Re: type based aliasing again

[Richard Stallman <rms at gnu dot org>]

    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.