On Thu, Feb 9, 2012 at 4:20 PM, Geert Bosch <bo...@adacore.com> wrote: > > On Feb 9, 2012, at 08:46, Andrew Haley wrote: >> n 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. > > It seems to be a persistent problem that we need to rely on a system-provided > C library, instead of a math library that is installed with the compiler. > > For non-C languages, such as Ada, we need to meet specific accuracy > requirements. However, we cannot distribute the latest glibc with the > compiler for each target platform. In particular, it is common for > users to install new compilers on (very) old OS versions. Even today, > we cannot rely on the standard libm to be C99 compliant and provide > a relative error of 2 epsilon (2-4 ulps) or better on ANY platform. > So, the only way to have a conforming Ada implementation is to roll > our own. It would be so much better if we could work on a common > library that can be used by all languages. > > Given the fact that GCC already needs to know pretty much everything > about these functions for optimizations and constant folding, and is > in the best situation to choose specific implementations (-ffast-math > or not, -frounding-math or not, -ftrapping-math or not, etc), specific > optimizations (latency/throughput/vector optimized) and perform > (partial) inlining, shouldn't we have a math library directly within GCC?
Yes, definitely! OTOH last time I added the toplevel libgcc-math directory and populated it with sources from glibc RMS objected violently and I had to remove it again. So we at least need to find a different source of math routines to start with (with proper licensing terms). I'd definitely like to see this library also to host vectorized routines and provide a myriad of entry-points. So - do you have an idea what routines we can start off with to get a full C99 set of routines for float, double and long double? The last time I was exploring the idea again I was looking at the BSD libm. Thanks, Richard. > -Geert
