[llvm] [WebAssembly] Reapply "[WebAssembly] Incorporate SCCs into WebAssemblyFixIrreducibleControlFlow" (#181755) (PR #184441)

[Heejin Ahn via llvm-commits]

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aheejin wrote:

> Then the assertions we have in place will get tripped (as they won't be assigned an SCC because they aren't connected to the function entry), and we can deal with that when it comes.

If we know there can be a single unreachable BB, why not two of them linked together? I think we should at least try to deal with all imaginable valid inputs, not just waiting for it to come up.

> It is generally expected that DCE clean up any MBBs not accessible from the entry before we get here.

That's what I thought too. Does this happen only with `llc -O0`? 

By the way, why did the previous code (without `SCCIterator`) handle this code and this doesn't?

https://github.com/llvm/llvm-project/pull/184441