[Bug gold/26678] New: construction vtable defined in a discarded section

[cassio.neri at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=26678

            Bug ID: 26678
           Summary: construction vtable defined in a discarded section
           Product: binutils
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: gold
          Assignee: ccoutant at gmail dot com
          Reporter: cassio.neri at gmail dot com
                CC: ian at airs dot com
  Target Milestone: ---

This might be a compiler bug but I get the felling it's the linker. To
reproduce you need g++ and clang++ and a quite strange example spread across 3
files:

// a.hpp
struct A {
    int i;
    ~A() { asm("" : "=r"(i)); }
};

struct B : public virtual A { };

struct C : public B { };

struct D {
    D(C);
};

// a.cpp
#include "a.hpp"

void f(C) {
}

D::D(C c) {
    f(c);
}

// main.cpp
#include "a.hpp"

int main() {
    C c;
    D d(c);
}

Build with:

g++ -O3 -fPIC -c a.cpp
clang++ -O3 -fPIC -c main.cpp
clang++ -fuse-ld=gold main.o a.o -o main

The output is:

a.o:a.cpp:function D::D(C) [clone .cold]: error: relocation refers to global
symbol "construction vtable for B-in-C", which is defined in a discarded
section
  section group signature: "_ZTV1C"
  prevailing definition is from main.o
clang-10: error: linker command failed with exit code 1 (use -v to see
invocation)

FWIW: All the ingredients in the example seem relevant (asm, virtual
inheritance, levels of indirection, compiler flags) and the issue goes away if
any is removed. More notable changes that make the issue go away are:

    1. Use the same compiler for all TUs, (It doesn't matter whether g++ or
clang++.)
    2. Link with ld (i.e., remove -fuse-ld=gold)
    3. Compile main.cpp without -O3.
    4. Compile main.cpp without -fPIC.
    5. Swap a.o and main.o in the linker command line.

For more information: https://stackoverflow.com/q/64118757/1137388

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