[gcc-4.4] Re: arch/x86/events/intel/uncore_snbep.o: warning: objtool: snbep_uncore_msr_enable_event()+0x2e: function has unreachable instruction

[Fengguang Wu <fengguang dot wu at intel dot com>]

On Thu, Jul 21, 2016 at 11:06:06AM +0800, Liang, Kan wrote:
> Hi Fengguang,
>
> I located the unreachable instruction which is ud2.
>
> This instruction will raise invalid opcode exception. So I think
> normally it should not be reached.  Also ud2 should be generated by
> compiler, not our codes.

It's great to know that!

> I have no idea how to fix it.
> Have you met similar issue before?
> Do you have any suggestions?

The warning shows up when we enabled gcc-4.4 for kernel build tests.
It emits this "function has unreachable instruction" warning for
~10000 kernel functions all over the places.

So it must be a toolchain specific problem.
Let's CC LKML and gcc lists for possible solutions.

> 0000000000000e0f <snbep_uncore_msr_enable_event>:
>     e0f:       55                      push   %rbp
>     e10:       48 89 e5                mov    %rsp,%rbp
>     e13:       53                      push   %rbx
>     e14:       83 be 80 01 00 00 ff    cmpl   $0xffffffff,0x180(%rsi)
>     e1b:       48 89 f3                mov    %rsi,%rbx
>     e1e:       74 2b                   je     e4b <snbep_uncore_msr_enable_event+0x3c>
>     e20:       31 f6                   xor    %esi,%esi
>     e22:       e8 00 00 00 00          callq  e27 <snbep_uncore_msr_enable_event+0x18>
>     e27:       48 89 c2                mov    %rax,%rdx
>     e2a:       8b b3 78 01 00 00       mov    0x178(%rbx),%esi
>     e30:       48 c1 ea 20             shr    $0x20,%rdx
>     e34:       48 83 3d 00 00 00 00    cmpq   $0x0,0x0(%rip)        # e3c <snbep_uncore_msr_enable_event+0x2d>
>     e3b:       00
>     e3c:       75 02                   jne    e40 <snbep_uncore_msr_enable_event+0x31>
>     e3e:       0f 0b                   ud2
>     e40:       89 f7                   mov    %esi,%edi
>     e42:       89 c6                   mov    %eax,%esi
>     e44:       ff 14 25 00 00 00 00    callq  *0x0
>
> Thanks,
> Kan
>
> > tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
> > master
> > head:   47ef4ad2684d380dd6d596140fb79395115c3950
> > commit: 3b94a891667c30fb4624221497d77fc65d950345
> > perf/x86/intel/uncore: Remove SBOX support for Broadwell server
> > date:   7 weeks ago
> > config: x86_64-randconfig-s0-07191857 (attached as .config)
> > compiler: gcc-4.4 (Debian 4.4.7-8) 4.4.7
> > reproduce:
> >         git checkout 3b94a891667c30fb4624221497d77fc65d950345
> >         # save the attached .config to linux build tree
> >         make ARCH=x86_64
> >
> > All warnings (new ones prefixed by >>):
> >
> > >> arch/x86/events/intel/uncore_snbep.o: warning: objtool:
> > snbep_uncore_msr_enable_event()+0x2e: function has unreachable
> > instruction
> >
> > ---
> > 0-DAY kernel test infrastructure                Open Source Technology Center
> > https://lists.01.org/pipermail/kbuild-all                   Intel Corporation