[Bug lto/61969] [4.8/4.9/5 Regression] wrong code by LTO on i?86-linux-gnu (affecting trunk, 4.9.x, and 4.8.x)

["andi-gcc at firstfloor dot org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61969

--- Comment #4 from Andi Kleen <andi-gcc at firstfloor dot org> ---

The problem is when returning a struct from func_52:

const struct S0 func_52 (uint32_t p_53)
{
  const struct S0 l_55 = { 4, 40290, 10, 4 };
  return l_55;
}

The main code stores the struct value from the stack into the global variable
and eventually prints it

 80482f4:       83 ec 38                sub    $0x38,%esp
 80482f7:       0f b6 15 4c da 04 08    movzbl 0x804da4c,%edx
 80482fe:       8b 1d 20 d1 04 08       mov    0x804d120,%ebx
 8048304:       0f b6 35 70 da 04 08    movzbl 0x804da70,%esi
 804830b:       e8 b0 0c 00 00          call   8048fc0 <func_52>
 8048310:       0f b7 45 d2             movzwl -0x2e(%ebp),%eax
 8048314:       ba 01 00 00 00          mov    $0x1,%edx
 8048319:       c7 05 20 d1 04 08 48    movl   $0x804da48,0x804d120
 8048320:       da 04 08 
 8048323:       66 a3 5c da 04 08       mov    %ax,0x804da5c


But func_52 has been completely optimized away and puts nothing onto the stack:


08048fc0 <func_52>:
 8048fc0:       f3 c3                   repz ret 
 8048fc2:       8d b4 26 00 00 00 00    lea    0x0(%esi,%eiz,1),%esi
 8048fc9:       8d bc 27 00 00 00 00    lea    0x0(%edi,%eiz,1),%edi

So the value is random stack garbage.