[mlir][acc] Introduce acc loop tiling pass (#171692)
This pass implements the OpenACC loop tiling transformation for acc.loop
operations that have the tile clause (OpenACC 3.4 spec, section 2.9.8).
The tile clause specifies that the iterations of the associated loops
should be divided into tiles (rectangular blocks). The pass transforms a
single or nested acc.loop with tile clauses into a structure of "tile
loops" (iterating over tiles) containing "element loops" (iterating
within tiles).
For example, tiling a 2-level nested loop with tile(T1, T2):
```
// Before tiling:
acc.loop tile(T1, T2) control(%i, %j) = ...
// After tiling:
acc.loop control(%i) step (s1*T1) { // tile loop 1
acc.loop control(%j) step (s2*T2) { // tile loop 2
acc.loop control(%ii) = (%i) to (min(ub1, %i+s1*T1)) {
acc.loop control(%jj) = (%j) to (min(ub2, %j+s2*T2)) {
// loop body using %ii, %jj
}
}
}
}
```
Key features:
- Handles constant tile sizes and wildcard tile sizes ('*') which use a
configurable default tile size
- Properly handles collapsed loops with tile counts exceeding collapse
count by uncollapsing loops before tiling
- Distributes gang/worker/vector attributes appropriately: gang -> tile
loops, vector -> element loops
- Validates that tile size types are not wider than loop IV types
- Emits optimization remarks for tiling decisions
Three test files are added:
- acc-loop-tiling.mlir: Tests single and nested loop tiling with
constant tile sizes, unknown tile sizes (*), and loops with collapse
attributes
- acc-loop-tiling-invalid.mlir: Tests error diagnostic when tile size
type is wider than the loop IV type
- acc-loop-tiling-remarks.mlir: Tests optimization remarks emitted for
tiling decisions including default tile size selection
Co-authored-by: Vijay Kandiah <vkandiah@nvidia.com>
