[Dominators] Assert if there is modification to DelBB while it is awaiting deletion

Summary:
Previously, passes use
```
DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
DTU.deleteBB(DelBB);
```
to delete a BasicBlock.
But passes which don't have the ability to update DomTree (e.g. tailcallelim, simplifyCFG) cannot recognize a DelBB awaiting deletion and will continue to process this DelBB.
This is a simple approach to notify devs of passes which may use DTU in the future to deal with deleted BasicBlocks under Lazy Strategy correctly.

Reviewers: kuhar, brzycki, dmgreen

Reviewed By: kuhar

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D49731

llvm-svn: 337891

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/IR/DomTreeUpdater.cpp b/llvm/lib/IR/DomTreeUpdater.cpp
index 9af22b16224..f035a86edda 100644
--- a/llvm/lib/IR/DomTreeUpdater.cpp
+++ b/llvm/lib/IR/DomTreeUpdater.cpp
@@ -136,6 +136,13 @@ bool DomTreeUpdater::forceFlushDeletedBB() {
     return false;
 
   for (auto *BB : DeletedBBs) {
+    // After calling deleteBB or callbackDeleteBB under Lazy UpdateStrategy,
+    // validateDeleteBB() removes all instructions of DelBB and adds an
+    // UnreachableInst as its terminator. So we check whether the BasicBlock to
+    // delete only has an UnreachableInst inside.
+    assert(BB->getInstList().size() == 1 &&
+           isa<UnreachableInst>(BB->getTerminator()) &&
+           "DelBB has been modified while awaiting deletion.");
     BB->removeFromParent();
     eraseDelBBNode(BB);
     delete BB;