On 08.05.2017 17:36, Aurelien Jarno wrote: > Hi, > > On 2017-05-08 15:58, Alex Bennée wrote: >> Hi, >> >> We've got a task coming up to implement half-precision floating point >> (FP16) for ARMv8.2. As you know pretty much all our floating point in >> QEMU is handled by our internal fork of John R. Hauser's BSD SoftFloat >> library. Our current implementation is based on version 2a which doesn't >> support FP16. >> >> As it happens there has been a new release of SoftFloat recently. >> Version 3 is a complete re-write which made a number of changes, some >> notable ones being: >> >> - Complete rewrite, different use license than earlier releases. >> - Renaming most types and functions, upgrading some algorithms >> - restructuring the source files, and making SoftFloat into a true >> library. >> - Added functions to convert between floating-point and unsigned integers, >> both 32-bit and 64-bit (uint32_t and uint64_t). >> - Added functions for fused multiply-add, for all supported floating-point >> formats except 80-bit double-extended-precision. >> - Added support for a fifth rounding mode, near_maxMag (round to nearest, >> with ties to maximum magnitude, away from zero). >> >> And in the most recent release as of February 2017, 3c: >> >> - Added optional rounding mode odd (round to odd, also known as jamming). >> - Implemented the common 16-bit “half-precision” floating-point format >> (float16_t) >> >> See: http://www.jhauser.us/arithmetic/SoftFloat-3c/doc/SoftFloat-history.html >> >> Of course the softfloat in QEMU's tree hasn't been static either. We've >> made numerous changes over the years to add and fix various features, >> including features that have since been added to the upstream softfloat. >> It seems unlikely we could switch to the newer softfloat without risking >> breaking something. However if we look at back-porting stuff from the >> newer library we essentially get to own our version of softfloat >> forever. > > There have been many many changes in our forked version of softfloat: > qNaN/sNaN, IEEE754-2008 functions, squash input denormal, many floatx80 > fixes, ...
Note that we've apparently also got plenty of bugs in our version of softloat left, for example: - https://bugs.launchpad.net/qemu/+bug/645662 - http://lists.nongnu.org/archive/html/qemu-ppc/2017-05/msg00187.html ... would be interesting to know whether such issues are fixed with the newer version of softfloat... Thomas
