3 ms·

I once thought "bit shuffling" was a special-purpose use-case you'd only ever need to deinterleave RGBA channels or something. Later I wanted to implement a sma

I once thought "bit shuffling" was a special-purpose use-case you'd only ever need to deinterleave RGBA channels or something. Later I wanted to implement a small look-up table (surely something more common) and realized that it's just a different name for the same operation. (I think "bit shuffling" instructions have evolved to be generic enough that they're just programmable LUTs now.)

no92 3y ago
I would have never thought to phrase bit-shuffling as deinterleaving, which makes so much more sense to my not-professionally-computer-related eyes and mind. Realistically though, how likely would a GCC/clang be to emit these instructions when I'm working on some lookup tables, assuming I permit it to use them (e.g. via `-march=native` on a machine that supports the extension)? My gut feeling would be that unless I specifically make sure to structure my code to be as close to the semantics of the instructions as possible, these instructions would never ever be emitted. Or has the world of compiler optimizers advanced enough that rewriting that is commonplace now?
galangalalgol 3y ago
Good for software defined radio too because real and imaginary are interleaved.
- tomcam 3y ago
  > because real and imaginary are interleaved. And so it is in life
  - galangalalgol 3y ago
    Yeah, I often talk about the "phase" of a schedule being close to 90. Managers are thinking % but I'm thinking degrees.
    - tomcam 3y ago
      Took me a while to comprehend that. NOYCE!
less_less 3y ago
Yeah, those vector permute instructions are super useful for both patterns. There are dedicated instructions for some specific permutations (shifting over by a constant number of bytes, and some interleavings) but you can easily end up needing the general case. And of course parallel LUTs are also very useful. Depending on what you're doing you could easily end up with both in the same algorithm.

no92 3y ago

I would have never thought to phrase bit-shuffling as deinterleaving, which makes so much more sense to my not-professionally-computer-related eyes and mind. Realistically though, how likely would a GCC/clang be to emit these instructions when I'm working on some lookup tables, assuming I permit it to use them (e.g. via `-march=native` on a machine that supports the extension)? My gut feeling would be that unless I specifically make sure to structure my code to be as close to the semantics of the instructions as possible, these instructions would never ever be emitted. Or has the world of compiler optimizers advanced enough that rewriting that is commonplace now?

galangalalgol 3y ago

Good for software defined radio too because real and imaginary are interleaved.

tomcam 3y ago
> because real and imaginary are interleaved. And so it is in life
- galangalalgol 3y ago
  Yeah, I often talk about the "phase" of a schedule being close to 90. Managers are thinking % but I'm thinking degrees.
  - tomcam 3y ago
    Took me a while to comprehend that. NOYCE!

less_less 3y ago

Yeah, those vector permute instructions are super useful for both patterns. There are dedicated instructions for some specific permutations (shifting over by a constant number of bytes, and some interleavings) but you can easily end up needing the general case. And of course parallel LUTs are also very useful. Depending on what you're doing you could easily end up with both in the same algorithm.