[Bug gold/19842] New: LTO build fails to write call address for weak symbol reference

[matt at godbolt dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=19842

            Bug ID: 19842
           Summary: LTO build fails to write call address for weak symbol
                    reference
           Product: binutils
           Version: 2.26
            Status: NEW
          Severity: normal
          Priority: P2
         Component: gold
          Assignee: ccoutant at gmail dot com
          Reporter: matt at godbolt dot org
                CC: ian at airs dot com
  Target Milestone: ---

I have a large, proprietary project which links with an open source project.
The large project ("hippo") is compiled with GCC 5.2, -O3, -flto, -fuse-ld=gold
and -fuse-linker-plugin. The open source project ("seasocks":
https://github.com/mattgodbolt/seasocks ) is compiled into a library archive
with "-O0" and a normal, non-LTO build.

Both projects use the exact same compiler version and -std=c++1y. No ODR
violations are noted during link, and we build cleanly with -Wall -Werror
-Wextra -pedantic

Both projects use (amongst many things) std::function<void()>. If I link the
two projects together, a call to std::function<void()>::~function as defined in
the non-LTO project seemingly is left as a "callq 0" :

Dump of assembler code for function std::_List_node<std::function<void ()>
>::~_List_node():
   0x0000000000763f50 <+0>:     push   %rbp
   0x0000000000763f51 <+1>:     mov    %rsp,%rbp
   0x0000000000763f54 <+4>:     sub    $0x10,%rsp
   0x0000000000763f58 <+8>:     mov    %rdi,-0x8(%rbp)
   0x0000000000763f5c <+12>:    mov    -0x8(%rbp),%rax
   0x0000000000763f60 <+16>:    add    $0x10,%rax
   0x0000000000763f64 <+20>:    mov    %rax,%rdi
   0x0000000000763f67 <+23>:    callq  0x0
   0x0000000000763f6c <+28>:    nop
   0x0000000000763f6d <+29>:    leaveq 
   0x0000000000763f6e <+30>:    retq   
End of assembler dump.

Obviously this causes a run-time crash when executed (here it is manifested
only in a std::list<std::function<void()>>).

If I link using the normal ld (identical flags, but dropping the
-fuse-ld=gold), then all works ok:

Dump of assembler code for function std::_List_node<std::function<void ()>
>::~_List_node():
   0x000000000077e880 <+0>:     push   %rbp
   0x000000000077e881 <+1>:     mov    %rsp,%rbp
   0x000000000077e884 <+4>:     sub    $0x10,%rsp
   0x000000000077e888 <+8>:     mov    %rdi,-0x8(%rbp)
   0x000000000077e88c <+12>:    mov    -0x8(%rbp),%rax
   0x000000000077e890 <+16>:    add    $0x10,%rax
   0x000000000077e894 <+20>:    mov    %rax,%rdi
   0x000000000077e897 <+23>:    callq  0x4ff020 <std::function<void
()>::~function()@plt>
   0x000000000077e89c <+28>:    nop
   0x000000000077e89d <+29>:    leaveq 
   0x000000000077e89e <+30>:    retq   
End of assembler dump.

I first reported this issue on the gcc-help mailing list:
https://gcc.gnu.org/ml/gcc-help/2016-03/msg00066.html and there I was directed
here. There is further information on that thread, in particular where I tried
using symbol tracing and debug options to gold.

Try as I might I've been unable to come up with a simple repro case. I
appreciate this makes this kind of bug very difficult.

I'm filing this in the hope someone can help me direct both my testing and
reproduction steps. I also appreciate there may still yet be some uncaught ODR
violation.

Many thanks in advance, Matt

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