Re: putting more smarts into a == b

[Andrei Alexandrescu]

Robert Jacques wrote:

On Sun, 27 Sep 2009 09:11:38 -0400, Andrei Alexandrescu <seewebsiteforem...@erdani.org> wrote:

Robert Jacques wrote:

On Sat, 26 Sep 2009 21:32:13 -0400, Andrei Alexandrescu <seewebsiteforem...@erdani.org> wrote:

Consider two objects a and b with a of class type. Currently, the expression a == b is blindly rewritten as a.opEquals(b). I argue it should be rewritten into a call to an (imaginary/inlined) function equalObjects(a, b), with the following definition:

bool equalObjects(T, U)(T a, U b) if (is(T == class))
{
     static if (is(U == class))
     {
         if (b is null) return a is null;
         if (a is null) return b is null;
     }
     else
     {
         enforce(a !is null);
     }
     return a.opEquals(b);
}

This hoists the identity test outside the opEquals call and also deals with null references. What do you think?

Andrei

I like this. I think optimizing away opEquals for identical objects would also be a good idea:

 static if (is(U == class))
    if(a is b || a is null || b  is null) return a is b;
else
    enforce(a !is null);

This code has an inefficiency, it seems, because it makes a bit more checks than necessary (e.g. checks a is b twice). Let's simplify:

bool equalObjects(T, U)(T a, U b) if (is(T == class))
{
      static if (is(U == class))
      {
          if (a is b) return true;
          if (b is null || a is null) return false;
      }
      else
      {
          enforce(a !is null);
      }
      return a.opEquals(b);
}


Andrei

Are the extra branch and return statement faster?

Just as fast. Short-circuit evaluation also generates code with branches.

Besides, I thought the optimizer would cache a is b:

auto a_is_b = a is b;
if (a_is_b || b is null || a is null) return a_is_b;

I'm trying to not rely on such...


Andrei