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From: brandis@inf.ethz.ch (Marc Brandis)
Newsgroups: comp.arch
Subject: Re: RISC reaching its limits?!?
Message-ID: <25696@neptune.inf.ethz.ch>
Date: 20 Feb 91 09:39:50 GMT
References: <1991Jan27.214225.26765@csusac.csus.edu> <780025@otter.hpl.hp.com>
Sender: news@neptune.inf.ethz.ch
Reply-To: brandis@inf.ethz.ch (Marc Brandis)
Organization: Departement Informatik, ETH, Zurich
Lines: 40

In article <780025@otter.hpl.hp.com> scu@otter.hpl.hp.com (Shankar Unni) writes:
>> In article <3416@uc.msc.umn.edu> dwm@msc.edu (Don Mears) writes:
>> The most
>> surprising feature to me was the hardware string support; IBM justified this
>> by pointing out profiling data showing that a large fraction of system time
>> is spent in string compares**.
>> 
>Also known as the DHRY instruction; named after a very popular but
>ultimately meaningless benchmark :-) :-).

It was not only the high number of string compares, but generally the high
number of string operations that IBM used as a justification. Whether Dhrystone
is a useful benchmark has been the cause of many flame wars and I do not want
to raise this again, but indeed there are many programs that contain a lot
of string operations. In one of the articles of "RISC System/6000 Technology"
they are talking about the compiler, which spends a considerable time in
string operations. I do not have profiling data, but I guess that many UNIX
utilities contain a lot of string operations too. And you should not forget
commercial data processing applications which are known to spend a lot of time
in string operations.

Talking about string operations and Dhrystone: Some of the benefit of the
string operations is lost in the C Dhrystone on the IBM S/6000 because 
procedures are called for the string operations. In the Pascal version, the
string operations are inlined, yielding a significant difference in the
rating. On the IBM S/6000 Model 530, I got the following results.

Dhrystone 2.1, C, all optimizations turned on		46729.0
Dhrystone 2.1, C, non-optimized				18764.7
Dhrystone 2.1, Pascal, all optimizations turned on	64892.9
Dhrystone 2.1, Pascal, non-optimized			21061.5

I had a look at the code and it was almost identical (no surprise, the 
optimizer is the same), except for the string operations.


Marc-Michael Brandis
Computer Systems Laboratory, ETH-Zentrum (Swiss Federal Institute of Technology)
CH-8092 Zurich, Switzerland
email: brandis@inf.ethz.ch