Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!rutgers!rochester!pt.cs.cmu.edu!andrew.cmu.edu!bader+
From: bader+@andrew.cmu.edu (Miles Bader)
Newsgroups: comp.sys.amiga.tech
Subject: Dividing by a power of two (Re: Aztec compiler ineffeciencies)
Message-ID: <YXXhTUy00UkaQ85bl7@andrew.cmu.edu>
Date: 26 Nov 88 16:12:16 GMT
References: <3013@sugar.uu.net>,
	<7949@dasys1.UUCP>
Organization: Carnegie Mellon
Lines: 20
In-Reply-To: <7949@dasys1.UUCP>

rodd@dasys1.UUCP (Rod Dorman) writes:
> As many compiler writers have found in the past and I'm sure many will
> rediscover in the future, this only works for positive values.  Try
> shifting -1 and you'll get -1 as a result, *not* zero as one would expect.

I think the problem is that shifting always rounds down, as opposed
to towards zero, like we expect.  So even in cases where you have a
signed, unkown value to divide, it would probably still be more
effecient to inline code.

So, when dividing a signed value, x, by a power of two, y:

if(x<0 && (x&(y-1))!=0)
    x|=y; /* pre-round negative numbers */
x>>=log2(y);

Which isn't quite as bad as it looks, since y is a known constant
power of two.

-Miles