From mboxrd@z Thu Jan  1 00:00:00 1970
From: Richard Stallman <rms@gnu.org>
To: jbuck@synopsys.com
Cc: mark@codesourcery.com, jbuck@synopsys.com, gcc@gcc.gnu.org
Subject: Re: type based aliasing again
Date: Thu, 30 Sep 1999 18:02:00 -0000
Message-ID: <199909100634.CAA01839@psilocin.gnu.org>
References: <199909090502.WAA24772@atrus.synopsys.com>
X-SW-Source: 1999-09n/msg00394.html
Message-ID: <19990930180200.joQ8hMYKsyzvP42nuYYNbGjzvCBqlQIiwnkXZoFgWHA@z>

    But this is just the reverse of what happens now; since -O2 will do
    more reordering than -O, we already have programs that work with -O
    and don't work with -O2 because of type-based aliasing.

There may also be some that work now with -O2 and not with -O.  I
don't know any examples, but if there are two instructions that could
be compiled in either order, it could be that -O chooses the order the
user doesn't want, and -O2 for random reasons chooses the order the
user does want.

In general, for any set of options, there will be a fairly large
number of cases of invalid aliasing that just happen to work,
because the optimizers decide not to do the possible optimizations
that could have changed the behavior.