[Bug tree-optimization/59932] New: spurious undefined behavior warning on valid code

[su at cs dot ucdavis.edu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59932

            Bug ID: 59932
           Summary: spurious undefined behavior warning on valid code
           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 issues a spurious undefined behavior warning when
compiling the following valid code at -Os and above in both 32-bit and 64-bit
mode. 

It does not affect gcc 4.8 though, so it is a regression from 4.8.x. 

Interestingly, the code also causes clang (from 3.2 to its current trunk) to
hang at -Os and above. 


$ 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 --prefix=/usr/local/gcc-trunk
--enable-languages=c,c++ --disable-werror --enable-multilib
Thread model: posix
gcc version 4.9.0 20140123 (experimental) [trunk revision 206958] (GCC) 
$ 
$ gcc-trunk -O1 small.c; a.out
$ gcc-4.8.2 -Os small.c; a.out
$ 
$ gcc-trunk -Os small.c
small.c: In function ‘main’:
small.c:27:16: warning: iteration 2147483646u invokes undefined behavior
[-Waggressive-loop-optimizations]
   for (; p1; p1++)
                ^
small.c:27:3: note: containing loop
   for (; p1; p1++)
   ^
$ 
$ timeout -s 9 30 clang-trunk -Os small.c
Killed
$ 
$ 


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


int a, b, c, e[1], f, g, h, i, k, m, n;
short j;

struct
{
  short f1;
} d;

static int
fn1 (int p1)
{
  int l = 64696;
  for (f = 0; f != 1; f--)
    {
      if (d.f1 > l)
    {
      h = p1;
      if (!h)
        l = 0;
    }
      else
    return b;
      for (; a; a++)
    if (e[a])
      break;
    }
  for (; p1; p1++)
    {
      if (h)
    continue;
      if (l)
    i = j = k = g && d.f1 != p1;
      for (; h != 1; h++)
    ;
    }
  return 0;
}

int
main ()
{
  m = c = 1;
  n = fn1 (c);
  return 0;
}