shifting

["Marko Mlinar" <markom at opencores dot org>]

Hello!

About some time I noticed gcc has strange "feature" with shifting.

Considering we have following program:

int main ()
{
  volatile int a = 20;
  volatile int b = 20;
  const int x = 0xdeaddead;
  const int c = 40;

  printf ("%08x %08x\n", (x >> a) >> b, x >> c);
  return 0;
}

when compining without optimizations we get:

$ gcc s.c -o s && ./s
ffffffff ffdeadde

and with optimizations:
$ gcc s.c -o s -O2 && ./s
s.c: In function `main':
s.c:8: warning: right shift count >= width of type
ffffffff ffffffff

The first result is quite scary. It may also cause serious problems with
different architectures
e.g. 32/64 bit architectures.
I cannot think of an example, but I have it somewhere back in my mind, it
was possible to produce warning,
but still do an errorneous output with -O2 or -O option.

If this issue is known about or it was already closed due to performance
considerations, please let me know.

best regards,
    Marko