swap(x,y)

[Anton Rang]

In article <433 at crdos1.crd.ge.COM> davidsen at crdos1.crd.ge.COM (Wm E Davidsen Jr) writes:
>  I would be willing to bet that there are more uses of swap than shift
>in applications programs, and yet we have shift.

  Shift can't be portably represented in a language without it; there
is no (simple) sequence of statements which will do a shift.  The
simple sequence:

		t = x;
		x = y;
		y = t;

will do a swap.  A reasonable compiler (on a reasonable machine) ought
to discover that T is only used in these three statements, and put it
in a register.  At this point, a peephole optimizer ought to discover
that there is a swap instruction applicable.

		MOVE X, R0
		MOVE Y, X
		MOVE R0, Y

can be transformed into

		SWAP X, Y

if the value in R0 is not used past this point.

			Anton
   
+----------------------------------+------------------+
| Anton Rang (grad student)        | rang at cs.wisc.edu |
| University of Wisconsin--Madison |                  |
+----------------------------------+------------------+

"You are in a twisty little maze of Unix versions, all different."