Created attachment 1473654 [details]
Tarball with minimal testcase.

Description of problem:

  Linker fails to resolve explitcitly referenced versioned symbol
  from an external DSO when building and linking an executable with
  LTO (-flto / -Wl,-flto) enabled.

  Doing the same with GCC everything works as expected.


Version-Release number of selected component (if applicable):

  clang-6.0.1-1.fc28

  clang version 6.0.1 (tags/RELEASE_601/final)
  Target: x86_64-unknown-linux-gnu
  Thread model: posix


How reproducible:

  100%  (see attached testcase)


Steps to Reproduce:

  1. Run `make` from testcase (use GCC, works).
  2. Run `make clean`.
  3. Run `CC=clang make` (use Clang).
  4. *BANG*


Actual results:

  Linker fails to resolve the explicitly referenced symbol from DSO.


Expected results:

  Works, as when using GCC.


Additional info:

  source.c:LINENO: undefined reference to `foo@BAR_123'
  /usr/bin/ld: out.a: No symbol version section for versioned
                      symbol `foo@BAR_123'
  /usr/bin/ld: final link failed: Nonrepresentable section on output


  DSO symbols when built using Clang:

  $nm -gD --with-symbol-versions --special-syms libtestcase.so
  0000000000000000 A BAR_123@@BAR_123
  0000000000000000 A BAR_234@@BAR_234
                   w __cxa_finalize.5
  00000000000005e0 T foo@BAR_123
  00000000000005f0 T foo@@BAR_234
                   w __gmon_start__
                   w _ITM_deregisterTMCloneTable
                   w _ITM_registerTMCloneTable


  DSO symbols when built using GCC:

  $nm -gD --with-symbol-versions --special-syms libtestcase.so
  0000000000000000 A BAR_123@@BAR_123
  0000000000000000 A BAR_234@@BAR_234
                   w __cxa_finalize.5
  00000000000005f0 T foo@BAR_123
  00000000000005e0 T foo@@BAR_234
                   w __gmon_start__
                   w _ITM_deregisterTMCloneTable
                   w _ITM_registerTMCloneTable

  (The start addresses of the functions differ in GCC build with LTO.)


  I suspect the problem comes from the LLVMgold / libLTO plugin for ld.


  Fedora 28+ is affected, too.