diff --git a/llvm/include/llvm/Analysis/BlockFrequencyInfoImpl.h b/llvm/include/llvm/Analysis/BlockFrequencyInfoImpl.h
--- a/llvm/include/llvm/Analysis/BlockFrequencyInfoImpl.h
+++ b/llvm/include/llvm/Analysis/BlockFrequencyInfoImpl.h
@@ -26,6 +26,7 @@
 #include "llvm/IR/BasicBlock.h"
 #include "llvm/Support/BlockFrequency.h"
 #include "llvm/Support/BranchProbability.h"
+#include "llvm/Support/CommandLine.h"
 #include "llvm/Support/DOTGraphTraits.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/ErrorHandling.h"
@@ -46,6 +47,8 @@
 
 #define DEBUG_TYPE "block-freq"
 
+extern llvm::cl::opt<bool> CheckBFIUnknownBlockQueries;
+
 namespace llvm {
 
 class BranchProbabilityInfo;
@@ -1043,6 +1046,15 @@
   computeMassInFunction();
   unwrapLoops();
   finalizeMetrics();
+
+  if (CheckBFIUnknownBlockQueries) {
+    // To detect BFI queries for unknown blocks, add entries for unreachable
+    // blocks, if any. This is to distinguish between known/existing unreachable
+    // blocks and unknown blocks.
+    for (const BlockT &BB : F)
+      if (!Nodes.count(&BB))
+        setBlockFreq(&BB, 0);
+  }
 }
 
 template <class BT>
diff --git a/llvm/lib/Analysis/BlockFrequencyInfoImpl.cpp b/llvm/lib/Analysis/BlockFrequencyInfoImpl.cpp
--- a/llvm/lib/Analysis/BlockFrequencyInfoImpl.cpp
+++ b/llvm/lib/Analysis/BlockFrequencyInfoImpl.cpp
@@ -40,6 +40,12 @@
 
 #define DEBUG_TYPE "block-freq"
 
+cl::opt<bool> CheckBFIUnknownBlockQueries(
+    "check-bfi-unknown-block-queries",
+    cl::init(false), cl::Hidden,
+    cl::desc("Check if block frequency is queried for an unknown block "
+             "for debugging missed BFI updates"));
+
 ScaledNumber<uint64_t> BlockMass::toScaled() const {
   if (isFull())
     return ScaledNumber<uint64_t>(1, 0);
@@ -550,8 +556,17 @@
 
 BlockFrequency
 BlockFrequencyInfoImplBase::getBlockFreq(const BlockNode &Node) const {
-  if (!Node.isValid())
+  if (!Node.isValid()) {
+#ifndef NDEBUG
+    if (CheckBFIUnknownBlockQueries) {
+      SmallString<256> Msg;
+      raw_svector_ostream OS(Msg);
+      OS << "*** Detected BFI query for unknown block " << getBlockName(Node);
+      report_fatal_error(OS.str());
+    }
+#endif
     return 0;
+  }
   return Freqs[Node.Index].Integer;
 }