correct code for pointer subtraction

[Tapani Tarvainen]

In article <PINKAS.89Jan3082456 at hobbit.intel.com> pinkas at hobbit.intel.com (Israel Pinkas ~) writes:

>>   > >         static int a[30000];
>>   > >         printf("%d\n",&a[30000]-a);

>                                                Let's ignore the fact that
>there is no a[30000] (and that taking its address is invalid). 

I have been told that dpANS explicitly states that the address of
"one-after-last" element of an array may be taken, and subtractions 
like the above are legal and should give correct result.
I do not have an access to the the dpANS - could somebody who does
please look this up?  
In any case all compilers I know of do it just fine
(unless some kind of overflow occurs, like in this very example -
but that's independent of how big the array is declared)
and a lot of existing code does rely on it.


Regarding the original problem, it *is* possible to do the subtraction
correctly, although not simply by using unsigned division.
Here is one way I think would work (on the left is what Turbo C
generates, for comparison):

                             	xor 	dx,dx
 mov    ax,&a[30000]         	mov 	ax,&a[30000]
 sub    ax,a                 	sub 	ax,a
 mov    bx,2                 	mov 	bx,2
 cwd                         	sbb 	dx,dx
 idiv   bx                   	idiv 	bx
        
I.e., take advantage of the fact that we can treat carry and
AX as one 17-bit register containing the result of subtraction.
It will cost a few clock cycles, I'm afraid.
In this particular case it can actually be done with
no speed penalty with the following trick:

 mov	ax,&a[30000]
 sub	ax,a
 rcr	ax

In general case it seems we must choose between doing it fast
and getting it right every time.  Perhaps a compiler option for
those who would otherwise use an old compiler version to save
the two cycles or whatever it costs...

Tapani Tarvainen
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