Agner`s CPU blog

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Proposal for instruction set - now on Github
Author:  Date: 2016-07-04 01:49
Hi Agner – Can you recap how you resolved the objection that Hubert had to unified int/floating point registers? He had argued for the benefits of dedicated FP registers, but I can't find the resolution (the thread became quite long).

Other questions:

2. How do you situate your ISA in the context of the current trend toward heterogeneous computing, GPGPU, AMD's HSA, OpenCL, OpenMP, OpenACC, FPGAs, etc.? Will the new ISA help, hinder, or be neutral to heterogeneous programming?

3. This is an interesting time to develop a new ISA because of the oft-claimed end of Moore's Law scaling. How do the anticipated physics of the next 10 – 20 years of microprocessor design impact ForwardCom? Is the physical substrate irrelevant? (I'm thinking of III-V semiconductors, mostly) Or is ForwardCom shaped by present-day silicon and fab assumptions? It seems like some aspects of an ISA must be affected by the underlying physics and engineering constraints. For example, whether unified registers can be efficiently implemented, or whether orthogonal instructions can be efficiently implemented, and definitely how variable-length vector registers will perform.

4. Is ForwardCom extensible to a unified RAM (volatile) and non-volatile memory space? There are a few proposals out now, including Moneta D, that rethink the file system and make calling solid state storage a lot like calling a memory address (or just the same thing). They mostly hinge on faster-than-SSD media, like Intel and Micron's new 3D XPoint, phase-change memory, etc. but you could probably do it with fast SSDs.

I think it would be interesting if applications could set a few custom constants at start time. A while back I asked about having useful constants hardwired in the CPU (e.g. π), and you guys explained why it probably wouldn't be worth it. I think letting each application tell the CPU to save a few constants that the application will often use might be more useful. The point is to be able to invoke the constant without an immediate, but just with a much smaller shortcode (two or three bits). It's kind of like an application-specific cache (an idea which Intel has implemented a bit), but much, much smaller. The programmer might not even need to know or designate the constants – it could be empirically determined by the compiler. This would only help certain kinds of applications, but that's true of many things – it would be a way to reduce code size but more importantly to have a sticky, stable cache immune from misses and eviction. An expanded version of this would be to be able to define and save specific, short code sequences. (I see your Chapter 6, but it seems like a placeholder for now.)

 
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