[Mlir-commits] [mlir] [mlir][async] Clamp per-axis trip counts in async-parallel-for (PR #225370)

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@llvm/pr-subscribers-mlir

Author: Shoryamishra61

<details>
<summary>Changes</summary>

### Problem

`AsyncParallelFor` computes each dimension's trip count using signed ceiling division and directly multiplies those values to form the flattened iteration count.

For an empty dimension with a positive step, the raw result can be negative. If multiple dimensions are empty, multiplying an even number of negative trip counts can produce a positive flattened count, causing work to be dispatched for an `scf.parallel` operation whose semantic iteration space is empty.

### Fix

Normalize each per-dimension trip count to a non-negative iteration cardinality before computing the flattened trip count:
```cpp
Value rawTripCount = b.createOrFold<arith::CeilDivSIOp>(range, step);
tripCounts[i] = b.createOrFold<arith::MaxSIOp>(rawTripCount, c0);
```

This ensures any empty dimension contributes zero to the Cartesian-product iteration space while preserving positive trip counts for non-empty dimensions.

### Tests

Add regression coverage to `mlir/test/Dialect/Async/async-parallel-for-async-dispatch.mlir`:
- `@<!-- -->empty_2d_dynamic`: Multidimensional empty iteration domain with dynamic bounds ensuring each dimension clamps to zero and takes the no-op branch.
- `@<!-- -->empty_2d_static`: Constant bounds verifying compile-time folding of empty dimensions to a no-op branch.

Fixes #<!-- -->225254.


---
Full diff: https://github.com/llvm/llvm-project/pull/225370.diff


2 Files Affected:

- (modified) mlir/lib/Dialect/Async/Transforms/AsyncParallelFor.cpp (+7-3) 
- (modified) mlir/test/Dialect/Async/async-parallel-for-async-dispatch.mlir (+40) 


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</details>


https://github.com/llvm/llvm-project/pull/225370