Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ukma!rutgers!rochester!pt.cs.cmu.edu!sei!firth
From: firth@sei.cmu.edu (Robert Firth)
Newsgroups: comp.arch
Subject: Complex Instructions
Keywords: RISC, CISC, HPS
Message-ID: <3169@bd.sei.cmu.edu>
Date: 13 Apr 89 13:01:59 GMT
References: <807@microsoft.UUCP> <92634@sun.uucp> <13322@steinmetz.ge.com> <573@loligo.cc.fsu.edu> <5064@hubcap.clemson.edu>
Reply-To: firth@sei.cmu.edu (Robert Firth)
Organization: Software Engineering Institute, Pittsburgh, PA
Lines: 16

In article <5064@hubcap.clemson.edu> mark@hubcap.clemson.edu (Mark Smotherman) writes:
 [quoting Clark & Strecker]

>  "Anecdotal accounts of irrational implementations are certainly
>   interesting.  Is it *typical*, however, that composite instructions
>   run more slowly than equivalent sequences of simple instructions?

In my experience, yes, it is indeed typical.  Taking only the example
of the VAX, I have found naive code faster than the block operations
(MOVC3, CMPC3), the loop instruction (ACB), INDEX, and, of course, the
call instructions.

Moving to RISC machines, one point to note is that the authors' assertion
that, other things being equal, several instructions cannot be faster than
one is false: the several instructions can be interleaved by the compiler
with other code, so as to overlap delays with useful work.