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

lkcl luke.leighton at gmail.com
Thu Aug 8 08:09:50 BST 2019

On Thursday, August 8, 2019 at 2:28:43 PM UTC+8, Luis Vitorio Cargnini(OURS/RiVAI) wrote:

> > * Embedded Platform (where it's entirely up to the implementor, as there will be no interaction with public APIs)
> No, IEEE, ARM is an embedded platform and they implement IEEE in all of them

I can see the sense in that one. I just thought that some 3D implementors, particularly say in specialist markets, would want the choice.

Hmmmm, perhaps a 3D Embedded spec, separate from "just" Embedded.

> > * UNIX Platform (which would require strict IEEE754 accuracy, for use in GNU libm, OR repeatable numericalist-acceptable accuracy)
> Standard IEEE, simple no 'new' on this sector.

Makes sense. Cannot risk noninteroperability, even if it means a higher gate count or larger latency.

> > * a *NEW* 3D Platform, where accuracy is defined by strict conformance to a high-profile standard e.g. OpenCL / Vulkan.
> No, simply use IEEE that it is all, and based on the IEEE standard you can measure the deviation in your system.

Ok, this is where that's not going to fly. As Mitch mentioned, full IEEE754 compliance would result in doubling the gate count and/or decreasing latency through longer pipelines.

In speaking with Jeff Bush from Nyuzi I learned that a GPU is insanely dominated by its FP ALU gate count: well over 50% of the entire chip.

If you double the gate count due to imposition of unnecessary accuracy (unnecessary because due to 3D Standards compliance all the shader algorithms are *designed* to lower accuracy requirements), the proposal will be flat-out rejected by adopters because products based around it will come with a whopping 100% power-performance penalty compared to industry standard alternatives.

So this is why I floated (ha ha) the idea of a new Platform Spec, to give implementors the space to meet industry-driven requirements and remain competitive.

Ironically our implementation will need to meet UNIX requirements, it is one of the quirks / penalties of a hybrid design.


More information about the libre-riscv-dev mailing list