Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/18/84; site rtp47.UUCP
Path: utzoo!watmath!clyde!bonnie!akgua!mcnc!rti-sel!rtp47!meissner
From: meissner@rtp47.UUCP (Michael Meissner)
Newsgroups: net.lang.c
Subject: Re: Code Generation
Message-ID: <260@rtp47.UUCP>
Date: Sat, 23-Nov-85 10:12:35 EST
Article-I.D.: rtp47.260
Posted: Sat Nov 23 10:12:35 1985
Date-Received: Mon, 25-Nov-85 07:12:16 EST
References: <3457@brl-tgr.ARPA>
Reply-To: meissner@rtp47.UUCP (Michael Meissner)
Organization: Data General, RTP, NC
Lines: 22
Summary: 

In article <3457@brl-tgr.ARPA> cottrell@nbs-vms.arpa (COTTRELL, JAMES) writes:
>/* Key: >>> Ron Natalie, >> Chris Torek
>> > It doesn't ignore register short definitions, it mearly does not put them
>> > in registers because registers can only be applied to types int and long
>> > on VAX.
>> 
>> 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.
>
>Why is that? For the moment lets forget about the floating types.
>From the way I read my manual, using the byte & word flavors of
>instruxions makes the registers appear to be 8 & 16 bits wide.
>So what's so difficult about that.
>

    I don't know much about the VAX pcc C compilers, but I would guess the
real problem is the semantics of C, which requires short/char items to be
converted to INT in any arithmetic expression.  Thus the 8 and 16 bit forms
of the instruction cannot be used (because they would truncate, etc.).  Also
such instructions assume that both operands are short/char.

	Michael Meissner, Data General