Index: lib/Transforms/Instrumentation/CFGMST.h
===================================================================
--- lib/Transforms/Instrumentation/CFGMST.h
+++ lib/Transforms/Instrumentation/CFGMST.h
@@ -78,6 +78,14 @@
     return *It->second.get();
   }
 
+  // Give BB, return the auxiliary information if it's available.
+  BBInfo *findBBInfo(const BasicBlock *BB) const {
+    auto It = BBInfos.find(BB);
+    if (It == BBInfos.end())
+      return nullptr;
+    return It->second.get();
+  }
+
   // Traverse the CFG using a stack. Find all the edges and assign the weight.
   // Edges with large weight will be put into MST first so they are less likely
   // to be instrumented.
Index: lib/Transforms/Instrumentation/PGOInstrumentation.cpp
===================================================================
--- lib/Transforms/Instrumentation/PGOInstrumentation.cpp
+++ lib/Transforms/Instrumentation/PGOInstrumentation.cpp
@@ -327,6 +327,9 @@
   // Return the auxiliary BB information.
   BBInfo &getBBInfo(const BasicBlock *BB) const { return MST.getBBInfo(BB); }
 
+  // Return the auxiliary BB information if available.
+  BBInfo *findBBInfo(const BasicBlock *BB) const { return MST.findBBInfo(BB); }
+
   // Dump edges and BB information.
   void dumpInfo(std::string Str = "") const {
     MST.dumpEdges(dbgs(), Twine("Dump Function ") + FuncName + " Hash: " +
@@ -386,7 +389,10 @@
     const TerminatorInst *TI = BB.getTerminator();
     for (unsigned I = 0, E = TI->getNumSuccessors(); I != E; ++I) {
       BasicBlock *Succ = TI->getSuccessor(I);
-      uint32_t Index = getBBInfo(Succ).Index;
+      auto BI = findBBInfo(Succ);
+      if (BI == nullptr)
+        continue;
+      uint32_t Index = BI->Index;
       for (int J = 0; J < 4; J++)
         Indexes.push_back((char)(Index >> (J * 8)));
     }
@@ -672,6 +678,11 @@
     return FuncInfo.getBBInfo(BB);
   }
 
+  // Return the auxiliary BB information if available.
+  UseBBInfo *findBBInfo(const BasicBlock *BB) const {
+    return FuncInfo.findBBInfo(BB);
+  }
+
 private:
   Function &F;
   Module *M;
@@ -857,27 +868,29 @@
     // For efficient traversal, it's better to start from the end as most
     // of the instrumented edges are at the end.
     for (auto &BB : reverse(F)) {
-      UseBBInfo &Count = getBBInfo(&BB);
-      if (!Count.CountValid) {
-        if (Count.UnknownCountOutEdge == 0) {
-          Count.CountValue = sumEdgeCount(Count.OutEdges);
-          Count.CountValid = true;
+      UseBBInfo *Count = findBBInfo(&BB);
+      if (Count == nullptr)
+        continue;
+      if (!Count->CountValid) {
+        if (Count->UnknownCountOutEdge == 0) {
+          Count->CountValue = sumEdgeCount(Count->OutEdges);
+          Count->CountValid = true;
           Changes = true;
-        } else if (Count.UnknownCountInEdge == 0) {
-          Count.CountValue = sumEdgeCount(Count.InEdges);
-          Count.CountValid = true;
+        } else if (Count->UnknownCountInEdge == 0) {
+          Count->CountValue = sumEdgeCount(Count->InEdges);
+          Count->CountValid = true;
           Changes = true;
         }
       }
-      if (Count.CountValid) {
-        if (Count.UnknownCountOutEdge == 1) {
-          uint64_t Total = Count.CountValue - sumEdgeCount(Count.OutEdges);
-          setEdgeCount(Count.OutEdges, Total);
+      if (Count->CountValid) {
+        if (Count->UnknownCountOutEdge == 1) {
+          uint64_t Total = Count->CountValue - sumEdgeCount(Count->OutEdges);
+          setEdgeCount(Count->OutEdges, Total);
           Changes = true;
         }
-        if (Count.UnknownCountInEdge == 1) {
-          uint64_t Total = Count.CountValue - sumEdgeCount(Count.InEdges);
-          setEdgeCount(Count.InEdges, Total);
+        if (Count->UnknownCountInEdge == 1) {
+          uint64_t Total = Count->CountValue - sumEdgeCount(Count->InEdges);
+          setEdgeCount(Count->InEdges, Total);
           Changes = true;
         }
       }
@@ -887,14 +900,22 @@
   DEBUG(dbgs() << "Populate counts in " << NumPasses << " passes.\n");
 #ifndef NDEBUG
   // Assert every BB has a valid counter.
-  for (auto &BB : F)
-    assert(getBBInfo(&BB).CountValid && "BB count is not valid");
+  for (auto &BB : F) {
+    auto BI = findBBInfo(&BB);
+    if (BI == nullptr)
+      continue;
+    assert(BI->CountValid && "BB count is not valid");
+  }
 #endif
   uint64_t FuncEntryCount = getBBInfo(&*F.begin()).CountValue;
   F.setEntryCount(FuncEntryCount);
   uint64_t FuncMaxCount = FuncEntryCount;
-  for (auto &BB : F)
-    FuncMaxCount = std::max(FuncMaxCount, getBBInfo(&BB).CountValue);
+  for (auto &BB : F) {
+    auto BI = findBBInfo(&BB);
+    if (BI == nullptr)
+      continue;
+    FuncMaxCount = std::max(FuncMaxCount, BI->CountValue);
+  }
   markFunctionAttributes(FuncEntryCount, FuncMaxCount);
 
   // Now annotate select instructions
@@ -974,7 +995,10 @@
   uint64_t SCounts[2];
   SCounts[0] = CountFromProfile[*CurCtrIdx]; // True count
   ++(*CurCtrIdx);
-  uint64_t TotalCount = UseFunc->getBBInfo(SI.getParent()).CountValue;
+  uint64_t TotalCount = 0;
+  auto BI = UseFunc->findBBInfo(SI.getParent());
+  if (BI != nullptr)
+    TotalCount = BI->CountValue;
   // False Count
   SCounts[1] = (TotalCount > SCounts[0] ? TotalCount - SCounts[0] : 0);
   uint64_t MaxCount = std::max(SCounts[0], SCounts[1]);
Index: test/Transforms/PGOProfile/Inputs/unreachable_bb.proftext
===================================================================
--- /dev/null
+++ test/Transforms/PGOProfile/Inputs/unreachable_bb.proftext
@@ -0,0 +1,9 @@
+# IR level Instrumentation Flag
+:ir
+foo
+# Func Hash:
+12884901887
+# Num Counters:
+1
+# Counter Values:
+0
Index: test/Transforms/PGOProfile/unreachable_bb.ll
===================================================================
--- /dev/null
+++ test/Transforms/PGOProfile/unreachable_bb.ll
@@ -0,0 +1,23 @@
+; RUN: llvm-profdata merge %S/Inputs/unreachable_bb.proftext -o %t.profdata
+; RUN: opt < %s -pgo-instr-use -pgo-test-profile-file=%t.profdata -S | FileCheck %s --check-prefix=USE
+; RUN: opt < %s -passes=pgo-instr-use -pgo-test-profile-file=%t.profdata -S | FileCheck %s --check-prefix=USE
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @foo() {
+entry:
+  call void @bar()
+  unreachable
+return:
+  ret void
+}
+
+declare void @bar()
+
+;USE: !0 = !{i32 1, !"ProfileSummary", !1}
+;USE: !1 = !{!2, !3, !4, !5, !6, !7, !8, !9}
+;USE: !2 = !{!"ProfileFormat", !"InstrProf"}
+;USE: !3 = !{!"TotalCount", i64 0}
+
+