do .. while loops ?

[Mirko Liss]

On Sat, 14 Jul 2001, Erik Max Francis wrote:
> do...while loops are a language feature that tend to be fairly
> controversial in some languages, for some reason -- probably because it
> seems more on the "fluff" side to many people.  They're easy enough to
> simulate with other control structures.

On the other hand, if you write do..while - loops, you can get rid 
of one jump per iteration. Rewriting while - loops into 
do ... while - loops is a common optimization.

Lets take a look at it:

Python 1.5.2 (#1, Jul 29 2000, 14:28:37)  [GCC 2.95.2 19991024 (release)]
on linux2
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
>>> import dis
>>> def testme(a):
...     while a > 0:
...           a = a -1
...
>>> dis.dis(testme)
          0 SET_LINENO          1
          3 SET_LINENO          2
          6 SETUP_LOOP         34 (to 43)
    >>    9 SET_LINENO          2
         12 LOAD_FAST           0 (a)
         15 LOAD_CONST          1 (0)
         18 COMPARE_OP          4 (>)
         21 JUMP_IF_FALSE      17 (to 41)
         24 POP_TOP        
         25 SET_LINENO          3
         28 LOAD_FAST           0 (a)
         31 LOAD_CONST          2 (1)
         34 BINARY_SUBTRACT
         35 STORE_FAST          0 (a)
         38 JUMP_ABSOLUTE       9
    >>   41 POP_TOP        
         42 POP_BLOCK      
    >>   43 LOAD_CONST          0 (None)
         46 RETURN_VALUE   
>>> 

Rewriting this to 
>>> def testme2(a):
...     while 1: 
...        a = a - 1
...        if a < 0 : break
... 
yields something like:
            ...
         35 LOAD_FAST           0 (a)
         38 LOAD_CONST          2 (0)
         41 COMPARE_OP          0 (<)
         44 JUMP_IF_FALSE       8 (to 55)
         47 POP_TOP        
         48 SET_LINENO          4
         51 BREAK_LOOP     
         52 JUMP_FORWARD        1 (to 56)
    >>   55 POP_TOP        
    >>   56 JUMP_ABSOLUTE       9
    >>   59 POP_TOP        
         60 POP_BLOCK      
            ...

Using a do..while - loop, you'd have one jump less per iteration,
but because of the if..break you get one additional jump.


regards,
	Mirko

PS: Sorry, if this has been discussed before.