Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!mcsun!unido!mpirbn!p554mve
From: p554mve@mpirbn.UUCP (Michael van Elst)
Newsgroups: comp.unix.wizards
Subject: Re: Should optimizing compilers automatically assign registers?
Message-ID: <952@mpirbn.UUCP>
Date: 6 May 90 21:02:05 GMT
References: <512@hhb.UUCP>
Reply-To: p554mve@mpirbn.UUCP (Michael van Elst)
Organization: Max-Planck-Institut fuer Radioastronomie, Bonn
Lines: 24

In article <512@hhb.UUCP> istvan@hhb.UUCP (Istvan Mohos) writes:
>Prog1 and prog2 are identical except that prog1 requests the
>assignment of the auto variable p to a hardware register.
>I mistakenly assumed that by using the -O flag, the compiler
>would optimize prog2, and generate identical objects from
>prog1 and prog2.  Of the systems I tried this on, the Pyramid90X,
>the Sparcstation1, and a SystemV 6300+ PC, proved this to be true.
>A VAX Ultrix and a Sun3 proved me wrong.  Here are some execution

I've simply checked this by looking at the cc -S output.

The Sun C compiler does not allocate registers when run with
optimizer level 1 but it does so with level 2. The -O option
just invokes a peephole optimizer.

The difference between Sparc and Sun-3 systems is that the Sparc
compiler will run -O2 if invoked with just -O while the Sun-3
defaults to -O1. Under SunOS 4.1, both systems will use -O2.

-- 
Michael van Elst
UUCP:     universe!local-cluster!milky-way!sol!earth!uunet!unido!mpirbn!p554mve
Internet: p554mve@mpirbn.mpifr-bonn.mpg.de
                                "A potential Snark may lurk in every tree."