[libre-riscv-dev] [isa-dev] Re: FP transcendentals (trigonometry, root/exp/log) proposal

lkcl luke.leighton at gmail.com
Mon Sep 9 12:01:23 BST 2019

i wrote it up 
formally, https://libre-riscv.org/ztrans_proposal/#requirements

comments and additions welcome (it is a publicly accessible wiki)

This proposal is designed to meet a wide range of extremely diverse needs, 
allowing implementors from all of them to benefit from the tools and 
hardware cost reductions associated with common standards adoption.

*There are four different, disparate platform's needs (two new)*:

   - 3D Embedded Platform 
   - Embedded Platform 
   - 3D UNIX Platform 
   - UNIX Platform 

3D Embedded will require significantly less accuracy and will need to make 
power budget and die area compromises that other platforms (including 
Embedded) will not need to make.

3D UNIX Platform has to be performance-price-competitive: subtly-reduced 
accuracy in FP32 is acceptable where, conversely, in the UNIX Platform, 
IEEE754 compliance is a hard requirement that would compromise power and 
efficiency on a 3D UNIX Platform.

*The use-cases are*:

   - 3D GPUs 
   - Numerical Computation 
   - (Potentially) A.I. / Machine-learning (1) 

(1) although approximations suffice in this field, making it more likely to 
use a custom extension. High-end ML would inherently definitely be excluded.

*The power and die-area requirements vary from*:

   - Ultra-low-power (smartwatches where GPU power budgets are in 
   - Mobile-Embedded (good performance with high efficiency for battery 
   - Desktop Computing 
   - Server / HPC (2) 

(2) Supercomputing is left out of the requirements as it is traditionally 
covered by Supercomputer Vectorisation Standards (such as RVV).

*The software requirements are*:

   - Full public integration into GNU math libraries (libm) 
   - Full public integration into well-known Numerical Computation systems 
   - Full public integration into upstream GNU and LLVM Compiler toolchains 
   - Full public integration into Khronos OpenCL SPIR-V compatible 
   Compilers seeking public Certification and Endorsement from the Khronos 
   Group under their Trademarked Certification Programme. 

*The "contra"-requirements are*:

   - The requirements are *not* for the purposes of developing a full 
   custom proprietary GPU with proprietary firmware. 
   - A full custom proprietary GPU ASIC Manufacturer *may* benefit from 
   this proposal however the fact that they typically develop proprietary 
   software that is not shared with the rest of the community likely to use 
   this proposal means that they have completely different needs. 

This proposal is for *sharing* of effort in reducing development costs, and 
as such needs to be entirely public, transparent, and fully integrated into 
public upstream libre-licensed libraries and toolchains.

*Overall this proposal is categorically and wholly unsuited to relegation 
of "custom" status*.


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