The current gcc trunk miscompiles the attached testcase on x86_64-linux at (only) -O1 in 64-bit mode. 

This is a regression from 4.8.x.

This one was quite tough to reduce. The attached testcase is the best I was able to get so far. 


$ 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 20131009 (experimental) [trunk revision 203302] (GCC) 
$ 
$ gcc-trunk -O0 small.c; a.out
0
0
1
$ gcc-4.8 -O1 small.c; a.out
0
0
1
$ gcc-trunk -m32 -O1 small.c; a.out
0
0
1
$ clang-trunk -O1 small.c; a.out
0
0
1
$ gcc-trunk -O1 small.c; a.out
0
0
0
$ 


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


int printf (const char *, ...);

#pragma pack(1)
struct S
{
  unsigned int f0:7;
  int f1:19;
  unsigned int f2:22;
  int f3:25;
} k[6][8];

int a, b, c, e, h, l, m, n, o, *p = &h, **q = &p, r, s;
unsigned int d[256];

static void
fn1 ()
{
  for (a = 0; a < 256; a++)
    d[a] = a;
}

static void
fn2 (unsigned int p1, char *p2, int p3)
{
  e = d[e ^ p1];
  printf ("%s%X\n", p2, e);
}

static int *
fn3 (int p1, int p2)
{
  for (s = 10; s > 2; s = -6)
    for (r = 0; r < 6; r++)
      for (n = 0; n < 8; n++)
	{
	  struct S t = { 1, -195, 321, 4857 };
	  k[r][n] = t;
	}
  return *q;
}

static int
fn4 ()
{
  *q = fn3 (l, b);
  return c;
}

int
main (int argc, char *argv[])
{
  int v = 0;
  if (argc == 2)
    v = 1;
  fn1 ();
  fn4 ();
  fn2 (m, "", v);
  fn2 (o, "", v);
  fn2 (k[0][0].f0, "", v);
  return 0;
}