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 jos...@codesourcery.com
