[Bug tree-optimization/68083] New: wrong code at -O3 on x86_64-linux-gnu

[Bug tree-optimization/68083] New: wrong code at -O3 on x86_64-linux-gnu

[su at cs dot ucdavis.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68083

            Bug ID: 68083
           Summary: wrong code at -O3 on x86_64-linux-gnu
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: su at cs dot ucdavis.edu
  Target Milestone: ---

The current gcc trunk miscompiles the following code on x86_64-linux-gnu at -O3
in both 32-bit and 64-bit modes. 

This is a regression from 5.2.x.


$ gcc-trunk -v
Using built-in specs.
COLLECT_GCC=gcc-trunk
COLLECT_LTO_WRAPPER=/usr/local/gcc-trunk/libexec/gcc/x86_64-pc-linux-gnu/6.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-trunk/configure --prefix=/usr/local/gcc-trunk
--enable-languages=c,c++ --disable-werror --enable-multilib
Thread model: posix
gcc version 6.0.0 20151023 (experimental) [trunk revision 229251] (GCC) 
$ 
$ gcc-trunk -O2 small.c; ./a.out
$ gcc-5.2 -O3 small.c; ./a.out
$ 
$ gcc-trunk -O3 small.c
$ ./a.out
Aborted (core dumped)
$ 


------------------------------------


int a = 2, b = 1, c = 1;

int
fn1 ()
{
  int d;
  for (; a; a--)
    {
      for (d = 0; d < 4; d++)
        {
          int k;
          if (c < 1)
            if (k)
              c = 0;
          if (b)
            continue;
          return 0;
        }
      b = !1;
    }
  return 0;
}

int
main ()
{
  fn1 ();

  if (a != 1) 
    __builtin_abort (); 

  return 0;
}

[Bug tree-optimization/68083] [6 Regression] wrong code at -O3 on x86_64-linux-gnu

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68083

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-10-26
      Known to work|                            |5.2.0
   Target Milestone|---                         |6.0
            Summary|wrong code at -O3 on        |[6 Regression] wrong code
                   |x86_64-linux-gnu            |at -O3 on x86_64-linux-gnu
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.

[Bug tree-optimization/68083] [6 Regression] wrong code at -O3 on x86_64-linux-gnu

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68083

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mpolacek at gcc dot gnu.org

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Started with r226901.

[Bug tree-optimization/68083] [6 Regression] wrong code at -O3 on x86_64-linux-gnu

[aoliva at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68083

--- Comment #3 from Alexandre Oliva <aoliva at gcc dot gnu.org> ---
This is a latent problem in ifcvt, that pulls the computation involving the
uninitialized k out of the always-false condition.  Things go down the hill
when k's default def gets coalesced with b, and rtl optimizers further mess
things up.

[Bug tree-optimization/68083] [6 Regression] wrong code at -O3 on x86_64-linux-gnu

[aoliva at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68083

--- Comment #4 from Alexandre Oliva <aoliva at gcc dot gnu.org> ---
s/ifcvt/ifcombine/ above, sorry

[Bug tree-optimization/68083] [6 Regression] wrong code at -O3 on x86_64-linux-gnu

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68083

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rguenth at gcc dot gnu.org

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
Ok, we shouldn't introduce uninitialized uses.  It's going to be interesting to
teach this to ifcombine ;)

[Bug tree-optimization/68083] [6 Regression] wrong code at -O3 on x86_64-linux-gnu

[aoliva at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68083

Alexandre Oliva <aoliva at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |aoliva at gcc dot gnu.org

--- Comment #6 from Alexandre Oliva <aoliva at gcc dot gnu.org> ---
Created attachment 36622
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=36622&action=edit
Patch I'm testing to fix the problem

This patch stops ifcombine from introducing this kind of undefined behavior. 
I'm testing it now, and I'll post it to the list if that completes
successfully.