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. > > A single bit necessarily conveys at most one bit of information. > That can indicate whether the MSB being zero is the criterion; > it cannot indicate what the criterion is if that is not the case.
There are only two conventions relevant to systems supported by GCC: the one where MSB of 1 indicates quiet NaN and 0 indicates signaling NaN (most systems) and the reverse one, where MSB of 0 indicates quiet NaN and 1 indicates signaling NaN (MIPS -mnan=legacy, HPPA). (Plus a few formats with no NaNs at all, but there __builtin_issignaling can just evaluate its argument for its side effects and return 0, like __builtin_isnan does.) -- Joseph S. Myers jos...@codesourcery.com
