Re: changing "configure" to default to "gcc -g -O2 -fwrapv ..."

["Richard Guenther" <richard dot guenther at gmail dot com>]

On 12/31/06, Robert Dewar <dewar@adacore.com> wrote:
> Paul Eggert wrote:
>
> > The question is not whether GCC should support wrapv
> > semantics; it already does, if you specify -fwrapv.
> > The question is merely whether wrapv should be the default
> > with optimization levels -O0 through -O2.
>
> That over simplifies, because it presents things as though
> there are only two possibilities
>
> 1. Allow "undefined" optimizations without restriction
>
> 2. Forbid all such optimizations by specifying fwrapv
>
> Quite a few of us (including certainly me, and I think
> Richard) argue for a mid ground where
>
> We don't insist on full fwrapv semantics (because we
> think it will hurt code equality, particularly in the
> loop invariant case).
>
> We are judcious in applying the optimization in other
> cases in a pragmatic attempt to keep "traditional"
> C code working in practice.

I think this is a fragile and not very practical approach.  How
do you define these "traditional" cases?  I guess you would
keep the other two as well, so have the mid ground default,
-fno-wrapv do what we have now and -fwrapv disable all
the optimizations.

I'd rather enable -fwrapv at -O1 and keep -O2 as is.  This is
what we also do for strict-aliasing, it's enabled at -O2 and beyond
only (for C, that is).  Of course with -O1 the VRP pass is not run, so
it might be that the reported problems do not show up with -O1 - and
as they are reportedly not for performance critical code they should
maybe just use -O1 as autoconf default.

Richard.