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

[Robert Dewar <dewar at adacore dot com>]

Vincent Lefevre wrote:
> On 2006-12-31 10:08:32 -0500, Richard Kenner wrote:
> > > Well, that's not equivalent. For instance, MPFR has many conditions
> > > that evaluate to TRUE or FALSE on some/many implementations (mainly
> > > because the type sizes depend on the implementation), even without
> > > the assumption that an overflow cannot occur.
> > Can you give an example of such a condition and show how an
> > optimization that assumed overflows were undefined could break that
> > code?
>
> This won't break the code.

OK, so your answer to Richard is simply no, helping to confirm
his pragmatic assertion that the code in GCC that assumes wrapping
is in practice (if not in theory) safe from the optimization efforts
in this area.

> But I'm saying that if the compiler assumes
> wrapping, even in some particular cases (e.g. code that *looks like*
> "overflow check"), it could miss some potential optimizations. That
> is, it is not possible to avoid breaking overflow checks *and*
> optimizing everything else.

Of course, but this has nothing to do with Richard's point