On 23 August 2017 at 13:23, Nicolai Hähnle <nhaeh...@gmail.com> wrote: > On 23.08.2017 13:07, Elie Tournier wrote: >> >> From: Elie Tournier <elie.tourn...@collabora.com> >> >> TL;DR >> This series is a "status update" of my work done for adding fp64 support >> on r600g. >> One of the biggest issue is due to a lake of accuracy on the rcp >> implementation. >> Divide relay on rcp. >> >> A branch is available on >> https://github.com/Hopetech/mesa/tree/glsl_arb_gpu_shader_fp64_v3 >> Comments and reviews are welcome. >> >> Patches 1-18: >> These few patches implement the basic fp64 operations. >> >> Patches 19-47: >> Lower operations using the builtin functions previously implemented. >> >> Known issues: >> - operations on matrix crash the system. >> - sqrt and d2f are not accurate enought so the piglit tests are failling. >> But sqrt and d2f are working correctly using softpipe. >> However, implementing sqrt64 as f2d(sqrt32(d2f()) seems to be good >> enought for Piglit. >> - rcp is define as pow(pow(x, -0.5), 2) >> NIR and NV convert the input in a fp32, realize a rcp, convert back to >> a fp64 and realize some Newton-Raphson step. >> This is not possible with GLSL IR because using fma will generate a >> massive builtin_float64.h file. > > > I don't understand this part. You need multiplication and addition anyway. > So if it's only fma which is the problem (why?), then why not just use > non-fused multiply-add? It may end up being slightly less accurate, but we > don't give any strong guarantees about rcp accuracy anyway, do we? > Pardon for dropping it like that. I'll try to explain things in a slightly different way.
Due to the fp64 <> fp32 conversion the accuracy of RCP is pretty bad. Thus a couple of Newton-Ralphson steps are used. Each one implemented via fma. There's no native fma thus we use normal multiply and add. As those get added to the generated file of built-ins (builtin_float64.h), it grows by ~20k LoC making compilation/linking quite slow. Noticeably bloating the final binary size as well (Elie has some crazy numbers from the very first experiments). -Emil _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/mesa-dev
