Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!ames!zodiac!joyce!sri-unix!quintus!ok
From: ok@quintus
Newsgroups: comp.arch
Subject: MC88000 multiply question
Keywords: answer, thanks
Message-ID: <207@quintus.UUCP>
Date: 29 Jul 88 04:12:47 GMT
Sender: news@quintus.UUCP
Reply-To: ok@quintus ()
Organization: Quintus Computer Systems, Inc.
Lines: 22

The best answer I have received to my question came from Marv Rubinstein,
who pointed out that
    To determine whether x*y would overflow:
	a = abs(x)
	b = abs(y)
	if (a != 0 && MAXINT/a < b) report overflow
will do the job, and one can then use the multiply instruction provided.
In the best RISC tradition, if one of the operands is a constant, the
division can be done by the compiler, reducing to
	if (y < -lower || y > upper) report_overflow.

Since quite a few integer multiplications are by constants (Does anyone have
measurements on this?  There should be some for MESA.) this is a reasonable
tradeoff, provided you assume that the common case should be to deliver
incorrect results without warning.  At least one Pascal compiler I use
generates code which will report an overflow if you do
	var x: 0..127; ... x := 100; x := x*x;
but will not report the overflow in
	var x: integer; ... x := 1000000000; x := x*x;
What a botch.  {The SunOS 3.2 pascal compiler catches both.  Thanks, SMI.}

    Richard (A bear of very little brain) O'Keefe.