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Async system calls; horizontal packing instruction
Author:  Date: 2016-12-14 06:13
Agner wrote:
Version 1.04 update today:

  • The instruction formats are made more consistent. Template E2 is modified.
  • The masking principle has been changed. Now there is an option to specify a fallback value to use when a mask bit is zero. The fallback value can be zero, or an input operand, or any other value specified by a register. Register r0 and v0 are allowed as masks to facilitate the use of boolean functions that have return values in r0 or v0.
  • The compare instruction has additional features with comparison of absolute values, and optional extra boolean operands.
  • Conditional jumps are modified.
  • Several other instructions are modified.


Hi Agner. I've been thinking about some interesting research I've read recently, and it raises a couple of questions for me:

1. Does ForwardCom assume synchronous system calls? Could it support a flexible async syscall architecture? I'm not sure where the boundary between an IAS and syscall architecture should lie, but it seems like it could lie wherever you want it to lie – you can choose to fold syscall specifications into an ISA, or to bracket it off as an OS concern.

I ask because this work on async sys calls is very compelling: https://www.usenix.org/legacy/events/osdi10/tech/full_papers/Soares.pdf

2. What do you think of the family of horizontal packing operations that the Parabix team pines for? parabix.costar.sfu.ca/wiki/ParabixTransform

(See their section headed "Ideal Implementation Using SIMD Packing" on the above-linked page.)

Would it be wise for ForwardCom to offer such instructions? Why or why not?


Finally, it would be nice to have a detailed comparison of ForwardCom and RISC-V, much of it in the form of comparison tables that get into specific features and decisions. I volunteer to help assemble such a comparison document if you're too busy.

 
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