Improve code generated for a switch statement (part 2)

[Andrew Stubbs]

Consider:

char
f (char *p)
{
  switch (*p)
    {
    case 32:
    case 13:
    case 10:
    case 9:
      return 0;
    default:
      return *p;
    }
}

"gcc -O2 -S test.c" generates

        ldrb r0, [r0, #0] @ zero_extendqisi2
        sub r3, r0, #9
        uxtb r2, r3
        cmp r2, #23
        bhi .L2
        sxtb r3, r3
        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

Note the three extensions -- ldrb, uxtb, and sxtb. We can optimize this sequence and remove uxtb and sxtb like so:

        ldrb r0, [r0, #0] @ zero_extendqisi2
        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

These extensions come from the fact that emit_case_bit_tests tries to do comparisons in the index type, which is char in this case. If we do the comparisons in "int" or "unsigned int", we don't have do extend as many times.

See LP:645267 for a closely related optimization opportunity.

[CodeSourcery Tracker ID #7900]