[Bug ld/20090] New: [Fix] clang fail on LTO phase with BFD linker (+patch)

[igor.venevtsev at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=20090

            Bug ID: 20090
           Summary: [Fix] clang fail on LTO phase with BFD linker (+patch)
           Product: binutils
           Version: 2.27 (HEAD)
            Status: NEW
          Severity: normal
          Priority: P2
         Component: ld
          Assignee: unassigned at sourceware dot org
          Reporter: igor.venevtsev at gmail dot com
  Target Milestone: ---

Created attachment 9256
  --> https://sourceware.org/bugzilla/attachment.cgi?id=9256&action=edit
reproducer

Reproducer is just trying to make an executable using one object and two
archives with LTO info inside. First arch, live.a, contains live() function
which is called from main(). The second one, dead.a, contains never called
function dead(). During linking phase, clang comply on 'Unexpected resolution'
and terminates with UNREACHABLE.

bash-4.3$ make

[...skip...]

Unexpected resolution
UNREACHABLE executed at
/users/ivvenevt/llvm/llvm/tools/gold/gold-plugin.cpp:759!
clang-3.9: error: unable to execute command: Aborted (core dumped)
clang-3.9: error: linker command failed due to signal (use -v to see
invocation)
Makefile:26: recipe for target 'test' failed

The reason is wrong resolution reported by ld.bfd for dead (never included to
link) symbols. When LTO plugin asks for dead symbol resolution via
get_symbols() callback, ld.bfd is trying to find BFD entry for this symbol. But
there is no BFD entry for dead symbol despite the symbol itself is defined
(sym.def != LDPK_UNDEF). As a result ld.bfd set resolution to LDPR_UNKNOWN and
confuses the LLVMgold.so plugin. Opposite, GOLD linker reports
LDPR_PREEMPTED_REG for dead symbols. (gold/plugin.cc:959)

Please find reproducer and patch in attachments.

To run reproducer you should specify locations for clang and bfd.ld via
CLANG_BIN and BINUTILS_BIN vars.

Thanks, Igor

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