Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ncar!tank!mimsy!chris
From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.lang.c
Subject: Re: forward references in typedefs
Message-ID: <18729@mimsy.UUCP>
Date: 25 Jul 89 06:28:58 GMT
References: <55480@tut.cis.ohio-state.edu> <1989Jul20.152935.14872@utzoo.uucp> <24CB9E07.9547@marob.masa.com>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 54

In article <24CB9E07.9547@marob.masa.com> cowan@marob.masa.com (John Cowan)
writes:
>... As someone (Chris Torek?) said earlier in this group, "all 'struct'
>pointers must 'smell' the same."  If this behavior is not guaranteed,
>forward references to structs would be unimplementable in one pass.

It was Doug Gwyn, but this is otherwise correct.

>Ufcawss, if you talk to the people who wrote VMS C, they'll tell you that
>all one-pass implementations of C are unacceptable!  (This has something to
>do with generating good code for the 'switch' statement.)

They are both right and wrong.  One-pass code generation means there
are few opportunities for optimsiation; but generating good switches
is easy.  Simply emit a branch at the top of the switch, code at each
of the labels, a branch at the bottom, and then generate the code
that actually implements the switch.  E.g., given

	switch (foo) {
	case 1: case1(); break;
	case 2: case2(); break;
	case 3: case3(); break;
	case 999: case999(); break;
	}
	onward();

one generates code of the form:

		jump	L1
	L3:	call	case1_
		jump	L2
	L4:	call	case2_
		jump	L2
	L5:	call	case3_
		jump	L2
	L6:	call	case999_
		jump	L2
	L1:	move	foo,r0
		compare	r0,#3
		jeq	L5
		jgt	L7
		compare	r0,#1
		jeq	L3
		jgt	L4	# here is the clever part
		jump	L2
	L7:	compare	r0,#999
		jeq	L6
	L2:	call	onward_

This sort of code is exactly what one gets from a one-pass compiler,
except that they tend not to have a clever part. :-)
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris