On Wed, 14 Oct 2015, Paul Rubin wrote:

> Ben Bacarisse <ben.usenet@bsb.me.uk> writes:
>> Do you mean that there are reasons to permit !(a == b) and a != b to
>> have different values in some cases?
>
> I think he meant they should both return the same value, but it is ok if
> they don't use the same algorithm.  Of course it may be possible for the
> compiler to optimize the first into the second.  The idea is just that
> if (a != b) { ... }   can be much faster than  if (a == b) { ... } .

As I seem to understand, Ben questioned that either the test on equality 
or on non-equality could be much faster than the other one.

In Ada syntax:

   function "="(A, B: T) return Boolean is
   begin
      if Hash(A) /= Hash(B) then
         return False
      else return Slow_Equality_Test(A, B)
   end "=";

   function "/="(A, B: T) return Boolean is
   begin
      if Hash(A) /= Hash(B) then
         return True
      else return Not Slow_Equality_Test(A, B)
   end "/="; -- if this where allowed in Ada

It seems quite clear that for any inputs X and Y, the expressions X = Y 
and X /= Y should evaluate at the same speed. Or what am I missing?

Actually, "/=" could be faster than "=" if either where defined as 
follows:

   function "="(A, B: T) return Boolean
      renames Slow_Equality_Test;

   function "/="(A, B: T) return Boolean is
     (Hash(A) /= Hash(B)); -- second case

Redefining "=" this way is just missing an optimization.

Redefining "/=" is that way is a bug.

So how could "/=" really be faster than "="?

My guess is that the devil's advocate, Maciej Sobczak, assumed that "=" is 
already in use as identifier, instead of another identifier for the 
Slow_Equality_Test, as I call it. Which means, we cannot employ the 
optimization for equality. An independent "/=" would then allow us to 
optimize non-equality.

But this would be a design flaw, and it might be easy to fix:

   function "="(A, B: T) return Boolean is
     -- optimized equality
   begin
      if Hash(A) /= Hash(B) then
         return False
      else return Outer_Package."="(A, B)
   end "=";


--------  I  love  the  taste  of  Cryptanalysis  in  the morning!  --------
www.uni-weimar.de/de/medien/professuren/mediensicherheit/people/stefan-lucks
----Stefan.Lucks (at) uni-weimar.de, Bauhaus-Universität Weimar, Germany----