Re: ‘?dup’ and ‘drop’

Oh, I get it now.  Block 24 (CF05) has a perfect example:
0: ?dup c031 2, ;
The ‘c031’ is ‘31 0c’ which is ‘xor eax, eax’
You want a zero put on the stack and the flags all reinitialised, so you 
use a ?dup just to put any old value on the stack—you don’t actually 
care that a dup really is dup-ing, so you’re happy to remove the ‘drop’ 
if there was a ‘drop’ in the word before—and then you xor it with itself 
to do the work.

You use ‘?dup’ when you want to create a new value on the stack and you 
don’t care what value because you are going to modify eax directly; 
you’re effectively just creating a space on top of the stack.  If you 
cared that it really was a duplicate then you would use the un-optimized 
‘dup’.

Jason

Jason Kemp said the following on 17/10/2008 17:55:
> Sorry: ignore that one.  Kristopher Johnson had asked it before and 
> got an answer. http://www.strangegizmo.com/forth/ColorForth/msg00193.html
>
> I've just spent quite a while trying to work it out but should have 
> looked at the archive first.
>
> Jason Kemp said the following on 17/10/2008 17:42:
>> Hi,
>>
>> I understand that ?dup only compiles a dup if the previous 
>> instruction was not a drop.  If it was a drop then the drop is 
>> removed and no dup is compiled either.  Karig, 
>> http://www.karig.net/os/cf/c.htm#cdrop explains this.
>>
>> But how can it be assumed that the TOS (that was to have been 
>> dropped) and NOS are identical so that we don't need a drop followed 
>> by a dup?
>>
>> Thanks,
>> Jason
>>
>>
>>
>>
>>
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