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Re: Reporting bugs: there is nothing to gain in frustrating reporters


On 2005-06-16 08:20:20 -0400, Daniel Berlin wrote:
> You deny that Bug 21809 is the same bug as Bug 323, which was closed
> in 1999?

Yes: Bug 323 as originally reported is really invalid. The C standard
doesn't guarantee that y and y2 should exactly be the same value.
However bug 21809 (like *some* other bugs duplicated to bug 323) shows
a bug in the compiler, since a variable must not have a precision (or
range) larger than the one specified by the type. A fix is *possible*:
just store the value to memory after a cast or an assignment. Of
course, this wouldn't necessarily be efficient, but there could be
options to allow the compiler to have a non-comforming behavior. Or
the user could get a processor that behaves differently.

Dynamical extended precision is allowed by IEEE-754 and a part of
the specifications of the x86 processors. So, you can't say that
such processors are buggy.

On 2005-06-16 10:11:39 -0400, Mark Hahn wrote:
> the question then becomes whether GCC is helped or harmed by 
> its current policy of unpredictable excess precision.

BTW, unpredictability, such as in bug 323, is not a bug (according
to the C standard). This may be seen as a bad behavior and changing
this behavior would be a great improvement, but I don't complain
about it here when saying "bug".

> the only constructive comment in this thread so far has been that libm
> might somehow rely on EP (ie can't use the _FPU_SET_CW workaround).
> which code is this?  I'd guess it might be related to using series 
> approximations, and the code could either set CW or accept 64b results.

Or the code could be changed: glibc has code that is guaranteed to
work in double precision (modulo some bugs), since it is used on
processors that have no extended precision.

-- 
Vincent Lefèvre <vincent@vinc17.org> - Web: <http://www.vinc17.org/>
100% accessible validated (X)HTML - Blog: <http://www.vinc17.org/blog/>
Work: CR INRIA - computer arithmetic / SPACES project at LORIA


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