Suppose you have this simple C snippet:

char c0, c1;


void some_function(void)
{

  if(c0>c1)
    c0_gt_c1();
  else
    c0_lte_c1();
}

What is the fastest implementation in assembly (on a mid-range pic)?
Note, a simple subtract is inadequate - this is a *signed* comparison.
(try for example, c0=-1, c1=1.  a -1 is encoded as 0xff. 0xff is greater
than 1 when doing a signed comparison. )

Here's an implementation guaranteed to be one of the worst:

   movf    c0,w
   subwf   c1,w

   btfss   c1,7
    goto   c1_pos

c1_neg:
   btfss   c0,7
    goto   c0_gt_c1

  ;; both negative

   skpnc
    goto   c0_lte_c1
   goto    c0_gt_c1

c1_pos
   btfsc   c0,7
    goto   c0_lte_c1

   skpnc
    goto   c0_lte_c1
   goto    c0_gt_c1

14 hideous instructions. (and ~11 cycles worst case execution).

Here's something better, but still pretty bad.

    movf   c0,w
    addlw  0x80    ; Convert from signed to unsigned
    movwf  temp

    movf   c1,w
    addlw  0x80

;; now do an unsigned comparison

    subwf  temp   ; c1 - c0

    skpnc
     goto  c0_lte_c1
    goto   c0_gt_c1

Can anyone improve this?

Scott

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