Re: [9fans] useful language extension, or no?

[Digby Tarvin <digbyt@acm.org>]

Ah - I see! It hadn't realised that the scope rules now included
that sort of nesting. When did that happen? I guess I am going to
have to trade in my much loved, dog eared K&R for a book on
standard C one of these days...

Presumably I need something along the lines of

  #include <stdio.h>
  int foo(int (*f)()) {
          (*f)();
  }
  
  int main() {
          int g(){; }
          foo(g);
  }

To see it used, producing:

foo:
        pushl %ebp
        movl %esp,%ebp
        subl $20,%esp
        pushl %ebx
        movl 8(%ebp),%ebx
        call *%ebx
.L2:
        movl -24(%ebp),%ebx
        movl %ebp,%esp
        popl %ebp
        ret

Presumably %esp is the stack pointer, %ebp is the frame pointer,
and %ebx is the a scratch register used for parameter passing
and jumping through to get to the trampoline. Makes me curious
about what the 'e' stands for - like I said, I am not an x86 expert...

This seems to require the somewhat less than transparent calling sequence:

main:
        pushl %ebp
        movl %esp,%ebp
        subl $40,%esp
        leal -28(%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 $-12,%esp
        leal -28(%ebp),%edx
        leal 15(%edx),%eax
        andb $240,%al
        pushl %eax
        call foo
        addl $16,%esp
.L3:
        movl %ebp,%esp
        popl %ebp
        ret

Presumably $185 generates the instruction to load the frame pointer,
and $233 creates the jump to the actual function.

Thanks for the explanation. The level of knowledge on this list never
ceases to impress me...

Regards,
DigbyT

> In order to call a lexically scoped nested function, a caller needs
> two pieces of information:
> 
> 	1.  the actual address address of the code
> 
> 	2.  a pointer to the stack frame for the containing function,
> 	    sometimes called the static chain pointer.
> 
> A traditional C "pointer to function" is just (1) since there is no
> containing function.
> 
> In order to make "pointer to nested function" have the same
> representation as "pointer to top level function", GCC creates a
> small bit of trampline code on the stack that sets up (2), then
> jumps to (1).  Then when a pointer to the nested function is actually
> needed, GCC passes out the pointer to the trampoline code.
> 
> The trampoline isn't needed when the call to the nested function is a
> direct call within the scope of the parent function--it's only needed
> when the nested function is being called via a function pointer from an
> unknown scope.  So if you never take the address of a nested function,
> GCC shouldn't generate any trampoline code.
> 
> The alternative to the trampoline would be for "pointer to function"
> to become a two-word object, or a pointer to a two-word object, with a
> changed calling sequence in either case.  This would be fine, the C
> standard would certainly allow it, but it would be binary incompatible
> with existing calling conventions in most systems.
> 


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