diff --git a/llvm/include/llvm/MC/MCWinEH.h b/llvm/include/llvm/MC/MCWinEH.h
--- a/llvm/include/llvm/MC/MCWinEH.h
+++ b/llvm/include/llvm/MC/MCWinEH.h
@@ -55,6 +55,9 @@
   const FrameInfo *ChainedParent = nullptr;
   std::vector<Instruction> Instructions;
   MapVector<MCSymbol*, std::vector<Instruction>> EpilogMap;
+#ifndef NDEBUG
+  MapVector<MCSymbol *, MCSymbol *> EpilogEnd;
+#endif
 
   FrameInfo() = default;
   FrameInfo(const MCSymbol *Function, const MCSymbol *BeginFuncEHLabel)
diff --git a/llvm/lib/MC/MCWin64EH.cpp b/llvm/lib/MC/MCWin64EH.cpp
--- a/llvm/lib/MC/MCWin64EH.cpp
+++ b/llvm/lib/MC/MCWin64EH.cpp
@@ -1189,7 +1189,8 @@
   return Count;
 }
 
-static uint32_t ARMCountOfInstructionBytes(ArrayRef<WinEH::Instruction> Insns) {
+static uint32_t ARMCountOfInstructionBytes(ArrayRef<WinEH::Instruction> Insns,
+                                           bool *HasCustom = nullptr) {
   uint32_t Count = 0;
   for (const auto &I : Insns) {
     switch (static_cast<Win64EH::UnwindOpcodes>(I.Operation)) {
@@ -1239,12 +1240,37 @@
       // We can't reason about what instructions this maps to; return a
       // phony number to make sure we don't accidentally do epilog packing.
       Count += 1000;
+      if (HasCustom)
+        *HasCustom = true;
       break;
     }
   }
   return Count;
 }
 
+#ifndef NDEBUG
+static void checkInstructions(MCStreamer &Streamer,
+                              ArrayRef<WinEH::Instruction> Insns,
+                              const MCSymbol *Begin, const MCSymbol *End,
+                              StringRef Name, StringRef Type) {
+  Optional<int64_t> MaybeDistance =
+      GetOptionalAbsDifference(Streamer, End, Begin);
+  if (!MaybeDistance)
+    return;
+  uint32_t Distance = (uint32_t)*MaybeDistance;
+  bool HasCustom = false;
+  uint32_t InstructionBytes = ARMCountOfInstructionBytes(Insns, &HasCustom);
+  if (HasCustom)
+    return;
+  if (Distance != InstructionBytes) {
+    errs() << "Incorrect size for " << Name << " " << Type << ": Symbol diff "
+           << Distance << " bytes, instructions map to " << InstructionBytes
+           << " bytes\n";
+    std::abort();
+  }
+}
+#endif
+
 // Unwind opcode encodings and restrictions are documented at
 // https://docs.microsoft.com/en-us/cpp/build/arm-exception-handling
 static void ARMEmitUnwindCode(MCStreamer &streamer, const MCSymbol *begin,
@@ -1906,6 +1932,21 @@
   streamer.emitLabel(Label);
   info->Symbol = Label;
 
+#ifndef NDEBUG
+  if (info->PrologEnd)
+    checkInstructions(streamer, info->Instructions, info->Begin, info->PrologEnd,
+                      info->Function->getName(), "prolog");
+  for (auto &I : info->EpilogMap) {
+    MCSymbol *EpilogStart = I.first;
+    auto &EpilogInstrs = I.second;
+    if (info->EpilogEnd.find(EpilogStart) != info->EpilogEnd.end()) {
+      checkInstructions(streamer, EpilogInstrs, EpilogStart,
+                        info->EpilogEnd[EpilogStart], info->Function->getName(),
+                        "epilog");
+    }
+  }
+#endif
+
   Optional<int64_t> RawFuncLength;
   const MCExpr *FuncLengthExpr = nullptr;
   if (!info->FuncletOrFuncEnd) {
diff --git a/llvm/lib/Target/ARM/MCTargetDesc/ARMWinCOFFStreamer.cpp b/llvm/lib/Target/ARM/MCTargetDesc/ARMWinCOFFStreamer.cpp
--- a/llvm/lib/Target/ARM/MCTargetDesc/ARMWinCOFFStreamer.cpp
+++ b/llvm/lib/Target/ARM/MCTargetDesc/ARMWinCOFFStreamer.cpp
@@ -252,6 +252,10 @@
   InEpilogCFI = false;
   WinEH::Instruction Inst = WinEH::Instruction(UnwindCode, nullptr, -1, 0);
   CurFrame->EpilogMap[CurrentEpilog].push_back(Inst);
+#ifndef NDEBUG
+  MCSymbol *Label = S.emitCFILabel();
+  CurFrame->EpilogEnd[CurrentEpilog] = Label;
+#endif
   CurrentEpilog = nullptr;
 }