On 02/09/2012 02:51 PM, James Courtier-Dutton wrote: > 2012/2/9 Andrew Haley <a...@redhat.com>: >> On 02/09/2012 01:38 PM, Tim Prince wrote: >>> x87 built-ins should be a fair compromise between speed, code size, and >>> accuracy, for long double, on most CPUs. As Richard says, it's >>> certainly possible to do better in the context of SSE, but gcc doesn't >>> know anything about the quality of math libraries present; it doesn't >>> even take into account whether it's glibc or something else. >> >> Yeah. On x86_64 glibc, we should really get the sincos bg fixed. And >> it really is a bug: cos() and sin() are correctly rounded but sincos() >> isn't. >> >> Andrew. > > Given: > > 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) > > It looks to me that only sin() is evaluation correctly using xmm > register instructions.
Yes. To be more precise, it uses the IBM Accurate Mathematical Library > sinl, sincos, and sincosl all use fprem and fsin and fsincos. That's right. > From my analysis, fprem is causing a lot of the problems. > Using a combination of xmm register instructions instead of fprem > might cure our problems with fsincos. > Alternatively, make both sin() and sinl() use xmm register intructions. I don't think that's possible in any reasonable way: the registers are only 64 bits wide. It would be better to do an accurate argument reduction on 80-bit extended reals. Andrew.
