Re: [PATCH] fix powerpc64le bootstrap failure caused by r243661 (PR 78817)

[Martin Sebor <msebor at gmail dot com>]

On 12/19/2016 10:31 AM, Jeff Law wrote:
> On 12/17/2016 02:55 PM, Martin Sebor wrote:
> > On 12/17/2016 01:01 PM, Markus Trippelsdorf wrote:
> >
> > I agree that these warnings should probably not be issued, though
> > it's interesting to see where they come from.  The calls are in
> > the code emitted by GCC, are reachable, and end up taking place
> > with the right Ubsan runtime recovery options.  It turns out that
> > Ubsan transforms calls to nonnull functions into conditional
> > branches testing the argument for null, like so:
> >
> >     if (s == 0)
> >       __builtin___ubsan_handle_nonnull_arg();
> >     n = strlen (s);
> >
> > and GCC then transforms those into
> >
> >     if (s == 0)
> >       {
> >         __builtin___ubsan_handle_nonnull_arg();
> >         n = strlen (NULL);
> >       }
> >
> > When the ubsan_handle_nonnull_arg function returns to the caller
> > the call to strlen(NULL) is made.
> So I'd like to see more complete dumps here.

The -Wnonnull warning can be reproduced with this C test case and
-fsantize=undefined:

  char* f (const char *s)
  {
    unsigned n = __builtin_strlen (s) + 1;
    char *d = __builtin_malloc (n);

    if (!d)
      return 0;

    __builtin_memcpy (d, s, n);
    return d;
  }

The sanitizer emits the following code (I snipped the rest after
the call to malloc):

  <bb 2> [0.00%]:
  if (s_8(D) == 0B)
    goto <bb 7>; [0.04%]
  else
    goto <bb 6>; [99.96%]

  <bb 7> [0.00%]:
  __builtin___ubsan_handle_nonnull_arg (&*.Lubsan_data0);

  <bb 6> [0.00%]:
  _1 = __builtin_strlen (s_8(D));
  _2 = (unsigned int) _1;
  n_9 = _2 + 1;
  _3 = (long unsigned int) n_9;
  d_11 = __builtin_malloc (_3);
  ...

This is then transformed by the third thread jumping pass into:

  <bb 2> [100.00%]:
  if (s_7(D) == 0B)
    goto <bb 3>; [0.04%]
  else
    goto <bb 8>; [99.96%]

  <bb 3> [0.04%]:
  __builtin___ubsan_handle_nonnull_arg (&*.Lubsan_data0);
  _24 = __builtin_strlen (0B);
  _25 = (unsigned int) _24;
  n_26 = _25 + 1;
  _27 = (long unsigned int) n_26;
  d_29 = __builtin_malloc (_27);
  if (d_29 == 0B)
    goto <bb 4>; [4.07%]
  else
    goto <bb 5>; [95.93%]

  <bb 4> [4.07%]:
  goto <bb 7>; [100.00%]

  <bb 5> [0.04%]:
  __builtin___ubsan_handle_nonnull_arg (&*.Lubsan_data2);

  <bb 6> [95.93%]:
  # _30 = PHI <_19(8), _27(5)>
  # d_31 = PHI <d_22(8), d_29(5)>
  __builtin_memcpy (d_31, s_7(D), _30);

  <bb 7> [100.00%]:
  # _4 = PHI <0B(4), d_31(6)>
  return _4;

  <bb 8> [99.96%]:
  _16 = __builtin_strlen (s_7(D));
  _21 = (unsigned int) _16;
  n_20 = _21 + 1;
  _19 = (long unsigned int) n_20;
  d_22 = __builtin_malloc (_19);
  if (d_22 == 0B)
    goto <bb 4>; [4.07%]
  else
    goto <bb 6>; [95.93%]

(If you'd like to see more context please let me know.)

Martin