[Mlir-commits] [flang] [mlir] [Flang][OpenMP] Don't generate code for unreachable target regions. (PR #178937)

[Pranav Bhandarkar]

https://github.com/bhandarkar-pranav commented:

Thank you for the PR, @abidh. This is a good first pass, but I have some thoughts
1. Any reason, you haven't styled this as a forward pass from the entry block to compute reachabililty and then check the omp.target for membership of the set of reachable block/ops? The reason, I say this is that  in `IsUnreachableBlock` we do not identify transitively dead (although perhaps rare) omp.target ops. Consider a CFG like 
```
bb0 
     cf.cond_br false, bb1, bb2
bb1
    cf.br bb3
bb2
    cf.br bb4
bb3:
     omp.target    // Since `IsUnreachableBlock` only checks predecessors of the block of  
                          // omp.target, and bb1 doesn't terminate with cf.cond_br this function
                           // would miss this case.
bb4:
    return
```
2. MLIR has `DominanceInfo::isReachableFromEntry()`. When combined with `SCCPPass` you could get what you want for free
```
pm.addPass(createSCCPPass());           // Fold constant branches
pm.addPass(createCanonicalizerPass());  // Clean up
pm.addPass(flangomp::createMarkUnreachableTargetsPass());  // now use DominanceInfo::isReachableFromEntry
```

https://github.com/llvm/llvm-project/pull/178937