QNaN and SNaN definitions

[Hartvig Ekner <hartvige at mips dot com>]

I have found the glibc definitions for QNaN and SNaN are wrong for MIPS
(and some other architectures).

Unfortunately, IEEE let it up to the implementations to decide how
the bit distinguishing between QNaN and SNaN was to be encoded, and
most of the world choose that particular bit==1 to mean QNaN. 

For MIPS, HPPA and maybe others, that bit is reversed, and has to be ==0
to mean QNaN.

As far as I can see, all cases inside glibc don't care whether things
are a QNaN or SNaN (eg. isnan(), fpclassify etc) and therefore work
correctly for both cases, but some of the public header files in glibc
which end up in /usr/include are wrong:

sysdeps/ieee754/bits/nan.h

	Specifically specifies a NaN (QNaN) to 7fc0.0000.

sysdeps/ieee754/ieee754.h:

	Defines 8 structures like:

    /* This format makes it easier to see if a NaN is a signalling NaN.  */
    struct
      {
#if     __BYTE_ORDER == __BIG_ENDIAN
        unsigned int negative:1;
        unsigned int exponent:8;
        unsigned int quiet_nan:1;
        unsigned int mantissa:22;

I can send in patches to fix this for MIPS, but I would think this is
inappropriate and that some of you glibc people would have to decide how
to deal with this. If you can tell me how you want it fixed, I will be
happy to deliver the patches, but I need a machine specific #define to
look at in these header files. I would like to avoid separate MIPS versions
of these header files if possible (maintenance issues).

In nan.h, it is clear what has to be done. 

In ieee754.h it is less clear. One can leave things as they are, but then
peoples apps will not work without them knowing the difference. Maybe define
machine-dep constants to be used with the quiet_nan field, or more 
drastically, rename the quiet_nan field to signalling_nan. Is this an
option at all?

/Hartvig

--
 _    _   _____  ____     Hartvig Ekner        Mailto:hartvige@mips.com
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-- 
 _    _   _____  ____     Hartvig Ekner        Mailto:hartvige@mips.com
 |\  /| | |____)(____                          Direct: +45 4486 5503
 | \/ | | |     _____)    MIPS Denmark         Switch: +45 4486 5555
T E C H N O L O G I E S   http://www.mips.com  Fax...: +45 4486 5556