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

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 avoids merging a BB into 
unreachable predecessor.

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);
@@ -1895,6 +1894,11 @@ bool JumpThreadingPass::maybeMergeBasicBlockIntoOnlyPred(BasicBlock *BB) {
       SinglePred == BB || hasAddressTakenAndUsed(BB))
     return false;
 
+  // MergeBasicBlockIntoOnlyPred may delete SinglePred, we need to avoid
+  // deleting a BB pointer from Unreachable.
+  if (Unreachable.count(SinglePred))
+    return false;
+
   // If SinglePred was a loop header, BB becomes one.
   if (LoopHeaders.erase(SinglePred))
     LoopHeaders.insert(BB);

llvm/test/Transforms/JumpThreading/pr62908.ll

@@ -5,7 +5,18 @@
 
 define i32 @test() {
 ; CHECK-LABEL: define i32 @test() {
-; CHECK-NEXT:  end:
+; CHECK-NEXT:  join.thread:
+; CHECK-NEXT:    br label [[END:%.*]]
+; CHECK:       unreachable:
+; CHECK-NEXT:    [[SH_PROM:%.*]] = zext i32 -1 to i64
+; CHECK-NEXT:    [[SHL:%.*]] = shl nsw i64 -1, [[SH_PROM]]
+; CHECK-NEXT:    [[CONV:%.*]] = trunc i64 [[SHL]] to i32
+; CHECK-NEXT:    br label [[JOIN:%.*]]
+; CHECK:       join:
+; CHECK-NEXT:    [[PHI:%.*]] = phi i32 [ [[CONV]], [[UNREACHABLE:%.*]] ]
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[PHI]], 0
+; CHECK-NEXT:    br i1 [[CMP]], label [[END]], label [[END]]
+; CHECK:       end:
 ; CHECK-NEXT:    ret i32 0
 ;
 entry: