[Bug gdb/16157] the function get_pc_function_start (CORE_ADDR pc) maybe inaccurate

["guosheng_gao at realsil dot com.cn" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=16157

--- Comment #2 from ggs334 <guosheng_gao at realsil dot com.cn> ---
> "bt" show when the program is stopped at the instruction just before lop2:
#0  _start () at crt0.S:93
> then stepi
(gdb) si
warning: GDB can't find the start of the function at 0xfffffffc.

> and then again "bt" when stepi to lop2?
#0  lop2 () at crt0.S:95
#1  0xfffffffe in ?? ()

> Sounds like something else might be tricking GDB into thinking you stepped 
> into a new function.  See the code just below "Check for subroutine calls." 
> part of infrun.c.  That's where the logic to detect if the program called a 
> new function is.

GDB use the frame id to "Check for subroutine calls", and the function
frame_id_eq() will check the .code_addr,If .code addresses are
different, the frames are different. If lop3 and lop2 are mistaken for function
start address, the code address are different. So GDB thinking the program
stepped into a new function

gdb/frame.c:
int
frame_id_eq (struct frame_id l, struct frame_id r)
{
  int eq;

  if (!l.stack_addr_p && l.special_addr_p
      && !r.stack_addr_p && r.special_addr_p)
    /* The outermost frame marker is equal to itself.  This is the
       dodgy thing about outer_frame_id, since between execution steps
       we might step into another function - from which we can't
       unwind either.  More thought required to get rid of
       outer_frame_id.  */
    eq = 1;
  else if (!l.stack_addr_p || !r.stack_addr_p)
    /* Like a NaN, if either ID is invalid, the result is false.
       Note that a frame ID is invalid iff it is the null frame ID.  */
    eq = 0;
  else if (l.stack_addr != r.stack_addr)
    /* If .stack addresses are different, the frames are different.  */
    eq = 0;
  else if (l.code_addr_p && r.code_addr_p && l.code_addr != r.code_addr)
    /* An invalid code addr is a wild card.  If .code addresses are
       different, the frames are different.  */
    eq = 0;
  else if (l.special_addr_p && r.special_addr_p
       && l.special_addr != r.special_addr)
    /* An invalid special addr is a wild card (or unused).  Otherwise
       if special addresses are different, the frames are different.  */
    eq = 0;
  else if (l.artificial_depth != r.artificial_depth)
    /* If artifical depths are different, the frames must be different.  */
    eq = 0;
  else
    /* Frames are equal.  */
    eq = 1;

  if (frame_debug)
    {
      fprintf_unfiltered (gdb_stdlog, "{ frame_id_eq (l=");
      fprint_frame_id (gdb_stdlog, l);
      fprintf_unfiltered (gdb_stdlog, ",r=");
      fprint_frame_id (gdb_stdlog, r);
      fprintf_unfiltered (gdb_stdlog, ") -> %d }\n", eq);
    }
  return eq;
}

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