Not just for cryptography

[Era Scarecrow]

> From: Don <nospam at nospam.com.au>
> Era Scarecrow wrote:
> >> From: Sean Kelly <sean at invisibleduck.org>
> >>
> >> bearophile wrote:
> >>> Another little story for people that think the
> >> multi-precision integers are mostly good for
> cryptography:
> >>
> http://dobbscodetalk.com/index.php?option=com_content&task=view&id=614&Itemid=
> >>
> >> Avoiding computation overflow is never a bad thing
> :)
<Snip>

> Actually that's (sort of) implemented in most hardware
> (X86, for 
> instance). The overflow flag is set if you get an int
> overflow (signed 
> ints). The carry flag is set if you get a uint overflow.
> 
> There are several branch instructions which make use of it.
> 
> >  Internally (Assembly) it would look something like
> this.
> >  if (a+b > 0){
> > ...
> > }
> > --becomes
> >  xor edx,edx        ;clear upper 32bits
> >  mov eax, [esp-12]  ;int a
> >  add eax, [esp-8]   ;int b
> >  adc edx, 0         ;add with carry
> 
> > 
> > --then following the compare 
> > 
> >  cmp edx,0
> >  jg inside_if  ;jump if greater than 0. More likely?
> >  jb outside_if ;below 0, so it's false
> >  cmp eax,0
> >  jbe outside_if
> > inside_if:
> > ;{...}
> > outside_if:
> 
> mov eax, [esp-12];
> add eax, [esp-8];
> jo error;
> jbe outside_if;
> 
> error: throw IntegerOverflowException.
> 
> Could be added in debug builds, at least.

 Sounds promising. From what i know in C, that is never brought up, so if i wanted to handle a overflow or a carry flag i'd have to use a long long. (For a unsigned unlimited Integer math library i made, but has problems)

 Would it be possible, to have a couple operators or a special inline functions we can use to be able to use those possible overflows? I'd rather not ASM in lines where it won't be portable :(

 int carry, a = int.umax;
 b = a + a;

 if (Register.Overflow || Register.Carry){...} //possible?
 math_overflow{...}
 //or
 math_carryOn{...}
//or??
 ASM(Intel)
{
jno notOverFlow
...
}
notOverFlow:
 //or??

 long long carry, a = int.umax;
 b = a + a;
if (b>int.umax) {...}

Era