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Re: What is wrong with Bugzilla? [Was: Re: GCC and Floating-Point]
On 2005-05-31 16:07:53 +0200, Andreas Schwab wrote:
> Vincent Lefevre <vincent+gcc@vinc17.org> writes:
>
> > On 2005-05-31 15:33:48 +0200, Andreas Schwab wrote:
> >> Vincent Lefevre <vincent+gcc@vinc17.org> writes:
> >> > On 2005-05-31 14:27:01 +0200, Andreas Schwab wrote:
> >> >> Vincent Lefevre <vincent+gcc@vinc17.org> writes:
> >> >> > But as I said on my page, this isn't much a problem since users
> >> >> > who really need *extended* precision can still set the rounding
> >> >> > precision to extended precision; this isn't portable, but extended
> >> >> > precision isn't portable anyway.
> >> >>
> >> >> What about LDBL_* from <float.h>?
> >> >
> >> > What do you mean here?
> >>
> >> They give you access to long double in a portable manner,
> >
> > No, this is not portable,
>
> Sure they are, since they are required since C89.
The "long double" type is required, but it is not required to be
extended precision.
> > since if extended precision is necessary to get correct results
> > for some application, the same application run on PowerPC, where
> > there is no extended precision, would give incorrect results.
>
> You can use <float.h> to find that out. That's what portability is
> about.
No, this is not sufficient.
> >> but if you change the rounding mode then the constants are no longer
> >> accurate.
> >
> > The implementation could provide another way to get the necessary
> > information; perhaps nextafterl()...
>
> But you still have to take care of (1.0 + LDBL_EPSILON) != 1.0, or the
> other required identities in <float.h>.
Once you change the rounding precision, this is no longer required,
since you are already working with an extension.
--
Vincent Lefèvre <vincent@vinc17.org> - Web: <http://www.vinc17.org/>
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Work: CR INRIA - computer arithmetic / SPACES project at LORIA