On Fri, Aug 7, 2020 at 9:00 AM Dennis Clarke via Gcc-help <gcc-help@xxxxxxxxxxx> wrote: > There is no such thng as an 80 byte double. The standards state that we > shall use floating point types which are of specific bit widths and all > of those are perfectly aligned on perfect power of two bit widths. These > range from 16 bits to 256 bit width. To date I have not seen hardware > which natively supports the 256-bit floating point type. The IBM POWER9 > processor as well as the RISC-V and Fujitsu SPARC provide hardware for > the 128-bit width and they seem to work very well indeed. The 80-bit > thing to which you refer is a hack put in place by the intel/x86 world > simply because the hardware can not properly handle anything larger. It > should not exist at all in if we use the floating point standards. The IEEE FP standard defines some basic formats which have fixed bit-widths, and extended formats which have implementation defined bit widths. The 80-bit x86 format meets the definition of extended binary64 in the 2008 standard, and extended double in the 1985 standard. Likewise the similar 96 bit (with 16-bits of zero padding) format used on 68k systems. Nowadays though, I think most people would rather have a proper 128-bit long double than an 80/96-bit extended double. But there is nothing hackish about the 80/96 bit extended doubles on x86/68k. They have just outlived their usefulness. Jim
