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

Authored by MaskRay on Jun 12 2019, 12:35 AM.

Description

[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