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From: lamaster@pioneer.arpa (Hugh LaMaster)
Newsgroups: comp.arch
Subject: Re: What should be in hardware but isn'
Message-ID: <2910@ames.arpa>
Date: Mon, 28-Sep-87 21:17:36 EDT
Article-I.D.: ames.2910
Posted: Mon Sep 28 21:17:36 1987
Date-Received: Wed, 30-Sep-87 00:58:24 EDT
References: <581@l.cc.purdue.edu> <28200048@ccvaxa>
Sender: usenet@ames.arpa
Reply-To: lamaster@ames.UUCP (Hugh LaMaster)
Organization: NASA Ames Research Center, Moffett Field, Calif.
Lines: 40

In article <28200048@ccvaxa> aglew@ccvaxa.UUCP writes:

>
>    important. Say that I do have a code that needs to count the number
>    of set bits in a bitstring, and it already uses a POP function.
>    Well, I can simply replace the POP function by my POP instruction,
>    can't I? Unfortunately, most $%^^#@!!! languages do not let you;
>    you either have to wrap the POP instruction in a function call,
>    which loses most of it's benefit, or you have to use asms after
>    putting stuff into register variables that you *KNOW* the compiler
>    maps to R7 and R6... Not good either way.
>	It would be nice if the compiler could be made to know about every
>    instruction in the machine, even though it didn't generate code
>    for them; it would be nice if you could say

As described, it is semi-machine-independent.  This is hard to do.  But, there
have been compilers that let you put instructions in in a machine dependent
way.  This is a nice compromise with assembly language code, because most of
the time you can let the compiler do the work.  The Cyber 205 has a set of
"Q8" calls, so called because they look like Q8MERGE say to use a "MERGE"
instruction.  The compiler uses program names to generate the addresses:
e.g.  CALL MERGE(A,B,C)  so you don't have to do anything wierd to get the
address.  It is a nice feature, as long as it doesn't encourage the compiler
writer to get lazy.  ("Well, I won't bother to recognize that special case and
generate the "MERGE" instruction, because if someone really needs it they can
call it directly.")  I wish more compilers had this feature; it eliminates the
need for a lot of assembly language coding.  (Or speeds up the process,
depending on how you look at it.)






  Hugh LaMaster, m/s 233-9,  UUCP {topaz,lll-crg,ucbvax}!
  NASA Ames Research Center                ames!pioneer!lamaster
  Moffett Field, CA 94035    ARPA lamaster@ames-pioneer.arpa
  Phone:  (415)694-6117      ARPA lamaster@pioneer.arc.nasa.gov

(Disclaimer: "All opinions solely the author's responsibility")