Fractional byte packing?

Tagged: bits

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NXTangl
Participant
July 20, 2024 at 10:38 am
Post count: 21
#3989 |
Any thoughts on instructions to quickly load and store bit-packed representations? They are commonly used in graphics programming, such as here and here, and in general should help with memory-bandwidth-bound tasks…but isn’t very useful for general code, I’ll admit.
Findecanor
Participant
July 20, 2024 at 7:40 pm
Post count: 34
#3990
I’m not sure what you mean. Do you mean bitfield insert/extract instructions, or SIMD instructions for operating on multiple elements of a short vector at a time?
From previous info, I think The Mill is supposed to have both.
- NXTangl
  Participant
  July 22, 2024 at 2:25 pm
  Post count: 21
  #3993
  Like, if you were to pack five six-bit values and two boolean flags into a 32-bit word, is there a clean way to extract them without clogging the ALU pipelines with a bunch of intermediary shift-and-adds?
  - Findecanor
    Participant
    July 23, 2024 at 7:11 am
    Post count: 34
    #3995
    You could do that with one bitfield extract instructions per bitfield you’d want to extract: in this case six.
    Those would all be independent, so max parallelism would be the number of execution units that implement that instruction.
    If a bitfield is at the top or the bottom of the word, then a shiftr{s|u} or andl respectively could be done instead, possibly in another execution unit freeing up a bitfield extract unit.
    If you have such a layout with one at bits 31:26 and one at 0:0, and there is a CPU with two execution units that do bitfield extract and one unit that does shiftru/andl, then full extraction should be possible with only two cycles of latency.
    (Speaking very generically of course. Someone from Mill Computing could fill in the details)
    This reply was modified 3 months, 3 weeks ago by Findecanor.
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