Views w/ shrink and flex about dahlia HOT 3 CLOSED

Alternative proposal: If shrink forces the step size to be the same as the width, the compilation looks like:

decl a: bit<10>[16 bank 8];
decl b: bit<10>[8];

for (let i = 0..8) {
  view v_a = shrink a[i:2]
  b[i] = v_a[0] + 2*v_a[1]
}

compiles to:

for (let i = 0..8) {
  b[i] = v_a[4*i + 0] + 2*v_a[4*i + 1];
}

I propose that we have the explicit syntax:

view v_a = shrink a[4 * i: 4];

where the programmer explicitly specifies 4 * i for the offset and 4 for the width. I think it would reduce mistakes like:

for (let i = 0..12) { // 12 will be OOB!
  view v_a = shrink a[i:2]
  b[i] = v_a[0] + 2*v_a[1]
}

Especially since we don't do any static bounds checking.

from dahlia.

1. I don't see any immediate problems with any of these three—they all seem like they should probably be fine? It could help to try drawing the "hardware interposer" for each to make sure they seem to have roughly predictable cost.

2. I think those restrictions make sense, yes.

3. Some rough ideas, based on previous discussions:

    let v = view a[0..+5];  // v is a view on a with offset 0 and width 5
    let a' = shrink a: int[10 bank 5];   // a' is like a, but with banking factor 5
    let a' = shrink a: int[10 bank 5][10 bank 5];   // for multi-dimensional arrays
   

   flex could work the same way. The idea behind the shrink syntax is that it might be nice, for clarity, to write out the new type of the "adapted" memory.

from dahlia.

Tracking in #124.

from dahlia.