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    SubjectRe: Kernel Development & Objective-C
    From
    Date

    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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