[Mlir-commits] [mlir] [mlir][OpenMP] Don't use label prefixes on linear variable rewrite (PR #200900)

[Tom Eccles]

https://github.com/tblah commented:

Thanks for the follow-up on this!

Codex gave me the following reproducer:
```
  llvm.func @wsloop_linear_post_use(%lb : i32, %ub : i32, %step : i32,
                                    %x : !llvm.ptr, %out : !llvm.ptr) {
    omp.wsloop linear(%x : !llvm.ptr = %step : i32) {
      omp.loop_nest (%iv) : i32 = (%lb) to (%ub) step (%step) {
        %cur = llvm.load %x : !llvm.ptr -> i32
        llvm.store %cur, %out : i32, !llvm.ptr
        omp.yield
      }
    } {linear_var_types = [i32]}
    %after = llvm.load %x : !llvm.ptr -> i32
    llvm.store %after, %out : i32, !llvm.ptr
    llvm.return
  }
```

The new traversal collects every successor reachable from startBB until it pops endBB, so it also collects blocks outside the actual loop-body region. (I think) This can be seen for translation of this reproducer.

I think one solution would be to use the `llvm::CanonicalLoopInfo` for the start and end blocks: as I understand it, `loopInfo->getBody()` gives the single entry block for the loop body, and `loopInfo->getLatch()` is the block reached after the body. All arbitrary control flow inside of the loop body should always eventually branch to the latch so those should work as safer start and end blocks for the walk.

Please also add a regression test for this change.

https://github.com/llvm/llvm-project/pull/200900