Messages in this thread | | | Subject | Re: Kernel Development & Objective-C | From | Nicholas Miell <> | Date | Fri, 30 Nov 2007 15:53:38 -0800 |
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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@comcast.net>
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