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From: baum@Apple.COM (Allen J. Baum)
Newsgroups: comp.arch
Subject: Re: loop unrolling (was:Re: Register Count)
Message-ID: <48303@apple.Apple.COM>
Date: 21 Jan 91 03:46:27 GMT
References: <PCG.91Jan16195742@teacho.cs.aber.ac.uk> <5869@labtam.labtam.oz> <PCG.91Jan19120851@odin.cs.aber.ac.uk>
Reply-To: baum@apple.UUCP (Allen Baum)
Organization: Apple Computer, Inc.
Lines: 18

[]
>In article <PCG.91Jan19120851@odin.cs.aber.ac.uk> pcg@cs.aber.ac.uk (Piercarlo Grandi) writes:
>2) registers are not free. Every doubling in size of the register file
>costs you a lengthening of the cycle time of a few percent; even worse,
>a lot of complication because such register files are flat and need to
>be multiported all the way. The same resources that go into doubling a
>register file might well be more productively employed elsewhere, just
>like all the real memory wasted because of the appalling inadequacy of
>the VM policies mentioned above.

I couldn't let this pass by. Doubling a register file size slows down register
access a few percent. It may or may not slow down cycle time-- when register
access is in the critical path.


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