From mboxrd@z Thu Jan  1 00:00:00 1970
Message-Id: <200207191253.NAA06845@cthulhu.dircon.co.uk>
Subject: Re: [9fans] useful language extension, or no?
In-Reply-To: <06b78404a549aa66986788bcb976b339@plan9.bell-labs.com> from David
 Gordon Hogan at "Jul 18, 2002 08:52:21 pm"
To: 9fans@cse.psu.edu
From: Digby Tarvin <digbyt@acm.org>
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Date: Fri, 19 Jul 2002 13:53:30 +0100
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> Interesting.  My guess is that it's creating a
> trampoline on the stack, which calls the real
> g(), which nm tells me is at:
> 
> 080485a6 t g.0
> Unfortunately gdb on the Linux server is unable to show
> me the instructions:
> 
Why not just get the compiler to generate the assembly language
for you.

It is interesting that the current compiler lets you do it (my old
BSD system won't swallow it) but I can't see any attempt to actually
execute any code on the stack.

For instance, adding code to actually call the function:
int bar(){
        int i;
        int g(){;}
        int h;
        printf("foo=%x\n",foo);
        printf("bar=%x\n",bar);
        printf("i=%x\n",&i);
        printf("g=%x\n",g);
        printf("h=%x\n",&h);
        g();
}

generates
		.
		.
        call printf
        addl $16,%esp
        movl %ebp,%ecx
        call g.6
.L3:
        movl %ebp,%esp
        popl %ebp
        ret

where g.6 is in the code segment:
g.6:
        pushl %ebp
        movl %esp,%ebp
        subl $24,%esp
        movl %ecx,-4(%ebp)
.L4:
        movl %ebp,%esp
        popl %ebp
        ret

and the address of the function is put on the stack, but not used
in the call:

bar:
        pushl %ebp
        movl %esp,%ebp
        subl $56,%esp
        leal -36(%ebp),%edx
        leal 15(%edx),%eax
        andb $240,%al
        movl $g.6,%edx
        leal 10(%eax),%ecx
        subl %ecx,%edx
        movb $185,(%eax)
        movl %ebp,1(%eax)
        movb $233,5(%eax)
        movl %edx,6(%eax)
        addl $-8,%esp
        pushl $foo
        pushl $.LC0
        call printf
		.
		.

This is assuming I am reading the Intel code correctly. I am much more
at home with Motorola or DEC assembly language, but I think these code
fragments seem pretty clear.

I don't really understand the need to have a pointer to the function
on the stack, but I am sure there is a good reason for it. Thre restricted
scope of the function name, which I guess could be useful, does not seem
to depend on it.

Regards,
DigbyT
-- 
Digby R. S. Tarvin                                              digbyt@acm.org
http://www.cthulhu.dircon.co.uk