Path: utzoo!attcan!uunet!husc6!mailrus!iuvax!pur-ee!hankd
From: hankd@pur-ee.UUCP (Hank Dietz)
Newsgroups: comp.arch
Subject: Re: A simple question on RISC
Summary: Code replication solves this.
Message-ID: <9867@pur-ee.UUCP>
Date: 22 Nov 88 16:20:00 GMT
References: <6544@xanth.cs.odu.edu> <75577@sun.uucp> <1618@imagine.PAWL.RPI.EDU> <11562@cup.portal.com>
Organization: Purdue University Engineering Computer Network
Lines: 34

In article <11562@cup.portal.com>, bcase@cup.portal.com (Brian bcase Case) writes:
> >        mov     lower,i
> >        bra     entry
> >body    foo
> >        add     step,i
> >entry   cmp     i,upper
> >        bge     body
>
> Ok, but notice the label in the middle of the add-compare-branch sequence?
> I don't think it is possible to specify the ACB instruction in a version
> that has a branch destination in the middle!  This is one of the points of
> RISC:  Hey, *I'M* the compiler, let *me* decide on the semantics of
> instruction sequences.

You can do the same thing by simply replicating the test code above the loop
rather than jumping into the loop.  It's a smidge harder to do, but it isn't
*that* hard, and it works even if the branching-backward loop becomes a
single instruction. The example would become:

        mov     lower,i
        cmp     i,upper		;replicated test
        blt     skipit		;opposite condition to bge
body    foo
        add     step,i
        cmp     i,upper
        bge     body
skipit

Which obviously doesn't mind having the code between body and skipit end up
being a single instruction.

					-hankd@ee.ecn.purdue.edu

PS:  I'm not saying I like ACB, but you're not *my* compiler (:-).