On 02/09/2012 03:55 PM, James Courtier-Dutton wrote: > Results for x86_64 > gcc -g -O0 -c -o sincos1.o sincos1.c > gcc -static -g -o sincos1 sincos1.o -lm > > ./sincos1 > sin = -8.52200849767188795e-01 (uses xmm register intructions) > sinl = 0.46261304076460176 (uses fprem and fsin) > sincos = 4.62613040764601746e-01 (uses fprem and fsin) > sincosl = 0.46261304076460176 (uses fprem and fsin) > > Only sin() gets an accurate answer. > > Results when compiled for 32bit x86. > gcc -m32 -g -O0 -c -o sincos1.o sincos1.c > gcc -m32 -static -g -o sincos1 sincos1.o -lm > > ./sincos1 > sin = 4.62613040764601746e-01 > sinl = 0.46261304076460176 > sincos = 4.62613040764601746e-01 > sincosl = 0.46261304076460176 > > Which are all inaccurate. > > So, we have a case of the same program compiled for 32bit might give > different floating point results than the same source compiled for > 64bit. > > From what I can tell, the xmm register instructions on x86_64 are > using 128bit precision which probably explains why the result is more > accurate.
That's not the reason. The reason is that x86_64 is using the IBM Accurate Mathematical Library. Have a look. It's in s_sin.c. Andrew.
