NaN format

Marcus Mascari wrote:

Sun Microsystem's Numerical Computation Guide
which covers the IEEE 754 standard, states:

"NaN's are often represented as floating-point numbers
with the exponent e max + 1 and non-zero significands."

and

SoftFloat, a free software implementation of the
standard,
http://HTTP.CS.Berkeley.EDU/~jhauser/arithmetic/softfloat.html,
defines the following:

default 32-bit NaN: 0xFFC00000
default 64-bit NaN: 0xFFF8000000000000

My i386 Linux's nan.h defines them as

Signal double NaN 0x7FF00000000F0000
Quiet double NaN 0x7FF8000000000000
Signal float NaN 0x7F800F00
Quiet float NaN 0x7FC00000

Assigning these values to int32_t and int64_t, and then
memcpying them to a float and a double representation
yeilded a TRUE result when testing with isnan().
If they are NaN under Linux/Intel implementation
(2.0.36) using gcc 2.7.2.3 and math library 2.0.7
of the IEEE 754 standard, is it not fair to say it
would be true for all implementations?

Even if your ones work with Linux too, I expect problems on
other implementations or at least on different hardware.

If we add a little check to the regression, I think we can
figure out most of the platform specific definitions for NaN
until v6.5 and put them into .../include/port/*.h with having
a fallback to a function that elog()'s why this test failed.

Jan

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