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

lkcl luke.leighton at gmail.com
Thu Aug 8 09:36:49 BST 2019

[mostly OT for the thread, but still relevant]

On Thursday, August 8, 2019 at 3:33:25 PM UTC+8, waterman wrote:

> It's not obvious that the proposed opcodes justify standardization.

It's outside of your area of expertise, Andrew. Just as for Luis, all the "metrics" that you use will be screaming "this is wrong, this is wrong!"

Both Jacob and I have Asperger's. In my case, I think in such different conceptual ways that I use language that bit differently, such that it needs "interpretation". Rogier demonstrated that really well a few months back, by "interpreting" something on one of the ISAMUX/ISANS threads.

Think of what I write as being a bit like the old coal mine "canaries". You hear "tweet tweet croak", and you don't understand what the bird said before it became daisy-food but you know to run like hell.

There are several aspects to this proposal. It covers multiple areas - multiple platforms, with different (conflicting) implementation requirements. 

It should be obvious that this is not going to fit the "custom" RISCV paradigm, as that's reserved for *private* (hard fork) toolchains and scenarios.

It should also be obvious that, as a public high profile open platform, the pressure on the compiler upstream developers could result in the Altivec SSE nightmare.

The RISCV Foundation has to understand that it is in everybody's best interests to think ahead, strategically on this one, despite it being well outside the core experience of the Founders.

Note, again, worth repeating: it is *NOT* intended or designed for EXCLUSIVE use by the Libre RISCV SoC. It is actually inspired by Pixilar's SIGGRAPH slides, where at the Bof there were over 50 attendees. The diversity of requirements of the attendees was incredible, and they're *really* clear about what they want.

Discussing this proposal as being a single platform is counterproductive and misses the point. It covers *MULTIPLE* platforms.

If you try to undermine or dismiss one area, it does *not* invalidate the other platforms's requirements and needs.

Btw some context, as it may be really confusing as to why we are putting forward a *scalar* proposal when working on a *vector* processor.

SV extends scalar operations. By proposing a mixed multi platform Trans / Trigonometric *scalar* proposal (suitable for multiple platforms other than our own), the Libre RISCV hybrid processor automatically gets vectorised [multi issue] versions of those "scalar" opcodes, "for free".

For a 3D GPU we still have yet to add Texture opcodes, Pixel conversion, Matrices, Z Buffer, Tile Buffer, and many more opcodes.  My feeling is that RVV's major opcode brownfield space simply will not cope with all of those, and going to 48 bit and 64 bit is just not an option, particularly for embedded low power scenarios, due to the increased I-cache power penalty.

I am happy for *someone else* to do the work necessary to demonstrate otherwise on that one: we have enough to do, already, if we are to keep within budget and on track).


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