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

[via llvm-commits]

weiguozhi wrote:

> Looking at this again, I'm not sure I fully understand what problem you're trying to solve here.
> 
> This code uses DTU so block deletion is deferred, so there are no problems with address reuse. Why is it problematic to merge into an unreachable block?

As you have noticed MergeBasicBlockIntoOnlyPred can mark an unreachable block for deletion. The deletion is deferred until 
DTU->flush() is called, then the unreachable BB is actually deleted, but the set Unreachable is not modified, so a deleted BB pointer is left in Unreachable. 

In later optimizations new basic blocks can be generated, if we are unlucky a new BB may be created at the deleted BB address, then the new BB pointer is automatically contained in Unreachable unexpectedly. And the new BB will be skipped by the following optimization. This part is a random behavior, so we got indeterministic result on our real code.


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