[Bug c++/84604] New: False positive "may be used uninitialized in this function" due to variable initialization eliminated as dead code

[cas43 at cs dot stanford.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84604

            Bug ID: 84604
           Summary: False positive "may be used uninitialized in this
                    function" due to variable initialization eliminated as
                    dead code
           Product: gcc
           Version: 8.0.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: cas43 at cs dot stanford.edu
  Target Milestone: ---

WHAT HAPPENS:

The compiler eliminates a=0; and b=0; from the code below during the dce2
optimization pass, resulting in a warning for use of uninitialized variable. 
Curiously, the generate code seems to be okay.

OFFENDING CODE:

void f();

struct S
{
    int a;
    int b;

    S() 
    {
        a=0;
        b=0;
    } 

    ~S()
    {
    }

    void g(int) 
    {
        if(a<10)
        {
            f();
            for(int i=0;i<b;i++){}
            this->~S();
            a=b+2;
        }
    } 
};

int foo() 
{
    S s;
    s.g(0);
    s.g(0);
    return s.a+s.b;
}

OPTIMIZED RESULT: (via -fdump-tree-all)

;; Function foo (_Z3foov, funcdef_no=7, decl_uid=2398, cgraph_uid=7,
symbol_order=7)

foo ()
{
  int s$b;
  int _5;
  int _16;

  <bb 2> [local count: 1073741825]:
  f ();
  _16 = s$b_12(D) + 2;
  if (_16 <= 9)
    goto <bb 4>; [33.00%]
  else
    goto <bb 3>; [67.00%]

  <bb 3> [local count: 1073741827]:
  _5 = s$b_12(D) + _16;
  return _5;

  <bb 4> [local count: 354334802]:
  f ();
  goto <bb 3>; [100.00%]

}

GENERATED ASM:

        .file   "prog.cpp"
        .text
        .p2align 4,,15
        .globl  _Z3foov
        .type   _Z3foov, @function
_Z3foov:
.LFB7:
        .cfi_startproc
        subq    $8, %rsp
        .cfi_def_cfa_offset 16
        call    _Z1fv
        call    _Z1fv
        movl    $2, %eax
        addq    $8, %rsp
        .cfi_def_cfa_offset 8
        ret
        .cfi_endproc
.LFE7:
        .size   _Z3foov, .-_Z3foov
        .ident  "GCC: (GNU) 8.0.1 20180227 (experimental)"
        .section        .note.GNU-stack,"",@progbits

COMPILER VERSION:

Using built-in specs.
COLLECT_GCC=/home/craig/new-gcc/i-trunk/bin/g++
COLLECT_LTO_WRAPPER=/home/craig/new-gcc/i-trunk/libexec/gcc/x86_64-pc-linux-gnu/8.0.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../s-trunk/configure --prefix=/home/craig/new-gcc/i-trunk
--disable-multilib
Thread model: posix
gcc version 8.0.1 20180227 (experimental) (GCC) 

COMPILATION FLAGS:

g++ prog.cpp -Wall -c -O2

OTHER VERSIONS that give false positev:

4.9.3 (Ubuntu 4.9.3-13ubuntu2)
5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.9)