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From: larus@paris.Berkeley.EDU.berkeley.edu (James Larus)
Newsgroups: comp.arch
Subject: Re: RISC v. CISC (was The NeXT problem)
Message-ID: <26618@ucbvax.BERKELEY.EDU>
Date: 29 Oct 88 22:29:59 GMT
References: <156@gloom.UUCP> <310@lynx.zyx.SE> <332@pvab.UUCP> <15964@agate.BERKELEY.EDU> <23367@amdcad.AMD.COM> <16003@agate.BERKELEY.EDU> <17208@ames.arc.nasa.gov> <17260@ames.arc.nasa.gov>
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Reply-To: larus@paris.Berkeley.EDU.UUCP (James Larus)
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In article <17260@ames.arc.nasa.gov> lamaster@ames.arc.nasa.gov.UUCP (Hugh LaMaster) writes:
>In article <17208@ames.arc.nasa.gov> lamaster@ames.arc.nasa.gov.UUCP (Hugh LaMaster) writes:
>>Correct.  Register windows seem to be a bad idea, but not because of
>
>I seem to have overstated my case.  I should have said, that I consider
>that it is unproven that register windows are a good idea relative to 
>other equivalent uses of the same real estate.  The only "problem"
>that I have with register windows is that, like "RISC" (whatever that is),
>some people make extraordinary unsubstantiated claims.

You might be interested in David Wall's paper "Register Windows vs.
Register Allocation," in the 1988 PLDI Conf.  He found that register
windows were generally as good as large register files (64 or more
entry) together with interprocedural register allocation.  However,
Wall argued that load/store in a large register file should be faster
so that the overall performance would be better without windows.
Given that the last assumption is questionable and that windows have
significant advantages for simpler compilers and for languages that
don't allow interprocedural allocation (e.g., Lisp), it is surprising
that more machines don't use them.

/Jim