[llvm] [SPIR-V] Fix block sorting with irreducible CFG (PR #116996)

[Steven Perron via llvm-commits]

https://github.com/s-perron commented:

I'm not sure if this works for irreducible regions because I am not sure what we are trying to accomplish.

This seems to be the strategy: if no node in `toVisit` has all of it predecessor ranked yet, then essentially randomly pick a node in `toVisit` as the next node.

Theoretically we could have:

A  -> B
A  -> C
A  -> D
B  -> C
B  -> D
C  -> B

In this case, you will get stuck with `toVisit = { B, C, D }`. Do we care if we choose `D` next, even if it is not part of the irreducible region? This would mean that `D` will have the same rank as `B` and `C`, and will have the earlier traversal number for the tie-breaker.

I tryed to find the restrictions on block order in spir-v. All I found was:

> The order of blocks in a function must satisfy the rule that blocks appear before all blocks they dominate.

If this is the only restriction, then we can do a much simpler algorithm: any pre-order traversal of the dominator tree.

I thought there was an restriction where a constructs has to be properly nested in each other for structured control flow. So we could not have H1, H2, M1, M2 where H<i> is a header with M<i> as its merge block. However, I cannot find that.



https://github.com/llvm/llvm-project/pull/116996