http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51446
--- Comment #8 from Dominique d'Humieres <dominiq at lps dot ens.fr> 2011-12-08 16:06:43 UTC --- > Does IEEE say anything about the sign of the qnan? >From "Draft 1.2.5 DRAFT Standard for Floating-Point Arithmetic P754 October 4, 2006" at http://www.validlab.com/754R/drafts/archive/2006-10-04.pdf : 8.2.1 NaN encodings in binary formats ... All binary NaN bitstrings have all the bits of the biased exponent field E set to 1 (see 5.4). A quiet NaN bitstring should be encoded with the first bit (d1) of the trailing significand field T being 1. A signaling NaN bitstring should be encoded with the first bit of the trailing significand field being 0. If the first bit of the trailing significand is 0, some other bit of the trailing significand field must be non-zero to distinguish the NaN from infinity. In the preferred encoding, a signaling NaN should be quieted by setting d1 to 1, leaving the remaining bits of T unchanged. ... 8.3 The sign bit 8.3.0 When either an input or result is NaN, this standard does not interpret the sign of a NaN. Note however that operations on bitstrings – copy, negate, abs, copySign – specify the sign bit of a NaN result, sometimes based upon the sign bit of a NaN operand. The logical predicate totalOrder is also affected by the sign bit of a NaN operand. For all other operations, this standard does not specify the sign bit of a NaN result, even when there is only one input NaN, or when the NaN is produced from an invalid operation. ...