[ELF][PPC64] Don't report "relocation refers to a discarded section" for .toc

Summary:
clang (as of 2019-06-12) / gcc (as of 8.2.1) PPC64 may emit a .rela.toc
which references an embedded switch table in a discarded .rodata/.text
section. The .toc and the .rela.toc are incorrectly not placed in the
comdat.

Technically a relocation from outside the group is not allowed by the ELF spec:

> A symbol table entry with STB_LOCAL binding that is defined relative
> to one of a group's sections, and that is contained in a symbol table
> section that is not part of the group, must be discarded if the group
> members are discarded. References to this symbol table entry from
> outside the group are not allowed.

Don't report errors to work around the bug.

This should fix the ppc64le-lld-multistage-test bot while linking llvm-tblgen:

    ld.lld: error: relocation refers to a discarded section: .rodata._ZNK4llvm3MVT13getSizeInBitsEv
    >>> defined in utils/TableGen/CMakeFiles/llvm-tblgen.dir/CodeGenRegisters.cpp.o
    >>> referenced by CodeGenRegisters.cpp
    >>> utils/TableGen/CMakeFiles/llvm-tblgen.dir/CodeGenRegisters.cpp.o:(.toc+0x0)

Some other PPC specific sections may have similar problems. We can blacklist more
section names when problems occur.

    // A simple program that reproduces the bug.
    // Note .rela.toc (outside the group) references a section symbol (STB_LOCAL) in a group.
    void puts(const char *);
    struct A {
      void foo(int a) {
        switch (a) {
        case 0: puts("0"); break;
        case 1: puts("1"); puts("1"); break;
        case 2: puts("2"); break;
        case 3: puts("3"); puts("4"); break;
        case 4: puts("4"); break;
        case 5: puts("5"); puts("5"); break;
        case 6: puts("6"); break;
        }
      }
      int a;
    };
    void foo(A x) { x.foo(x.a); }

Reviewers: ruiu, sfertile, espindola

Reviewed By: ruiu

Subscribers: emaste, nemanjai, arichardson, kbarton, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63182

llvm-svn: 363126
3 files changed
tree: ad435e335aa83cb880ce1a15f93a80dcfffa6fe4
  1. .arcconfig
  2. .clang-format
  3. .clang-tidy
  4. .gitignore
  5. README.md
  6. clang-tools-extra/
  7. clang/
  8. compiler-rt/
  9. debuginfo-tests/
  10. libclc/
  11. libcxx/
  12. libcxxabi/
  13. libunwind/
  14. lld/
  15. lldb/
  16. llgo/
  17. llvm/
  18. openmp/
  19. parallel-libs/
  20. polly/
  21. pstl/
README.md

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This directory and its subdirectories contain source code for LLVM, a toolkit for the construction of highly optimized compilers, optimizers, and runtime environments.