Built-in testing for signaling nan?

[Joseph S. Myers]

On Wed, 6 Nov 2013, N.M. Maclaren wrote:

> Yes, due to the poor quality of the IEEE 754 specifications.  In 1984,
> the distinction was left completely unspecified (even in intent).  In
> 2008, there is a recommendation (no more) that the top bit of the payload
> is used, with no specification of what to do if that is zero (which is
> the most obvious default).  That appears to be the Intel specification,
> though it may not have always been so even for x86.  I have certainly
> seen more than one convention for different architectures, but have no
> idea how many are extant nor how many are documented in the relevant
> architecture manuals.

In GCC, the qnan_msb_set bit of struct real_format is what specifies the 
convention in use for a particular floating-point mode.

-- 
Joseph S. Myers
joseph@codesourcery.com