Improve code generated for a switch statement

[Andrew Stubbs]

Consider:

unsigned int
f (unsigned int a)
{
  switch (a)
    {
    case 32:
    case 13:
    case 10:
    case 9:
      return 0;
    default:
      return a;
    }
}

"arm-none-eabi-gcc -mcpu=cortex-m3 -mthumb O2 -S kazu.c" generates

        sub r3, r0, #9
        cmp r3, #23
        bhi .L2
        movs r1, #1
        lsls r2, r1, r3
        movw r1, #:lower16:8388627
        movt r1, #:upper16:8388627
        and r3, r2, r1
        cmp r3, #0
        it ne
        movne r0, #0
.L2:
        bx lr

which is equivalent to:

  a -= 9;
  if (a > 23)
    return a;
  if (0x800013 & (1 << a))
    return 0;
  return a;

We can improve this sequence by shifting the magic constant to left
and test the sign bit like so:

  a -= 9;
  if (a > 23)
    return a;
  if (((signed int) 0xc8000100 << a) < 0)
    return 0;
  return a;

The assembly would be:

        sub r3, r0, #9
        cmp r3, #23
        bhi .L2
        movw r2, #:lower16:-939523840
        movt r2, #:upper16:-939523840
        lsls r2, r2, r3
        it mi
        movmi r0, #0
.L2:
        bx lr

Note that the "and" and "cmp r3,#0" have disappeared.

[CodeSourcery Tracker ID #7899]