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

[Vincent Lefevre]

On 2006-12-31 08:25:23 -0500, Richard Kenner wrote:
> > > > I suppose there is
> > > > 
> > > >       *hv = (HOST_WIDE_INT) -(unsigned HOST_WIDE_INT) h1;
> > > > 
> > > > to make it safe.
> > > 
> > > Can't that conversion overflow?
> > 
> > Not on a two's complement machine, 
> 
> Then I'm confused about C's arithmetic rules. Suppose h1 is 1. It's
> cast to unsigned, so stays as 1. Now we do unary minus in unsigned.
> That's ~0 because it's done mod 2**32, but interpreted as a POSITIVE
> number. Now, that positive number overflows when converted to signed
> (the intent is for that overflow make it negative). Am I missing
> something (quite likely!)?

No, this is not portable. On the same kind of code, even the wrap
may hide a bug, e.g. when one has:

  *hv = -(unsigned HOST_WIDE_INT) h1;

where *hv is signed, but not the same type as the original one.
For instance, such a code will work on a 32-bit machine, but not
necessarily on a 64-bit machine. Now, will GCC do anything to
make such code work on such machines? I doubt that.

-- 
Vincent Lefèvre <address@hidden> - Web: <http://www.vinc17.org/>
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Work: CR INRIA - computer arithmetic / Arenaire project (LIP, ENS-Lyon)