Path: utzoo!mnetor!uunet!husc6!uwvax!umn-d-ub!umn-cs!hall!dtj
From: dtj@hall.cray.com (Dean Johnson)
Newsgroups: comp.arch
Subject: Re: The advantages of FREQUENCY
Message-ID: <3783@hall.cray.com>
Date: 3 Feb 88 03:07:51 GMT
References: <8801142212.AA04401@decwrl.dec.com>
Reply-To: dtj@hall.UUCP (Dean Johnson)
Organization: Cray Research, Inc., Mendota Heights, MN
Lines: 13

The performance benefits of FREQUENCY in large scale architectures would
likely be very slight. In more complex architectures with significant
instruction buffers (such as supercomputers), jumps are very expensive
and, unless you moved the less frequent block totally out of the flow of
the program, you would be jumping around one of the blocks either way.

If you moved the less frequent case out of the flow of the program, you
would incur 2 jumps (to and from the block) to get back into the flow of
the program, thus (atleast) doubling the cost of the less frequent case.

There is potential for a small increase in performance by scheduling
the most common case so that it would get in a few extra instructions
before it had to jump, but it probably wouldn't be worth the hassle.