[JumpThreading] Remove deleted BB from Unreachable

Although an unreachable BB is skipped by processBlock, its successor can
still be handled by processBlock, and maybeMergeBasicBlockIntoOnlyPred
may merge the two BBs and delete the unreachable BB. Then the garbage
pointer is left in Unreachable set. This patch removes the invalid BB
pointer from the Unreachable set.

Author: weiguozhi

llvm/include/llvm/Transforms/Scalar/JumpThreading.h

@@ -94,6 +94,10 @@ class JumpThreadingPass : public PassInfoMixin<JumpThreadingPass> {
   SmallPtrSet<const BasicBlock *, 16> LoopHeaders;
 #endif
 
+  // JumpThreading must not processes blocks unreachable from entry. It's a
+  // waste of compute time and can potentially lead to hangs.
+  SmallPtrSet<BasicBlock *, 16> Unreachable;
+
   unsigned BBDupThreshold;
   unsigned DefaultBBDupThreshold;
 

llvm/lib/Transforms/Scalar/JumpThreading.cpp

@@ -307,12 +307,11 @@ bool JumpThreadingPass::runImpl(Function &F_, FunctionAnalysisManager *FAM_,
   else
     BBDupThreshold = DefaultBBDupThreshold;
 
-  // JumpThreading must not processes blocks unreachable from entry. It's a
-  // waste of compute time and can potentially lead to hangs.
-  SmallPtrSet<BasicBlock *, 16> Unreachable;
   assert(DTU && "DTU isn't passed into JumpThreading before using it.");
   assert(DTU->hasDomTree() && "JumpThreading relies on DomTree to proceed.");
   DominatorTree &DT = DTU->getDomTree();
+
+  Unreachable.clear();
   for (auto &BB : *F)
     if (!DT.isReachableFromEntry(&BB))
       Unreachable.insert(&BB);
@@ -1902,6 +1901,9 @@ bool JumpThreadingPass::maybeMergeBasicBlockIntoOnlyPred(BasicBlock *BB) {
   LVI->eraseBlock(SinglePred);
   MergeBasicBlockIntoOnlyPred(BB, DTU.get());
 
+  if (Unreachable.count(SinglePred) && DTU->isBBPendingDeletion(SinglePred))
+    Unreachable.erase(SinglePred);
+
   // Now that BB is merged into SinglePred (i.e. SinglePred code followed by
   // BB code within one basic block `BB`), we need to invalidate the LVI
   // information associated with BB, because the LVI information need not be