commit af183e7611cb707ec622c7deef2299fd120fd8cc
author Fangrui Song <maskray@google.com> Wed Jun 12 07:35:42 2019 +0000
committer Fangrui Song <maskray@google.com> Wed Jun 12 07:35:42 2019 +0000
tree dd5b7affd97f2ff8859f896cbc1cfdff7edfa9ac
parent 564a3cb89238563c6e16e60c2bd5943d9af91fe1

[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

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@363126 91177308-0d34-0410-b5e6-96231b3b80d8