Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!purdue!tut.cis.ohio-state.edu!YAHI.STANFORD.EDU!tiemann
From: tiemann@YAHI.STANFORD.EDU (Michael Tiemann)
Newsgroups: gnu.g++
Subject: address of virtual function (revisited)
Message-ID: <8905100026.AA27112@yahi.stanford.edu>
Date: 10 May 89 00:26:08 GMT
References: <6590118@hplsla.HP.COM>
Sender: daemon@tut.cis.ohio-state.edu
Reply-To: tiemann@lurch.stanford.edu
Distribution: gnu
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   So, for example, right now, with my combination of cfront and
   back end compiler, Jerry's approach, where you violate the encapsulation
   of an object, access its instance variables directly, and lock
   future derived classes into using one approach to returning
   "size", and force a syntax of p->size rather than p->size() --
   then generates code like such:
   
	   movel	a2@,sp@-
   
   Where as if one takes the "Object Oriented" approach of using
   virtual functions to return these values, so that some future
   class -- a linked list maybe -- can have a virtual function
   that calculates "p->size()" on the fly, then the [minimum]
   code that gets generated is:
   
	   movel	a3@(4),a0
	   addqw	#8,a0
	   addw	a0@,a3
	   movel	a3,sp@-
	   movel	a0@(4),a0
	   jbsr	a0@
	    |
	    |
	    V
	   link	a6,#0
	   movel	a6@(8),a0
	   movel	a0@,d0
	   unlk	a6
	   rts
	    |
	    |
	    V
	   movel	d0,sp@-
      
For crying out loud!  If you are so concerned about performance that
you would be willing to rape your code if it saved you a few cycles,
how about this radical approach: write in the object oriented style,
AND GET A REAL MACHINE.  On a RISC machine, such as the Sparc, your
code example compiles into this:

	ld	[%i0+4],%o0	// coax vtable out of `this'
				// with GNU C++, this may
				// already be in a register
	ld	[%o0+8],%o1	// get address of virtual function
   |	call	%o1		// start call to function
   |	mov	%i0,%o0		// put `this' in first parameter
   V |	ret			// trivial function, start return
     |	ld	[%o0+8],%o0
     V	mov	%o0,%o3		// `push' parameter

Note that this sequence of instructions takes about as long on a
SparcStation1 as this one:

	   movel	a2@,sp@-

on a 68020 machine costing about the same money, yet it came from
using code "written in the object oriented style".

Let's think about the future before we write a bunch of code which
will only deserve the name "dusty deck" when it finally makes it to
market.  RISC machines are now cheap.  Let's take advantage of their
power and scalability to write code which will last longer (or at
least hold up better) than our predecessors' did.  Reusable code isn't
reusable if it's not usable...

Michael