Date:	Fri, 30 Nov 2007 15:53:38 -0800
From:	Nicholas Miell <nmiell@...cast.net>
To:	"J.A." Magallón <jamagallon@....com>
Subject: Re: Kernel Development & Objective-C


On Sat, 2007-12-01 at 00:19 +0100, J.A. Magallón wrote:

> An vtable in C++ takes exactly the same space that the function
> table pointer present in every driver nowadays... and probably
> the virtual method call that C++ does itself with
> 
> 	thing->do_something(with,this)
> 
> like
> 	push thing
> 	push with
> 	push this
> 	call THING_vtable+indexof(do_something) // constants at compile time
> 
> is much more efficient that what gcc can mangle to do with
> 
> 	thing->do_something(with,this,thing)
> 
> 	push with
> 	push this
> 	push thing
> 	get thing+offsetof(do_something) // not constant at compile time
> 	dereference it
> 	call it
> 
> (that is, get a generic field on a structure and use it as jump address)
> 
> In short, the kernel is object oriented, implements OO programming by
> hand, but the compiler lacks the knowledge that it is object oriented
> programming so it could do some optimizations.

        struct test;
        struct testVtbl
        {
        	int (*fn1)(struct test *t, int x, int y);
        	int (*fn2)(struct test *t, int x, int y);
        };
        struct test
        {
        	struct testVtbl *vtbl;
        	int x, y;
        };
        void testCall(struct test *t, int x, int y)
        {
        	t->vtbl->fn1(t, x, y);
        	t->vtbl->fn2(t, x, y);
        }

and

        struct test
        {
        	virtual int fn1(int x, int y);
        	virtual int fn2(int x, int y);
        
        	int x, y;
        };
        
        void testCall(struct test *t, int x, int y)
        {
        	t->fn1(x, y);
        	t->fn2(x, y);
        }
        
generate instruction-for-instruction identical code.

-- 
Nicholas Miell <nmiell@...cast.net>

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