http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57484
--- Comment #20 from Uroš Bizjak <ubizjak at gmail dot com> --- (In reply to Charles L. Wilcox from comment #19) > (In reply to Uroš Bizjak from comment #11) > > On an x86 target using the legacy x87 instructions and the 80-bit registers, > > a load of a 64-bit or 32-bit value in memory into the 80-bit registers > > counts as a format conversion and an signaling NaN input will turn into a > > quiet NaN in the register format. > > Does this mean if a 80-bit sNaN was generated and loaded into a register it > was still have the signaling bit set correctly? And if so, could this value 80-bit load is not considered as a format conversion, so signalling bit will be set correctly. > then be down-converted to a 32 or 64-bit float? In C++: > float float32_snan const > = static_cast< float >( std::numeric_limits< long double > >::signaling_NaN() ); > double float64_snan const > = static_cast< double >( std::numeric_limits< long double > >::signaling_NaN() ); > Or, is the "cast" here a format conversion, causing the signaling NaN to > convert to a quiet NAN? I don't know the c++ details, but compiler can spill the value out from the register stack using 32- or 64-bit float moves. In any case, signalling bit in x87 is unreliable and should not be used.
