The following code causes an ICE when compiled with the current gcc trunk at -O3 on x86_64-linux-gnu in both 32-bit and 64-bit modes.

It is a regression from 5.3.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/7.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-source-trunk/configure --enable-languages=c,c++,lto --prefix=/usr/local/gcc-trunk --disable-bootstrap
Thread model: posix
gcc version 7.0.0 20160423 (experimental) [trunk revision 235384] (GCC) 
$ 
$ gcc-trunk -O2 -c small.c
$ gcc-5.3 -O3 -c small.c
$ 
$ gcc-trunk -O3 -c small.c
small.c: In function ‘fn2’:
small.c:17:1: internal compiler error: Segmentation fault
 fn2 ()
 ^~~
0xb6718f crash_signal
        ../../gcc-source-trunk/gcc/toplev.c:333
0x74e89c vec_safe_length<loop*, va_gc>
        ../../gcc-source-trunk/gcc/vec.h:524
0x74e89c loop_depth
        ../../gcc-source-trunk/gcc/cfgloop.h:444
0x74e89c flow_loop_nested_p(loop const*, loop const*)
        ../../gcc-source-trunk/gcc/cfgloop.c:64
0xdb7837 vect_is_simple_reduction
        ../../gcc-source-trunk/gcc/tree-vect-loop.c:2627
0xdb9011 vect_force_simple_reduction
        ../../gcc-source-trunk/gcc/tree-vect-loop.c:3026
0xdb9011 vect_analyze_scalar_cycles_1
        ../../gcc-source-trunk/gcc/tree-vect-loop.c:865
0xdbc807 vect_analyze_scalar_cycles
        ../../gcc-source-trunk/gcc/tree-vect-loop.c:939
0xdbc807 vect_analyze_loop_2
        ../../gcc-source-trunk/gcc/tree-vect-loop.c:1845
0xdbc807 vect_analyze_loop(loop*)
        ../../gcc-source-trunk/gcc/tree-vect-loop.c:2264
0xdd7611 vectorize_loops()
        ../../gcc-source-trunk/gcc/tree-vectorizer.c:532
Please submit a full bug report,
with preprocessed source if appropriate.
Please include the complete backtrace with any bug report.
See <http://gcc.gnu.org/bugs.html> for instructions.
$ 
$ 


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


struct S
{
  int f1;
  int f2;
} a;

int b, c, d, e;
short f;

int
fn1 (int p1, unsigned p2)
{
  return p1 + p2;
}

void
fn2 ()
{
  struct S g;
  int h;
  for (; c; c++)
    for (f = -3; f < 3; f = fn1 (f, 8))
      {
        a.f1 = e;
        if (b)
          a = g;
        else
          for (; h; h++)
            d = b;
      }
}