This bug is present with at least gcc-2.95.3, gcc-3.3.2 and gcc-3.4.

mhx@r2d2 ~ $ uname -a
Linux r2d2.mhxnet.de 2.4.22-gentoo-r2 #2 Sun Dec 21 18:25:49 MET 2003 i686
Intel(R) Pentium(R) III Mobile CPU      1000MHz GenuineIntel GNU/Linux

mhx@r2d2 ~ $ gcc-3.4 -v
Reading specs from
/home/mhx/gcc/gcc-3.4-20031217/lib/gcc/i686-pc-linux-gnu/3.4/specs
Configured with: ./configure --prefix=/home/mhx/gcc/gcc-3.4-20031217
--enable-languages=c --program-suffix=-3.4
Thread model: posix
gcc version 3.4 20031217 (experimental)

mhx@r2d2 ~ $ gcc-3.3.2 -v
Reading specs from /home/mhx/gcc/gcc-3.3.2/lib/gcc-lib/i686-pc-linux-gnu/3.3.2/specs
Configured with: ./configure --prefix=/home/mhx/gcc/gcc-3.3.2
--program-suffix=-3.3.2 --enable-languages=c,c++
Thread model: posix
gcc version 3.3.2

mhx@r2d2 ~ $ gcc-2.95.3 -v
Reading specs from
/home/mhx/gcc/gcc-2.95.3/lib/gcc-lib/i686-pc-linux-gnu/2.95.3/specs
gcc version 2.95.3 20010315 (release)

When casting a "long long" variable to "short" or "char" and storing
the result back to the same "long long" variable, the value that is
being stored is wrong. Casting to "int" or "long" works. Not storing
to the same variable also works. Disabling optimizations (-O0) also
makes it work. When optimizations are enables, it doesn't matter if
-O1, -O2 or -O3 is given.

The code that fails is e.g.:

  long long ll2s_1(long long x)
  {
    x = (short) x;
    return x;
  }

However, this works correctly:

  long long ll2s_2(long long x)
  {
    return (short) x;
  }

I'll try to attach the preprocessed code that causes the failure.
The executable has been generated with:

  gcc-3.4 -save-temps -O1 -o test test.c

The output of the executable on my machine is as follows:

           0          12           0          12           0           0           0
 10000001000          12         -24          12       -6168  1410066408  1410066408
 20000002000          12         -48          12      -12336 -1474834480 -1474834480
 30000003000          12         -72          12      -18504   -64768072   -64768072
 40000004000          12         -96          12      -24672  1345298336  1345298336
 50000005000          12        -120          12      -30840 -1539602552 -1539602552
 60000006000          12         112          12       28528  -129536144  -129536144
 70000007000          12          88          12       22360  1280530264  1280530264
 80000008000          12          64          12       16192 -1604370624 -1604370624
 90000009000          12          40          12       10024  -194304216  -194304216

Obviously, while columns 2/3 and 4/5 should be identical, they aren't.
Disabling optimizations (turning -O1 into -O0) gives the expected result:

           0           0           0           0           0           0           0
 10000001000         -24         -24       -6168       -6168  1410066408  1410066408
 20000002000         -48         -48      -12336      -12336 -1474834480 -1474834480
 30000003000         -72         -72      -18504      -18504   -64768072   -64768072
 40000004000         -96         -96      -24672      -24672  1345298336  1345298336
 50000005000        -120        -120      -30840      -30840 -1539602552 -1539602552
 60000006000         112         112       28528       28528  -129536144  -129536144
 70000007000          88          88       22360       22360  1280530264  1280530264
 80000008000          64          64       16192       16192 -1604370624 -1604370624
 90000009000          40          40       10024       10024  -194304216  -194304216

Here's the source code (just in case attaching won't work, this is
my first bug report here...):

----8<-------------------------------------------------------------------

#include <stdio.h>

long long ll2c_1(long long x)
{
  x = (signed char) x;
  return x;
}

long long ll2c_2(long long x)
{
  return (signed char) x;
}

long long ll2s_1(long long x)
{
  x = (short) x;
  return x;
}

long long ll2s_2(long long x)
{
  return (short) x;
}

long long ll2i_1(long long x)
{
  x = (int) x;
  return x;
}

long long ll2i_2(long long x)
{
  return (int) x;
}

int main(void)
{
  long long a;
  for (a = 0LL; a < 100000000000LL; a += 10000001000LL)
  {
    printf("%12lld", a);
    printf("%12lld", ll2c_1(a));
    printf("%12lld", ll2c_2(a));
    printf("%12lld", ll2s_1(a));
    printf("%12lld", ll2s_2(a));
    printf("%12lld", ll2i_1(a));
    printf("%12lld", ll2i_2(a));
    printf("\n");
  }
  return 0;
}