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From: chris@umcp-cs.UUCP (Chris Torek)
Newsgroups: net.lang.c
Subject: Re: Code Generation
Message-ID: <2327@umcp-cs.UUCP>
Date: Mon, 25-Nov-85 07:53:08 EST
Article-I.D.: umcp-cs.2327
Posted: Mon Nov 25 07:53:08 1985
Date-Received: Tue, 26-Nov-85 20:53:37 EST
References: <3457@brl-tgr.ARPA> <462@graffiti.UUCP>
Organization: U of Maryland, Computer Science Dept., College Park, MD
Lines: 33

In article <462@graffiti.UUCP> peter@graffiti.UUCP (Peter da Silva) writes:

[me]
> > > You can indeed apply
> > > registers to `short's and `char's and even `float's and `double's;
> > > it is merely much more difficult to get the code generation right.

[Jim Cottrell]
> > Why is that? ...

> The registers are not wide enough to hold a double.

No, but then they never were.  The double precision instructions
operate on register pairs (shades of the 370! :-) ), when they
operate on registers at all.

> Consider a case where { register short i; register long j; short c; }
> are in registers.  Perform { i=c; ... j=i; }.  You have to do more
> than move the low word of the register.  You have to either explicitly
> clear the high word of i when you store c in it, have explicitly cleared
> it when you entered the block, or clear the high word of j after doing
> a 16-bit move from i to j.

That is not really necessary.  All you need is `cvtwl rI,rJ'.

But all this just goes to demonstrate my original point.  The code
generation gets messier, and that makes it harder:  That is the main
reason no one has done it, except maybe Tartan Labs---and they are
not exactly giving it away....
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 4251)
UUCP:	seismo!umcp-cs!chris
CSNet:	chris@umcp-cs		ARPA:	chris@mimsy.umd.edu