIONS=ON` so that you don't need -Wa,-mrelax-relocations=yes

  % llvm-objdump -dr file2.rlib
  ...
         8: ff 15 00 00 00 00             callq   *(%rip)
                  000000000000000a:  R_X86_64_GOTPCREL    __tls_get_addr-4

Repository:
  rL LLVM

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  https://reviews.llvm.org/D62106/new/

https://reviews.llvm.org/D62106