[llvm] [JumpThreading] Remove deleted BB from Unreachable (PR #126984)

[via llvm-commits]

weiguozhi wrote:


> ```
>   if (Unreachable.count(SinglePred) && DTU->isBBPendingDeletion(SinglePred))
>     Unreachable.erase(SinglePred);
> ```
> 
> I have a few questions about this:
> 
> * Is this the only place where a live basic block appears to be "Unreachable"?

A live basic block in Unreachable is not a problem. A deleted basic block in Unreachable is a problem. Here the SinglePred has not been deleted, but it is marked so in DTU. It will be actually deleted when we call DTU->flush() later.

> * If `DTU->isBBPendingDeletion(SinglePred)` above is false, are we OK?  Does somebody else remove `SinglePred` from `Unrechable` for us?

`DTU->isBBPendingDeletion(SinglePred)` is false means SinglePred will not be deleted, so it is OK for us. Current implementation never delete any pointer from `Unrechable`.

> * Is it feasible to keep all basic blocks without erasing them?  I'm guessing that this might be difficult because we sometimes call external utility functions to manipulate basic blocks, and those utility functions might remove basic blocks.

Maybe @nikic's suggestion can accomplish this. I will try it.


https://github.com/llvm/llvm-project/pull/126984