[fpc-devel] Constant eval (and dead code removal) - comparing 2 methods for being the same.

[Martin Frb]

In the code

   if true then begin
     write(1);
   end
   else begin
     write(2);
   end;

the "else" block will not generate code (at least O2 and higher).

But

   TFoo = class
     procedure MyFoo;
   end;
   X = class(TFoo)
   end;


   //if @X.MyFoo = @TFoo.MyFoo then begin
   if @TFoo.MyFoo = @TFoo.MyFoo then begin
     write(1);
   end
   else begin
     write(2);
   end;

Does not detect this at compile time.

Note, that this is not on an instance, this is on the actual class => so 
it is known.

Is there any way to get that optimize the "else" block away (or the 
"then" if the compare goes the other way)?
I can't just compare the classes, as subclasses can have the same method 
(if it isn't overridden/reintroduced)


Background:
This seems useless in the above example.
But I want to to that in a generic, and I am testing if the method on 
the generic param is the same method as on a specific known class.

The best part is, that in the specialized generic (probably by the 
peephole opt) the asm goes
#  if @A.MyFoo = @TFoo.MyFoo then begin
     leaq    P$PROJECT1$_$TFOO_$__$$_MYFOO(%rip),%rax
     cmpq    %rax,%rax
     jne    .Lj14

And rax is always equal to rax, so the conditional jump always does the 
same. But it still is a conditional jump (though I guess the branch 
prediction in the CPU will be overjoyed).

----------------------------------------------

program project1;
{$Mode objfpc}

type

   TFoo = class
     procedure MyFoo;
   end;
   TFoo2 = class
     procedure MyFoo;
   end;

   generic TBar<A> = class
     procedure MyBar;
   end;

procedure TFoo.MyFoo;
begin
   write;
end;

procedure TFoo2.MyFoo;
begin
   write;
end;

procedure TBar.MyBar;
begin
   if @A.MyFoo = @TFoo.MyFoo then begin
     write(1);
   end
   else begin
     write(2);
   end;
end;

type
   X = class(TFoo) end;
   TB1 = specialize TBar<TFoo>;
   TB2 = specialize TBar<TFoo2>;


procedure xyz;
begin
   //if @X.MyFoo = @TFoo.MyFoo then begin
   if @TFoo.MyFoo = @TFoo.MyFoo then begin
     write(1);
   end
   else begin
     write(2);
   end;
end;

procedure xyz2;
begin
   if true then begin
     write(1);
   end
   else begin
     write(2);
   end;
end;

var
   b1: TB1;
   b2: TB2;

begin
   xyz;
   xyz2;
   b1.MyBar;
   b2.MyBar;
end.