----- Original Message ----- > On 09/24/2012 02:53 PM, Jose Fonseca wrote: > >> From signbit manpage: > > > > This is not the same as x< 0.0, because IEEE 754 floating point > > allows zero to be signed. The comparison -0.0< 0.0 is > > false, but signbit(-0.0) will return a nonzero value. > > > > I think that for consistency (and especially because MSVC ends not > > being as widely tested as gcc), we should match the spec. This can > > easily be done using an inline and an union. > > The DIFFERENT_SIGNS and IS_NEGATIVE macro are only used in the tnl > clipping code and I don't think the signbit() of 0 vs -0 would make > any real difference in the end. > > But I see your point. Who knows where signbit might be used in the > future and consistency is good. The question then is how closely do > we want to follow Linux's example? The code in math.h checks > sizeof(x) to handle float and double (and long double) specially: > > /* Return nonzero value if sign of X is negative. */ > # ifdef __NO_LONG_DOUBLE_MATH > # define signbit(x) \ > (sizeof (x) == sizeof (float) ? __signbitf (x) : __signbit (x)) > # else > # define signbit(x) \ > (sizeof (x) == sizeof (float) \ > ? __signbitf (x) > \ > : sizeof (x) == sizeof (double) > \ > ? __signbit (x) : __signbitl (x)) > # endif
I see, it's polymorphic macro, that works with all float types... One MSVC implement one can often see in google is # define signbit(x) (_copysign(1.0, (x))) floats are converted to double so it is correct for at least float and doubles (and I don't think we'll ever need to care for long doubles). I'm not sure if it is slower for single precision due to the double converstion though... Jose _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
