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From: ado@elsie.UUCP (Arthur David Olson)
Newsgroups: net.unix-wizards,net.wanted
Subject: Help wanted with C/4.?BSD/VAX register short problem
Message-ID: <5300@elsie.UUCP>
Date: Fri, 29-Nov-85 19:11:24 EST
Article-I.D.: elsie.5300
Posted: Fri Nov 29 19:11:24 1985
Date-Received: Sun, 1-Dec-85 03:21:03 EST
Followup-To: poster
Organization: NIH-LEC, Bethesda, MD
Lines: 26
Keywords: C VAX BSD register short
Xref: watmath net.unix-wizards:15922 net.wanted:7664

As documented on the 4.1BSD "cc" manual page, "The compiler currently ignores
advice to put char, unsigned char, short or unsigned hsort variables in
registers.  It previously produced poor, and in some cases incorrect, code for
such declarations."

Since the smart folks at Berkeley were unable to fix the problem, I figure
the best way to deal with it is a work-around.  The notion I've come up with
is to allow the "first pass" (which lives in the same executable as the
"second pass" on 4.1BSD systems) to put shorts (et al.) into registers.
Then, just before code is generated in the "second pass" (specifically, at the
beginning of "p2compile") you'd change nodes whose "op"s were "REG" and whose
"type"s were "short" (et al.) into "NAME" nodes--and would fill the "name" field
of the node with the string "r5" or "r6" or whatever.  The theory is that if
the compiler produces correct code for non-register shorts (et al.) then it
will produces correct code for register shorts provided that you make them
look like non-register shorts.

In testing out this approach, it would be a help to have samples of source code
that caused the compiler to produce "poor, and in some cases incorrect, code."
If you have such samples, I'd appreciate you mailing them to me.  Thanks.
--
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