[Numpy-discussion] Solaris Sparc build broken

[Charles R Harris]

On Thu, Nov 5, 2009 at 12:39 AM, David Cournapeau <
david@ar.media.kyoto-u.ac.jp> wrote:

> Charles R Harris wrote:
> >
> >
> > On Thu, Nov 5, 2009 at 12:47 AM, Charles R Harris
> > <charlesr.harris@gmail.com <mailto:charlesr.harris@gmail.com>> wrote:
> >
> >
> >
> >     On Thu, Nov 5, 2009 at 12:09 AM, David Cournapeau
> >     <david@ar.media.kyoto-u.ac.jp
> >     <mailto:david@ar.media.kyoto-u.ac.jp>> wrote:
> >
> >         Charles R Harris wrote:
> >         >
> >         >
> >         > On Wed, Nov 4, 2009 at 11:39 PM, David Cournapeau
> >         > <david@ar.media.kyoto-u.ac.jp
> >         <mailto:david@ar.media.kyoto-u.ac.jp>
> >         <mailto:david@ar.media.kyoto-u.ac.jp
> >         <mailto:david@ar.media.kyoto-u.ac.jp>>>
> >         > wrote:
> >         >
> >         >     Charles R Harris wrote:
> >         >     >
> >         >     >
> >         >     > On Wed, Nov 4, 2009 at 11:30 PM, David Cournapeau
> >         >     > <david@ar.media.kyoto-u.ac.jp
> >         <mailto:david@ar.media.kyoto-u.ac.jp>
> >         >     <mailto:david@ar.media.kyoto-u.ac.jp
> >         <mailto:david@ar.media.kyoto-u.ac.jp>>
> >         >     <mailto:david@ar.media.kyoto-u.ac.jp
> >         <mailto:david@ar.media.kyoto-u.ac.jp>
> >         >     <mailto:david@ar.media.kyoto-u.ac.jp
> >         <mailto:david@ar.media.kyoto-u.ac.jp>>>>
> >         >     > wrote:
> >         >     >
> >         >     >     Charles R Harris wrote:
> >         >     >
> >         >     >     >
> >         >     >     > I don't think it's that bad. Leaving out the ppc
> and
> >         >     sticking to
> >         >     >     ieee,
> >         >     >     > there is only double precision, extended
> >         precision and quad
> >         >     >     precision
> >         >     >     > versions of long double and they  are easily
> >         determined at
> >         >     run time.
> >         >     >
> >         >     >     How would you determine this at runtime ?
> >         >     >
> >         >     >
> >         >     > Excepting the PPC, just loop adding a number to one,
> >         dividing it by
> >         >     > two at each iteration, and stop when the result is
> >         equal to one.
> >         >
> >         >     But that's not what I need. I need to know exactly the
> >         binary
> >         >     representation: how many bits in the mantissa/exponent
> >         and where, the
> >         >     exponent, where does subnormals start, the range of NAN
> >         >     representations,
> >         >     etc...
> >         >
> >         >
> >         > It tells you how many bits are in the mantissa, and given
> >         ieee the
> >         > rest follows. We only support ieee anyway.
> >
> >         But is this reliable ? It does not seem to work for long double
> in
> >         intel, for example (but seems to work for sparc64, at least
> >         using qemu).
> >
> >
> >     Works fine here:
> >
> >     #include <stdio.h>
> >
> >     int main(int argc, char **args)
> >     {
> >         long double tol;
> >         int i;
> >
> >         for (i = 0, tol = 1; 1 + tol != 1; tol /=2, i++);
> >         printf("count: %d\n", i - 1);
> >         return 0;
> >     }
> >
> >
> >     $[charris@ubuntu ~]$ gcc precision.c
> >     $[charris@ubuntu ~]$ ./a.out
> >     count: 63
> >
> >     That's 63+1 for the mantissa, which is what intel extended
> >     precision is.
> >
> >
> > Googling around, it seems that the SUN quad precision is done in
> > software, not hardware and is available but not used by the compilers
> > in the current Intel based SUN systems, but will be in the next OS
> > version. So it looks dependent on the compiler, meaning we probably
> > need a run time check.
>
> Now that I think about it, if we only support quad precision, double ==
> long double and 80 bits Intel format, we could just check for the size
> and be done with it.
>
>
Won't work. For alignment, 80 bits is stored in 128 bits on 64 bit machines,
the same as quad precision. I've been complaining about not having a decent
distinction between those two types for several years ;) But the runtime
determination could just set some function pointers in a jump table if you
want to simplify the interface.

Chuck
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