[Bug tree-optimization/58731] New: wrong code (hangs) at -O3 on x86_64-linux-gnu

[Bug tree-optimization/58731] New: wrong code (hangs) at -O3 on x86_64-linux-gnu

[su at cs dot ucdavis.edu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58731

            Bug ID: 58731
           Summary: wrong code (hangs) at -O3 on x86_64-linux-gnu
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: su at cs dot ucdavis.edu

The current gcc trunk and gcc 4.8 produce wrong code that hangs for the
following testcase on x86_64-linux when compiled at -O3 (in both 32-bit and
64-bit modes). 

This is a regression from 4.7.x.

The testcase also triggers a different issue at -O2: a spurious "undefined
behavior" warning (see below). 

$ gcc-trunk -v
Using built-in specs.
COLLECT_GCC=gcc-trunk
COLLECT_LTO_WRAPPER=/usr/local/gcc-trunk/libexec/gcc/x86_64-unknown-linux-gnu/4.9.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc-trunk/configure
--enable-languages=c,c++,objc,obj-c++,fortran,lto --disable-werror
--enable-checking=release --with-gmp=/usr/local/gcc-trunk
--with-mpfr=/usr/local/gcc-trunk --with-mpc=/usr/local/gcc-trunk
--with-cloog=/usr/local/gcc-trunk --prefix=/usr/local/gcc-trunk
Thread model: posix
gcc version 4.9.0 20131014 (experimental) [trunk revision 203572] (GCC) 
$ 
$ gcc-trunk -O2 small.c; a.out
small.c: In function ‘main’:
cc1: warning: iteration 5u invokes undefined behavior
[-Waggressive-loop-optimizations]
small.c:6:3: note: containing loop
   for (b = 4; b > -30; b--)
   ^
$ 
$ gcc-trunk -Os small.c; a.out
$
$ gcc-trunk -O3 small.c; a.out
^C
$ gcc-4.8 -O3 small.c; a.out
^C
$
$ gcc-4.7 -O3 small.c; a.out
$  


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


int a, b, c, d, e;

int
main ()
{
  for (b = 4; b > -30; b--)
    for (; c;)
      for (;;)
    {
      e = a > 2147483647 - b;
      if (d)
        break;
    }
  return 0;
}
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From: "su at cs dot ucdavis.edu" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/58732] New: wrong code at -O3 on x86_64-linux-gnu
Date: Tue, 15 Oct 2013 03:43:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idX732

            Bug ID: 58732
           Summary: wrong code at -O3 on x86_64-linux-gnu
           Product: gcc
           Version: 4.8.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: su at cs dot ucdavis.edu

The attached testcase is miscompiled by gcc 4.8.1 on x86_64-linux-gnu at -O3 in
both 32-bit and 64-bit modes.

It does not seem to affect the current gcc trunk, 4.6.x, and 4.7.x.

$ gcc-4.8 -v
Using built-in specs.
COLLECT_GCC=gcc-4.8
COLLECT_LTO_WRAPPER=/usr/local/gcc-4.8/libexec/gcc/x86_64-unknown-linux-gnu/4.8.1/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc-4.8.1/configure
--enable-languages=c,c++,objc,obj-c++,fortran,lto --disable-checking
--with-gmp=/usr/local/gcc-4.8 --with-mpfr=/usr/local/gcc-4.8
--with-mpc=/usr/local/gcc-4.8 --with-cloog=/usr/local/gcc-4.8
--prefix=/usr/local/gcc-4.8
Thread model: posix
gcc version 4.8.1 (GCC)
$
$ gcc-4.8 -O2 small.c; a.out
$
$ gcc-4.8 -O3 small.c; a.out
Aborted (core dumped)
$
$ gcc-trunk -O3 small.c; a.out
$ gcc-4.6 -O3 small.c; a.out
$ gcc-4.7 -O3 small.c; a.out
$


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


static int a = -1, c, d;
int b[10];

int
main ()
{
  for (d = 1; d >= 0; d--)
    {
      b[8] = 0;
      for (c = 0; c; c++)
    ;
      b[d+8] = a;
    }

  if (b[8] != -1)
    __builtin_abort();

  return 0;
}

[Bug tree-optimization/58731] wrong code (hangs) at -O3 on x86_64-linux-gnu

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58731

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Disabling LIM fixes this, disabling PRE not (and PRE alone is not enough to
trigger it - even though it clearly may).

*** This bug has been marked as a duplicate of bug 58143 ***