int opEquals(Object), and other legacy ints (!)

[Frits van Bommel]

Bruno Medeiros wrote:
> And uuuh.., I've checked gcc's generated code for a C++'s Eq1, and it 
> was only 2 instructions too, CMP and SETE ! :
> 
> Eq1:
>     ...
>     cmp     EAX,0Ch[ECX]
>     sete    EAX;
> 
> (http://www.cs.tut.fi/~siponen/upros/intel/instr/sete_setz.html)
> It seems to me perfectly valid, is there any problem here?

Interesting instruction. Seems to have the exact semantics needed for 
these situations. You'd almost think CPU designers care about what 
people want to do with their products :P.


> What does the original Eq1 even do? :
Step by step:
 >     mov     ECX,0Ch[EAX]
(You skipped this one) Loads this.bar into ECX.
>     sub     ECX,0Ch[EDX]
Subtracts foo.bar from ECX.
>     cmp     ECX,1       // Huh?
Among other things, sets borrow (aka carry) flag if ECX == 0 (i.e. if 
foo.bar == this.bar), clears it otherwise.
>     sbb     EAX,EAX
Subtracts (EAX + borrow) from EAX, setting it to either -1 (if carry == 
1) or 0 (if carry == 0).
>     neg     EAX
Negates EAX.

A bit weird at first glance, but it works as advertised :).


But indeed, a cmp/sete combo seems to do the same in less instructions.