Fw: RFC on floating point output work

[Jerry Quinn]

Thanks for the replies.

Richard Henderson writes:
 > On Wed, Sep 10, 2003 at 04:56:08AM -0500, Benjamin Kosnik wrote:
 > > Richard, this might interest or disgust you, not quite sure which. Since
 > > you've recently had the priviledge of becoming overly-familiar with
 > > gcc's FP representation, perhaps you could give him some hints?
 > 
 > :-)
 > 
 > > Begin forwarded message:
 > > 
 > > Date: Wed, 10 Sep 2003 01:36:40 -0400
 > > From: Jerry Quinn <jlquinn@optonline.net>
 > > To: libstdc++@gcc.gnu.org
 > > Subject: RFC on floating point output work
 > [...]
 > > The only differences at this
 > > point are ones that appear to me to be bugs in glibc, according to the
 > > printf manpages.
 > 
 > Careful.  Man pages aren't normative.  Look at standards.
 > I kinda doubt the bugs are in glibc.

Can someone send me the relevant section?  I have the C++ standard,
but not the C standard.

 > You'd do well to use instead the nice cleanups we've done in
 > std_limits.h in the last year.

Will do.

 > > static int HIWORD = -1, LOWORD;
 > > static void test_endianness()
 > 
 > When integrated, this should either use defines from the compiler
 > proper, or from stdc++ config headers.  A runtime test is not good.

How do I pull this from the compiler?  I'll check the c++ config
headers first, but would like to know anyway.

 > 
 > > #define Storeinc(a,b,c) (((unsigned short *)a)[1] = (unsigned short)b, \
 > > ((unsigned short *)a)[0] = (unsigned short)c, a++)
 > 
 > Strict aliasing failures.  You need to use unions.

Thanks.  I'll fix up this one.

 > > // qnan: 1,rest of mantissa
 > 
 > Not necessarily true.  MIPS is backwards: 1->SNaN, 0->QNaN.


 > >  union {
 > >    double d;
 > >    unsigned int b[2];
 > >  } x;
 > 
 > (1) unsigned int is not always 32-bit.
 > (2) it'd be nice to use 64-bit types on 64-bit systems.

Oops.

I'll look at it, but I don't have any 64-bit systems to test out on.


 > > hi0bits (register unsigned32 x)
 > 
 > __builtin_clz(x)
 > 
 > > lo0bits (unsigned32 *y)
 > 
 >   int ret = __builtin_ctz(*y);
 >   *y = *y >> ret;

Cool!

 
 > > struct Bigint {
 > [...]
 > > class bigint
 > 
 > It's definitely confusing that you've got both new and old code
 > in the same file.

True.  This is still a work in progress.  My plan is to clean up the
bigint stuff as well.

 > >  * We want k to be too large rather than too small.
 > >  * The error in the first-order Taylor series approximation
 > >  * is in our favor, so we just round up the constant enough
 > >  * to compensate for any error in the multiplication of
 > >  * (i) by 0.301029995663981; since |i| <= 1077,
 > >  * and 1077 * 0.30103 * 2^-52 ~=~ 7.2e-14,
 > 
 > The comment should be updated for long double.  I guess the
 > overestimate is still ok, since while |i| <= 16881 is much
 > larger, 2^-64 and 2^-113 are much smaller.

Can I just use the text and numbers above?  Numerical analysis is not
my strong suit.

 > You might do well to templatize this function, since it is
 > otherwise identical between double and long double.

Good point.  

I'll repost after some more work.

Jerry