[OT] Walter about compilers

[Era Scarecrow]

On Tuesday, 22 January 2013 at 17:10:35 UTC, Thiez wrote:
> On Tuesday, 22 January 2013 at 16:31:20 UTC, Era Scarecrow 
> wrote:
>> I really wouldn't want to have to use BigInt for everything 
>> that can't overflow and then check to make sure I can fit it 
>> in my smaller variables afterwards along with the extra move. 
>> I wouldn't want to use BigInts everywhere, and long's aren't 
>> needed everywhere either.
>
> Since D aims to emulate C in this aspect, overflow with uints 
> is probably defined as a wrap-around (like C). In this case it 
> seems to me the check for overflow would simply be '(a+b)<a', 
> no need to cast to longs and BigInts and all that. Of course 
> this may not apply to signed ints...

  That merely shortens the size of the check, not where you need 
to place the checks or how often. Truthfully, in almost all cases 
the wrap-around or overflow/underflow is an error, usually 
unchecked. If 1 million were the max, then 1,000,000 + 1 should 
equal 1,000,001 and not <=0, and if 0 is the minimum, 0 - 1 
should not equal >=0.

  The only real time I can find overflow wanted is while making 
something that watches for it explicitly to make use of it. Say 
we emulate or write the 'ucent' types. That could be done as:

   //addition example obviously
   void add(const uint[4] lhs, const uint[4] rhs) {
     uint[4] val;
     bool carry = false;
     foreach(i, ref v; val) {
       uint tmp = lhs[i];

       v = lhs[i] + rhs[i] + (carry ? 1 : 0);
       carry = v < tmp;
     }

     assert(!carry); //could fail. How to handle this? Ignore?
  }

  Now let's say there's a for loop which someone decides they 
would be clever and use a ubyte (unsigned char) as an index or 
counter.

  for(ubyte i = 0; i < 1000; i++) {
    writeln(i);
  }

  The overflow is an error because the wrong type was selected but 
doesn't change the obvious logic behind it. You can hide the type 
behind an alias or similar but that doesn't change the fact it's 
a bug, and can be easier to detect if we are aware the overflow 
is happening at all rather than it getting stuck and having to 
manually kill the process or step through it in a debugger. If it 
wasn't outputting in some way you could identify it's much harder 
to find.

  Encryption may make use of the overflow/wrap around, but far 
more likely they use xor or binary operations which don't have 
those problems.